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Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT

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  Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-19 23:40 +0200
    Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Michal Hocko <mhocko@kernel.org> - 2015-08-20 10:00 +0200
      Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-20 19:10 +0200
        Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Michal Hocko <mhocko@kernel.org> - 2015-08-21 09:30 +0200
          Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-21 20:40 +0200
            Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Konstantin Khlebnikov <koct9i@gmail.com> - 2015-08-24 12:20 +0200
              Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Vlastimil Babka <vbabka@suse.cz> - 2015-08-24 15:40 +0200
                Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Konstantin Khlebnikov <koct9i@gmail.com> - 2015-08-24 16:00 +0200
                  Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Vlastimil Babka <vbabka@suse.cz> - 2015-08-24 16:30 +0200
                    Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-24 17:10 +0200
                      Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Konstantin Khlebnikov <koct9i@gmail.com> - 2015-08-24 17:50 +0200
                        Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-24 18:00 +0200
                          Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Konstantin Khlebnikov <koct9i@gmail.com> - 2015-08-24 18:30 +0200
                            Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-24 19:10 +0200
                              Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Konstantin Khlebnikov <koct9i@gmail.com> - 2015-08-24 21:00 +0200
                                Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-24 22:30 +0200
            Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Michal Hocko <mhocko@kernel.org> - 2015-08-25 15:50 +0200
              Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Vlastimil Babka <vbabka@suse.cz> - 2015-08-25 16:00 +0200
                Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Michal Hocko <mhocko@kernel.org> - 2015-08-25 16:30 +0200
              Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Konstantin Khlebnikov <koct9i@gmail.com> - 2015-08-25 16:00 +0200
              Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-25 16:30 +0200
                Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Michal Hocko <mhocko@kernel.org> - 2015-08-25 21:00 +0200
                  Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-08-25 21:10 +0200
                    Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Michal Hocko <mhocko@kernel.org> - 2015-08-26 09:30 +0200
                      Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Vlastimil Babka <vbabka@suse.cz> - 2015-08-26 17:40 +0200
    Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Vlastimil Babka <vbabka@suse.cz> - 2015-08-20 10:00 +0200

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#1210035 — Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT

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On Wed, 12 Aug 2015, Michal Hocko wrote:

> On Sun 09-08-15 01:22:53, Eric B Munson wrote:
> > The cost of faulting in all memory to be locked can be very high when
> > working with large mappings.  If only portions of the mapping will be
> > used this can incur a high penalty for locking.
> > 
> > For the example of a large file, this is the usage pattern for a large
> > statical language model (probably applies to other statical or graphical
> > models as well).  For the security example, any application transacting
> > in data that cannot be swapped out (credit card data, medical records,
> > etc).
> > 
> > This patch introduces the ability to request that pages are not
> > pre-faulted, but are placed on the unevictable LRU when they are finally
> > faulted in.  The VM_LOCKONFAULT flag will be used together with
> > VM_LOCKED and has no effect when set without VM_LOCKED.
> 
> I do not like this very much to be honest. We have only few bits
> left there and it seems this is not really necessary. I thought that
> LOCKONFAULT acts as a modifier to the mlock call to tell whether to
> poppulate or not. The only place we have to persist it is
> mlockall(MCL_FUTURE) AFAICS. And this can be handled by an additional
> field in the mm_struct. This could be handled at __mm_populate level.
> So unless I am missing something this would be much more easier
> in the end we no new bit in VM flags would be necessary.
> 
> This would obviously mean that the LOCKONFAULT couldn't be exported to
> the userspace but is this really necessary?

Sorry for the latency here, I was on vacation and am now at plumbers.

I am not sure that growing the mm_struct by another flags field instead
of using available bits in the vm_flags is the right choice.  After this
patch, we still have 3 free bits on 32 bit architectures (2 after the
userfaultfd set IIRC).  The group which asked for this feature here
wants the ability to distinguish between LOCKED and LOCKONFAULT regions
and without the VMA flag there isn't a way to do that.

Do we know that these last two open flags are needed right now or is
this speculation that they will be and that none of the other VMA flags
can be reclaimed?

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#1210247

On Wed 19-08-15 17:33:45, Eric B Munson wrote:
[...]
> The group which asked for this feature here
> wants the ability to distinguish between LOCKED and LOCKONFAULT regions
> and without the VMA flag there isn't a way to do that.

Could you be more specific on why this is needed?

> Do we know that these last two open flags are needed right now or is
> this speculation that they will be and that none of the other VMA flags
> can be reclaimed?

I do not think they are needed by anybody right now but that is not a
reason why it should be used without a really strong justification.
If the discoverability is really needed then fair enough but I haven't
seen any justification for that yet.

-- 
Michal Hocko
SUSE Labs
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#1210635

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On Thu, 20 Aug 2015, Michal Hocko wrote:

> On Wed 19-08-15 17:33:45, Eric B Munson wrote:
> [...]
> > The group which asked for this feature here
> > wants the ability to distinguish between LOCKED and LOCKONFAULT regions
> > and without the VMA flag there isn't a way to do that.
> 
> Could you be more specific on why this is needed?

They want to keep metrics on the amount of memory used in a LOCKONFAULT
region versus the address space of the region.

> 
> > Do we know that these last two open flags are needed right now or is
> > this speculation that they will be and that none of the other VMA flags
> > can be reclaimed?
> 
> I do not think they are needed by anybody right now but that is not a
> reason why it should be used without a really strong justification.
> If the discoverability is really needed then fair enough but I haven't
> seen any justification for that yet.

To be completely clear you believe that if the metrics collection is
not a strong enough justification, it is better to expand the mm_struct
by another unsigned long than to use one of these bits right?

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#1210962

On Thu 20-08-15 13:03:09, Eric B Munson wrote:
> On Thu, 20 Aug 2015, Michal Hocko wrote:
> 
> > On Wed 19-08-15 17:33:45, Eric B Munson wrote:
> > [...]
> > > The group which asked for this feature here
> > > wants the ability to distinguish between LOCKED and LOCKONFAULT regions
> > > and without the VMA flag there isn't a way to do that.
> > 
> > Could you be more specific on why this is needed?
> 
> They want to keep metrics on the amount of memory used in a LOCKONFAULT
> region versus the address space of the region.

/proc/<pid>/smaps already exports that information AFAICS. It exports
VMA flags including VM_LOCKED and if rss < size then this is clearly
LOCKONFAULT because the standard mlock semantic is to populate. Would
that be sufficient?

Now, it is true that LOCKONFAULT wouldn't be distinguishable from
MAP_LOCKED which failed to populate but does that really matter? It is
LOCKONFAULT in a way as well.

> > > Do we know that these last two open flags are needed right now or is
> > > this speculation that they will be and that none of the other VMA flags
> > > can be reclaimed?
> > 
> > I do not think they are needed by anybody right now but that is not a
> > reason why it should be used without a really strong justification.
> > If the discoverability is really needed then fair enough but I haven't
> > seen any justification for that yet.
> 
> To be completely clear you believe that if the metrics collection is
> not a strong enough justification, it is better to expand the mm_struct
> by another unsigned long than to use one of these bits right?

A simple bool is sufficient for that. And yes I think we should go with
per mm_struct flag rather than the additional vma flag if it has only
the global (whole address space) scope - which would be the case if the
LOCKONFAULT is always an mlock modifier and the persistance is needed
only for MCL_FUTURE. Which is imho a sane semantic.
-- 
Michal Hocko
SUSE Labs
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#1211295

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On Fri, 21 Aug 2015, Michal Hocko wrote:

> On Thu 20-08-15 13:03:09, Eric B Munson wrote:
> > On Thu, 20 Aug 2015, Michal Hocko wrote:
> > 
> > > On Wed 19-08-15 17:33:45, Eric B Munson wrote:
> > > [...]
> > > > The group which asked for this feature here
> > > > wants the ability to distinguish between LOCKED and LOCKONFAULT regions
> > > > and without the VMA flag there isn't a way to do that.
> > > 
> > > Could you be more specific on why this is needed?
> > 
> > They want to keep metrics on the amount of memory used in a LOCKONFAULT
> > region versus the address space of the region.
> 
> /proc/<pid>/smaps already exports that information AFAICS. It exports
> VMA flags including VM_LOCKED and if rss < size then this is clearly
> LOCKONFAULT because the standard mlock semantic is to populate. Would
> that be sufficient?
> 
> Now, it is true that LOCKONFAULT wouldn't be distinguishable from
> MAP_LOCKED which failed to populate but does that really matter? It is
> LOCKONFAULT in a way as well.

Does that matter to my users?  No, they do not use MAP_LOCKED at all so
any VMA with VM_LOCKED set and rss < size is lock on fault.  Will it
matter to others?  I suspect so, but these are likely to be the same
group of users which will be suprised to learn that MAP_LOCKED does not
guarantee that the entire range is faulted in on return from mmap.

> 
> > > > Do we know that these last two open flags are needed right now or is
> > > > this speculation that they will be and that none of the other VMA flags
> > > > can be reclaimed?
> > > 
> > > I do not think they are needed by anybody right now but that is not a
> > > reason why it should be used without a really strong justification.
> > > If the discoverability is really needed then fair enough but I haven't
> > > seen any justification for that yet.
> > 
> > To be completely clear you believe that if the metrics collection is
> > not a strong enough justification, it is better to expand the mm_struct
> > by another unsigned long than to use one of these bits right?
> 
> A simple bool is sufficient for that. And yes I think we should go with
> per mm_struct flag rather than the additional vma flag if it has only
> the global (whole address space) scope - which would be the case if the
> LOCKONFAULT is always an mlock modifier and the persistance is needed
> only for MCL_FUTURE. Which is imho a sane semantic.

I am in the middle of implementing lock on fault this way, but I cannot
see how we will hanlde mremap of a lock on fault region.  Say we have
the following:

    addr = mmap(len, MAP_ANONYMOUS, ...);
    mlock(addr, len, MLOCK_ONFAULT);
    ...
    mremap(addr, len, 2 * len, ...)

There is no way for mremap to know that the area being remapped was lock
on fault so it will be locked and prefaulted by remap.  How can we avoid
this without tracking per vma if it was locked with lock or lock on
fault?

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#1212059

On Fri, Aug 21, 2015 at 9:31 PM, Eric B Munson <emunson@akamai.com> wrote:
> On Fri, 21 Aug 2015, Michal Hocko wrote:
>
>> On Thu 20-08-15 13:03:09, Eric B Munson wrote:
>> > On Thu, 20 Aug 2015, Michal Hocko wrote:
>> >
>> > > On Wed 19-08-15 17:33:45, Eric B Munson wrote:
>> > > [...]
>> > > > The group which asked for this feature here
>> > > > wants the ability to distinguish between LOCKED and LOCKONFAULT regions
>> > > > and without the VMA flag there isn't a way to do that.
>> > >
>> > > Could you be more specific on why this is needed?
>> >
>> > They want to keep metrics on the amount of memory used in a LOCKONFAULT
>> > region versus the address space of the region.
>>
>> /proc/<pid>/smaps already exports that information AFAICS. It exports
>> VMA flags including VM_LOCKED and if rss < size then this is clearly
>> LOCKONFAULT because the standard mlock semantic is to populate. Would
>> that be sufficient?
>>
>> Now, it is true that LOCKONFAULT wouldn't be distinguishable from
>> MAP_LOCKED which failed to populate but does that really matter? It is
>> LOCKONFAULT in a way as well.
>
> Does that matter to my users?  No, they do not use MAP_LOCKED at all so
> any VMA with VM_LOCKED set and rss < size is lock on fault.  Will it
> matter to others?  I suspect so, but these are likely to be the same
> group of users which will be suprised to learn that MAP_LOCKED does not
> guarantee that the entire range is faulted in on return from mmap.
>
>>
>> > > > Do we know that these last two open flags are needed right now or is
>> > > > this speculation that they will be and that none of the other VMA flags
>> > > > can be reclaimed?
>> > >
>> > > I do not think they are needed by anybody right now but that is not a
>> > > reason why it should be used without a really strong justification.
>> > > If the discoverability is really needed then fair enough but I haven't
>> > > seen any justification for that yet.
>> >
>> > To be completely clear you believe that if the metrics collection is
>> > not a strong enough justification, it is better to expand the mm_struct
>> > by another unsigned long than to use one of these bits right?
>>
>> A simple bool is sufficient for that. And yes I think we should go with
>> per mm_struct flag rather than the additional vma flag if it has only
>> the global (whole address space) scope - which would be the case if the
>> LOCKONFAULT is always an mlock modifier and the persistance is needed
>> only for MCL_FUTURE. Which is imho a sane semantic.
>
> I am in the middle of implementing lock on fault this way, but I cannot
> see how we will hanlde mremap of a lock on fault region.  Say we have
> the following:
>
>     addr = mmap(len, MAP_ANONYMOUS, ...);
>     mlock(addr, len, MLOCK_ONFAULT);
>     ...
>     mremap(addr, len, 2 * len, ...)
>
> There is no way for mremap to know that the area being remapped was lock
> on fault so it will be locked and prefaulted by remap.  How can we avoid
> this without tracking per vma if it was locked with lock or lock on
> fault?

remap can count filled ptes and prefault only completely populated areas.

There might be a problem after failed populate: remap will handle them
as lock on fault. In this case we can fill ptes with swap-like non-present
entries to remember that fact and count them as should-be-locked pages.
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#1212192

On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
>>
>> I am in the middle of implementing lock on fault this way, but I cannot
>> see how we will hanlde mremap of a lock on fault region.  Say we have
>> the following:
>>
>>      addr = mmap(len, MAP_ANONYMOUS, ...);
>>      mlock(addr, len, MLOCK_ONFAULT);
>>      ...
>>      mremap(addr, len, 2 * len, ...)
>>
>> There is no way for mremap to know that the area being remapped was lock
>> on fault so it will be locked and prefaulted by remap.  How can we avoid
>> this without tracking per vma if it was locked with lock or lock on
>> fault?
>
> remap can count filled ptes and prefault only completely populated areas.

Does (and should) mremap really prefault non-present pages? Shouldn't it 
just prepare the page tables and that's it?

> There might be a problem after failed populate: remap will handle them
> as lock on fault. In this case we can fill ptes with swap-like non-present
> entries to remember that fact and count them as should-be-locked pages.

I don't think we should strive to have mremap try to fix the inherent 
unreliability of mmap (MAP_POPULATE)?
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#1212211

On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
> On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
>>>
>>>
>>> I am in the middle of implementing lock on fault this way, but I cannot
>>> see how we will hanlde mremap of a lock on fault region.  Say we have
>>> the following:
>>>
>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
>>>      mlock(addr, len, MLOCK_ONFAULT);
>>>      ...
>>>      mremap(addr, len, 2 * len, ...)
>>>
>>> There is no way for mremap to know that the area being remapped was lock
>>> on fault so it will be locked and prefaulted by remap.  How can we avoid
>>> this without tracking per vma if it was locked with lock or lock on
>>> fault?
>>
>>
>> remap can count filled ptes and prefault only completely populated areas.
>
>
> Does (and should) mremap really prefault non-present pages? Shouldn't it
> just prepare the page tables and that's it?

As I see mremap prefaults pages when it extends mlocked area.

Also quote from manpage
: If  the memory segment specified by old_address and old_size is locked
: (using mlock(2) or similar), then this lock is maintained when the segment is
: resized and/or relocated.  As a  consequence, the amount of memory locked
: by the process may change.

>
>> There might be a problem after failed populate: remap will handle them
>> as lock on fault. In this case we can fill ptes with swap-like non-present
>> entries to remember that fact and count them as should-be-locked pages.
>
>
> I don't think we should strive to have mremap try to fix the inherent
> unreliability of mmap (MAP_POPULATE)?

I don't think so. MAP_POPULATE works only when mmap happens.
Flag MREMAP_POPULATE might be a good idea. Just for symmetry.
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#1212240

On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
> On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
>> On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
>>>>
>>>>
>>>> I am in the middle of implementing lock on fault this way, but I cannot
>>>> see how we will hanlde mremap of a lock on fault region.  Say we have
>>>> the following:
>>>>
>>>>       addr = mmap(len, MAP_ANONYMOUS, ...);
>>>>       mlock(addr, len, MLOCK_ONFAULT);
>>>>       ...
>>>>       mremap(addr, len, 2 * len, ...)
>>>>
>>>> There is no way for mremap to know that the area being remapped was lock
>>>> on fault so it will be locked and prefaulted by remap.  How can we avoid
>>>> this without tracking per vma if it was locked with lock or lock on
>>>> fault?
>>>
>>>
>>> remap can count filled ptes and prefault only completely populated areas.
>>
>>
>> Does (and should) mremap really prefault non-present pages? Shouldn't it
>> just prepare the page tables and that's it?
>
> As I see mremap prefaults pages when it extends mlocked area.
>
> Also quote from manpage
> : If  the memory segment specified by old_address and old_size is locked
> : (using mlock(2) or similar), then this lock is maintained when the segment is
> : resized and/or relocated.  As a  consequence, the amount of memory locked
> : by the process may change.

Oh, right... Well that looks like a convincing argument for having a 
sticky VM_LOCKONFAULT after all. Having mremap guess by scanning 
existing pte's would slow it down, and be unreliable (was the area 
completely populated because MLOCK_ONFAULT was not used or because the 
process aulted it already? Was it not populated because MLOCK_ONFAULT 
was used, or because mmap(MAP_LOCKED) failed to populate it all?).

The only sane alternative is to populate always for mremap() of 
VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information as 
a limitation of mlock2(MLOCK_ONFAULT). Which might or might not be 
enough for Eric's usecase, but it's somewhat ugly.

>>
>>> There might be a problem after failed populate: remap will handle them
>>> as lock on fault. In this case we can fill ptes with swap-like non-present
>>> entries to remember that fact and count them as should-be-locked pages.
>>
>>
>> I don't think we should strive to have mremap try to fix the inherent
>> unreliability of mmap (MAP_POPULATE)?
>
> I don't think so. MAP_POPULATE works only when mmap happens.
> Flag MREMAP_POPULATE might be a good idea. Just for symmetry.

Maybe, but please do it as a separate series.
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#1212295

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On Mon, 24 Aug 2015, Vlastimil Babka wrote:

> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
> >>>>
> >>>>
> >>>>I am in the middle of implementing lock on fault this way, but I cannot
> >>>>see how we will hanlde mremap of a lock on fault region.  Say we have
> >>>>the following:
> >>>>
> >>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
> >>>>      mlock(addr, len, MLOCK_ONFAULT);
> >>>>      ...
> >>>>      mremap(addr, len, 2 * len, ...)
> >>>>
> >>>>There is no way for mremap to know that the area being remapped was lock
> >>>>on fault so it will be locked and prefaulted by remap.  How can we avoid
> >>>>this without tracking per vma if it was locked with lock or lock on
> >>>>fault?
> >>>
> >>>
> >>>remap can count filled ptes and prefault only completely populated areas.
> >>
> >>
> >>Does (and should) mremap really prefault non-present pages? Shouldn't it
> >>just prepare the page tables and that's it?
> >
> >As I see mremap prefaults pages when it extends mlocked area.
> >
> >Also quote from manpage
> >: If  the memory segment specified by old_address and old_size is locked
> >: (using mlock(2) or similar), then this lock is maintained when the segment is
> >: resized and/or relocated.  As a  consequence, the amount of memory locked
> >: by the process may change.
> 
> Oh, right... Well that looks like a convincing argument for having a
> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
> existing pte's would slow it down, and be unreliable (was the area
> completely populated because MLOCK_ONFAULT was not used or because
> the process aulted it already? Was it not populated because
> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to
> populate it all?).

Given this, I am going to stop working in v8 and leave the vma flag in
place.

> 
> The only sane alternative is to populate always for mremap() of
> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information
> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not
> be enough for Eric's usecase, but it's somewhat ugly.
> 

I don't think that this is the right solution, I would be really
surprised as a user if an area I locked with MLOCK_ONFAULT was then
fully locked and prepopulated after mremap().

> >>
> >>>There might be a problem after failed populate: remap will handle them
> >>>as lock on fault. In this case we can fill ptes with swap-like non-present
> >>>entries to remember that fact and count them as should-be-locked pages.
> >>
> >>
> >>I don't think we should strive to have mremap try to fix the inherent
> >>unreliability of mmap (MAP_POPULATE)?
> >
> >I don't think so. MAP_POPULATE works only when mmap happens.
> >Flag MREMAP_POPULATE might be a good idea. Just for symmetry.
> 
> Maybe, but please do it as a separate series.
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

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#1212321

On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@akamai.com> wrote:
> On Mon, 24 Aug 2015, Vlastimil Babka wrote:
>
>> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
>> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
>> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
>> >>>>
>> >>>>
>> >>>>I am in the middle of implementing lock on fault this way, but I cannot
>> >>>>see how we will hanlde mremap of a lock on fault region.  Say we have
>> >>>>the following:
>> >>>>
>> >>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
>> >>>>      mlock(addr, len, MLOCK_ONFAULT);
>> >>>>      ...
>> >>>>      mremap(addr, len, 2 * len, ...)
>> >>>>
>> >>>>There is no way for mremap to know that the area being remapped was lock
>> >>>>on fault so it will be locked and prefaulted by remap.  How can we avoid
>> >>>>this without tracking per vma if it was locked with lock or lock on
>> >>>>fault?
>> >>>
>> >>>
>> >>>remap can count filled ptes and prefault only completely populated areas.
>> >>
>> >>
>> >>Does (and should) mremap really prefault non-present pages? Shouldn't it
>> >>just prepare the page tables and that's it?
>> >
>> >As I see mremap prefaults pages when it extends mlocked area.
>> >
>> >Also quote from manpage
>> >: If  the memory segment specified by old_address and old_size is locked
>> >: (using mlock(2) or similar), then this lock is maintained when the segment is
>> >: resized and/or relocated.  As a  consequence, the amount of memory locked
>> >: by the process may change.
>>
>> Oh, right... Well that looks like a convincing argument for having a
>> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
>> existing pte's would slow it down, and be unreliable (was the area
>> completely populated because MLOCK_ONFAULT was not used or because
>> the process aulted it already? Was it not populated because
>> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to
>> populate it all?).
>
> Given this, I am going to stop working in v8 and leave the vma flag in
> place.
>
>>
>> The only sane alternative is to populate always for mremap() of
>> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information
>> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not
>> be enough for Eric's usecase, but it's somewhat ugly.
>>
>
> I don't think that this is the right solution, I would be really
> surprised as a user if an area I locked with MLOCK_ONFAULT was then
> fully locked and prepopulated after mremap().

If mremap is the only problem then we can add opposite flag for it:

"MREMAP_NOPOPULATE"
- do not populate new segment of locked areas
- do not copy normal areas if possible (anonymous/special must be copied)

addr = mmap(len, MAP_ANONYMOUS, ...);
mlock(addr, len, MLOCK_ONFAULT);
...
addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE);
...

>
>> >>
>> >>>There might be a problem after failed populate: remap will handle them
>> >>>as lock on fault. In this case we can fill ptes with swap-like non-present
>> >>>entries to remember that fact and count them as should-be-locked pages.
>> >>
>> >>
>> >>I don't think we should strive to have mremap try to fix the inherent
>> >>unreliability of mmap (MAP_POPULATE)?
>> >
>> >I don't think so. MAP_POPULATE works only when mmap happens.
>> >Flag MREMAP_POPULATE might be a good idea. Just for symmetry.
>>
>> Maybe, but please do it as a separate series.
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@kvack.org.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
--
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the body of a message to majordomo@vger.kernel.org
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#1212323

[Multipart message — attachments visible in raw view] — view raw

On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:

> On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@akamai.com> wrote:
> > On Mon, 24 Aug 2015, Vlastimil Babka wrote:
> >
> >> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
> >> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
> >> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
> >> >>>>
> >> >>>>
> >> >>>>I am in the middle of implementing lock on fault this way, but I cannot
> >> >>>>see how we will hanlde mremap of a lock on fault region.  Say we have
> >> >>>>the following:
> >> >>>>
> >> >>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
> >> >>>>      mlock(addr, len, MLOCK_ONFAULT);
> >> >>>>      ...
> >> >>>>      mremap(addr, len, 2 * len, ...)
> >> >>>>
> >> >>>>There is no way for mremap to know that the area being remapped was lock
> >> >>>>on fault so it will be locked and prefaulted by remap.  How can we avoid
> >> >>>>this without tracking per vma if it was locked with lock or lock on
> >> >>>>fault?
> >> >>>
> >> >>>
> >> >>>remap can count filled ptes and prefault only completely populated areas.
> >> >>
> >> >>
> >> >>Does (and should) mremap really prefault non-present pages? Shouldn't it
> >> >>just prepare the page tables and that's it?
> >> >
> >> >As I see mremap prefaults pages when it extends mlocked area.
> >> >
> >> >Also quote from manpage
> >> >: If  the memory segment specified by old_address and old_size is locked
> >> >: (using mlock(2) or similar), then this lock is maintained when the segment is
> >> >: resized and/or relocated.  As a  consequence, the amount of memory locked
> >> >: by the process may change.
> >>
> >> Oh, right... Well that looks like a convincing argument for having a
> >> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
> >> existing pte's would slow it down, and be unreliable (was the area
> >> completely populated because MLOCK_ONFAULT was not used or because
> >> the process aulted it already? Was it not populated because
> >> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to
> >> populate it all?).
> >
> > Given this, I am going to stop working in v8 and leave the vma flag in
> > place.
> >
> >>
> >> The only sane alternative is to populate always for mremap() of
> >> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information
> >> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not
> >> be enough for Eric's usecase, but it's somewhat ugly.
> >>
> >
> > I don't think that this is the right solution, I would be really
> > surprised as a user if an area I locked with MLOCK_ONFAULT was then
> > fully locked and prepopulated after mremap().
> 
> If mremap is the only problem then we can add opposite flag for it:
> 
> "MREMAP_NOPOPULATE"
> - do not populate new segment of locked areas
> - do not copy normal areas if possible (anonymous/special must be copied)
> 
> addr = mmap(len, MAP_ANONYMOUS, ...);
> mlock(addr, len, MLOCK_ONFAULT);
> ...
> addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE);
> ...
> 

But with this, the user must remember what areas are locked with
MLOCK_LOCKONFAULT and which are locked the with prepopulate so the
correct mremap flags can be used.

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#1212337

On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson <emunson@akamai.com> wrote:
> On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
>
>> On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@akamai.com> wrote:
>> > On Mon, 24 Aug 2015, Vlastimil Babka wrote:
>> >
>> >> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
>> >> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
>> >> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
>> >> >>>>
>> >> >>>>
>> >> >>>>I am in the middle of implementing lock on fault this way, but I cannot
>> >> >>>>see how we will hanlde mremap of a lock on fault region.  Say we have
>> >> >>>>the following:
>> >> >>>>
>> >> >>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
>> >> >>>>      mlock(addr, len, MLOCK_ONFAULT);
>> >> >>>>      ...
>> >> >>>>      mremap(addr, len, 2 * len, ...)
>> >> >>>>
>> >> >>>>There is no way for mremap to know that the area being remapped was lock
>> >> >>>>on fault so it will be locked and prefaulted by remap.  How can we avoid
>> >> >>>>this without tracking per vma if it was locked with lock or lock on
>> >> >>>>fault?
>> >> >>>
>> >> >>>
>> >> >>>remap can count filled ptes and prefault only completely populated areas.
>> >> >>
>> >> >>
>> >> >>Does (and should) mremap really prefault non-present pages? Shouldn't it
>> >> >>just prepare the page tables and that's it?
>> >> >
>> >> >As I see mremap prefaults pages when it extends mlocked area.
>> >> >
>> >> >Also quote from manpage
>> >> >: If  the memory segment specified by old_address and old_size is locked
>> >> >: (using mlock(2) or similar), then this lock is maintained when the segment is
>> >> >: resized and/or relocated.  As a  consequence, the amount of memory locked
>> >> >: by the process may change.
>> >>
>> >> Oh, right... Well that looks like a convincing argument for having a
>> >> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
>> >> existing pte's would slow it down, and be unreliable (was the area
>> >> completely populated because MLOCK_ONFAULT was not used or because
>> >> the process aulted it already? Was it not populated because
>> >> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to
>> >> populate it all?).
>> >
>> > Given this, I am going to stop working in v8 and leave the vma flag in
>> > place.
>> >
>> >>
>> >> The only sane alternative is to populate always for mremap() of
>> >> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information
>> >> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not
>> >> be enough for Eric's usecase, but it's somewhat ugly.
>> >>
>> >
>> > I don't think that this is the right solution, I would be really
>> > surprised as a user if an area I locked with MLOCK_ONFAULT was then
>> > fully locked and prepopulated after mremap().
>>
>> If mremap is the only problem then we can add opposite flag for it:
>>
>> "MREMAP_NOPOPULATE"
>> - do not populate new segment of locked areas
>> - do not copy normal areas if possible (anonymous/special must be copied)
>>
>> addr = mmap(len, MAP_ANONYMOUS, ...);
>> mlock(addr, len, MLOCK_ONFAULT);
>> ...
>> addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE);
>> ...
>>
>
> But with this, the user must remember what areas are locked with
> MLOCK_LOCKONFAULT and which are locked the with prepopulate so the
> correct mremap flags can be used.
>

Yep. Shouldn't be hard. You anyway have to do some changes in user-space.


Much simpler for users-pace solution is a mm-wide flag which turns all further
mlocks and MAP_LOCKED into lock-on-fault. Something like
mlockall(MCL_NOPOPULATE_LOCKED).
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

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#1212353

[Multipart message — attachments visible in raw view] — view raw

On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:

> On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson <emunson@akamai.com> wrote:
> > On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
> >
> >> On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@akamai.com> wrote:
> >> > On Mon, 24 Aug 2015, Vlastimil Babka wrote:
> >> >
> >> >> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
> >> >> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
> >> >> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
> >> >> >>>>
> >> >> >>>>
> >> >> >>>>I am in the middle of implementing lock on fault this way, but I cannot
> >> >> >>>>see how we will hanlde mremap of a lock on fault region.  Say we have
> >> >> >>>>the following:
> >> >> >>>>
> >> >> >>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
> >> >> >>>>      mlock(addr, len, MLOCK_ONFAULT);
> >> >> >>>>      ...
> >> >> >>>>      mremap(addr, len, 2 * len, ...)
> >> >> >>>>
> >> >> >>>>There is no way for mremap to know that the area being remapped was lock
> >> >> >>>>on fault so it will be locked and prefaulted by remap.  How can we avoid
> >> >> >>>>this without tracking per vma if it was locked with lock or lock on
> >> >> >>>>fault?
> >> >> >>>
> >> >> >>>
> >> >> >>>remap can count filled ptes and prefault only completely populated areas.
> >> >> >>
> >> >> >>
> >> >> >>Does (and should) mremap really prefault non-present pages? Shouldn't it
> >> >> >>just prepare the page tables and that's it?
> >> >> >
> >> >> >As I see mremap prefaults pages when it extends mlocked area.
> >> >> >
> >> >> >Also quote from manpage
> >> >> >: If  the memory segment specified by old_address and old_size is locked
> >> >> >: (using mlock(2) or similar), then this lock is maintained when the segment is
> >> >> >: resized and/or relocated.  As a  consequence, the amount of memory locked
> >> >> >: by the process may change.
> >> >>
> >> >> Oh, right... Well that looks like a convincing argument for having a
> >> >> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
> >> >> existing pte's would slow it down, and be unreliable (was the area
> >> >> completely populated because MLOCK_ONFAULT was not used or because
> >> >> the process aulted it already? Was it not populated because
> >> >> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to
> >> >> populate it all?).
> >> >
> >> > Given this, I am going to stop working in v8 and leave the vma flag in
> >> > place.
> >> >
> >> >>
> >> >> The only sane alternative is to populate always for mremap() of
> >> >> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information
> >> >> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not
> >> >> be enough for Eric's usecase, but it's somewhat ugly.
> >> >>
> >> >
> >> > I don't think that this is the right solution, I would be really
> >> > surprised as a user if an area I locked with MLOCK_ONFAULT was then
> >> > fully locked and prepopulated after mremap().
> >>
> >> If mremap is the only problem then we can add opposite flag for it:
> >>
> >> "MREMAP_NOPOPULATE"
> >> - do not populate new segment of locked areas
> >> - do not copy normal areas if possible (anonymous/special must be copied)
> >>
> >> addr = mmap(len, MAP_ANONYMOUS, ...);
> >> mlock(addr, len, MLOCK_ONFAULT);
> >> ...
> >> addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE);
> >> ...
> >>
> >
> > But with this, the user must remember what areas are locked with
> > MLOCK_LOCKONFAULT and which are locked the with prepopulate so the
> > correct mremap flags can be used.
> >
> 
> Yep. Shouldn't be hard. You anyway have to do some changes in user-space.
> 

Sorry if I wasn't clear enough in my last reply, I think forcing
userspace to track this is the wrong choice.  The VM system is
responsible for tracking these attributes and should continue to be.

> 
> Much simpler for users-pace solution is a mm-wide flag which turns all further
> mlocks and MAP_LOCKED into lock-on-fault. Something like
> mlockall(MCL_NOPOPULATE_LOCKED).

This set certainly adds the foundation for such a change if you think it
would be useful.  That particular behavior was not part of my inital use
case though.

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#1212422

On Mon, Aug 24, 2015 at 8:00 PM, Eric B Munson <emunson@akamai.com> wrote:
> On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
>
>> On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson <emunson@akamai.com> wrote:
>> > On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
>> >
>> >> On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@akamai.com> wrote:
>> >> > On Mon, 24 Aug 2015, Vlastimil Babka wrote:
>> >> >
>> >> >> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
>> >> >> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
>> >> >> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
>> >> >> >>>>
>> >> >> >>>>
>> >> >> >>>>I am in the middle of implementing lock on fault this way, but I cannot
>> >> >> >>>>see how we will hanlde mremap of a lock on fault region.  Say we have
>> >> >> >>>>the following:
>> >> >> >>>>
>> >> >> >>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
>> >> >> >>>>      mlock(addr, len, MLOCK_ONFAULT);
>> >> >> >>>>      ...
>> >> >> >>>>      mremap(addr, len, 2 * len, ...)
>> >> >> >>>>
>> >> >> >>>>There is no way for mremap to know that the area being remapped was lock
>> >> >> >>>>on fault so it will be locked and prefaulted by remap.  How can we avoid
>> >> >> >>>>this without tracking per vma if it was locked with lock or lock on
>> >> >> >>>>fault?
>> >> >> >>>
>> >> >> >>>
>> >> >> >>>remap can count filled ptes and prefault only completely populated areas.
>> >> >> >>
>> >> >> >>
>> >> >> >>Does (and should) mremap really prefault non-present pages? Shouldn't it
>> >> >> >>just prepare the page tables and that's it?
>> >> >> >
>> >> >> >As I see mremap prefaults pages when it extends mlocked area.
>> >> >> >
>> >> >> >Also quote from manpage
>> >> >> >: If  the memory segment specified by old_address and old_size is locked
>> >> >> >: (using mlock(2) or similar), then this lock is maintained when the segment is
>> >> >> >: resized and/or relocated.  As a  consequence, the amount of memory locked
>> >> >> >: by the process may change.
>> >> >>
>> >> >> Oh, right... Well that looks like a convincing argument for having a
>> >> >> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
>> >> >> existing pte's would slow it down, and be unreliable (was the area
>> >> >> completely populated because MLOCK_ONFAULT was not used or because
>> >> >> the process aulted it already? Was it not populated because
>> >> >> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to
>> >> >> populate it all?).
>> >> >
>> >> > Given this, I am going to stop working in v8 and leave the vma flag in
>> >> > place.
>> >> >
>> >> >>
>> >> >> The only sane alternative is to populate always for mremap() of
>> >> >> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information
>> >> >> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not
>> >> >> be enough for Eric's usecase, but it's somewhat ugly.
>> >> >>
>> >> >
>> >> > I don't think that this is the right solution, I would be really
>> >> > surprised as a user if an area I locked with MLOCK_ONFAULT was then
>> >> > fully locked and prepopulated after mremap().
>> >>
>> >> If mremap is the only problem then we can add opposite flag for it:
>> >>
>> >> "MREMAP_NOPOPULATE"
>> >> - do not populate new segment of locked areas
>> >> - do not copy normal areas if possible (anonymous/special must be copied)
>> >>
>> >> addr = mmap(len, MAP_ANONYMOUS, ...);
>> >> mlock(addr, len, MLOCK_ONFAULT);
>> >> ...
>> >> addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE);
>> >> ...
>> >>
>> >
>> > But with this, the user must remember what areas are locked with
>> > MLOCK_LOCKONFAULT and which are locked the with prepopulate so the
>> > correct mremap flags can be used.
>> >
>>
>> Yep. Shouldn't be hard. You anyway have to do some changes in user-space.
>>
>
> Sorry if I wasn't clear enough in my last reply, I think forcing
> userspace to track this is the wrong choice.  The VM system is
> responsible for tracking these attributes and should continue to be.

Userspace tracks addresses and sizes of these areas. Plus mremap obviously
works only with page granularity so memory allocator in userspace have to know
a lot about these structures. So keeping one more bit isn't a rocket science.

>
>>
>> Much simpler for users-pace solution is a mm-wide flag which turns all further
>> mlocks and MAP_LOCKED into lock-on-fault. Something like
>> mlockall(MCL_NOPOPULATE_LOCKED).
>
> This set certainly adds the foundation for such a change if you think it
> would be useful.  That particular behavior was not part of my inital use
> case though.
>

This looks like much easier solution: you don't need new syscall and after
enabling that lock-on-fault mode userspace still can get old behaviour simply
by touching newly locked area.
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To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
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#1212491

[Multipart message — attachments visible in raw view] — view raw

On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:

> On Mon, Aug 24, 2015 at 8:00 PM, Eric B Munson <emunson@akamai.com> wrote:
> > On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
> >
> >> On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson <emunson@akamai.com> wrote:
> >> > On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
> >> >
> >> >> On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@akamai.com> wrote:
> >> >> > On Mon, 24 Aug 2015, Vlastimil Babka wrote:
> >> >> >
> >> >> >> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
> >> >> >> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
> >> >> >> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
> >> >> >> >>>>
> >> >> >> >>>>
> >> >> >> >>>>I am in the middle of implementing lock on fault this way, but I cannot
> >> >> >> >>>>see how we will hanlde mremap of a lock on fault region.  Say we have
> >> >> >> >>>>the following:
> >> >> >> >>>>
> >> >> >> >>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
> >> >> >> >>>>      mlock(addr, len, MLOCK_ONFAULT);
> >> >> >> >>>>      ...
> >> >> >> >>>>      mremap(addr, len, 2 * len, ...)
> >> >> >> >>>>
> >> >> >> >>>>There is no way for mremap to know that the area being remapped was lock
> >> >> >> >>>>on fault so it will be locked and prefaulted by remap.  How can we avoid
> >> >> >> >>>>this without tracking per vma if it was locked with lock or lock on
> >> >> >> >>>>fault?
> >> >> >> >>>
> >> >> >> >>>
> >> >> >> >>>remap can count filled ptes and prefault only completely populated areas.
> >> >> >> >>
> >> >> >> >>
> >> >> >> >>Does (and should) mremap really prefault non-present pages? Shouldn't it
> >> >> >> >>just prepare the page tables and that's it?
> >> >> >> >
> >> >> >> >As I see mremap prefaults pages when it extends mlocked area.
> >> >> >> >
> >> >> >> >Also quote from manpage
> >> >> >> >: If  the memory segment specified by old_address and old_size is locked
> >> >> >> >: (using mlock(2) or similar), then this lock is maintained when the segment is
> >> >> >> >: resized and/or relocated.  As a  consequence, the amount of memory locked
> >> >> >> >: by the process may change.
> >> >> >>
> >> >> >> Oh, right... Well that looks like a convincing argument for having a
> >> >> >> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
> >> >> >> existing pte's would slow it down, and be unreliable (was the area
> >> >> >> completely populated because MLOCK_ONFAULT was not used or because
> >> >> >> the process aulted it already? Was it not populated because
> >> >> >> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to
> >> >> >> populate it all?).
> >> >> >
> >> >> > Given this, I am going to stop working in v8 and leave the vma flag in
> >> >> > place.
> >> >> >
> >> >> >>
> >> >> >> The only sane alternative is to populate always for mremap() of
> >> >> >> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information
> >> >> >> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not
> >> >> >> be enough for Eric's usecase, but it's somewhat ugly.
> >> >> >>
> >> >> >
> >> >> > I don't think that this is the right solution, I would be really
> >> >> > surprised as a user if an area I locked with MLOCK_ONFAULT was then
> >> >> > fully locked and prepopulated after mremap().
> >> >>
> >> >> If mremap is the only problem then we can add opposite flag for it:
> >> >>
> >> >> "MREMAP_NOPOPULATE"
> >> >> - do not populate new segment of locked areas
> >> >> - do not copy normal areas if possible (anonymous/special must be copied)
> >> >>
> >> >> addr = mmap(len, MAP_ANONYMOUS, ...);
> >> >> mlock(addr, len, MLOCK_ONFAULT);
> >> >> ...
> >> >> addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE);
> >> >> ...
> >> >>
> >> >
> >> > But with this, the user must remember what areas are locked with
> >> > MLOCK_LOCKONFAULT and which are locked the with prepopulate so the
> >> > correct mremap flags can be used.
> >> >
> >>
> >> Yep. Shouldn't be hard. You anyway have to do some changes in user-space.
> >>
> >
> > Sorry if I wasn't clear enough in my last reply, I think forcing
> > userspace to track this is the wrong choice.  The VM system is
> > responsible for tracking these attributes and should continue to be.
> 
> Userspace tracks addresses and sizes of these areas. Plus mremap obviously
> works only with page granularity so memory allocator in userspace have to know
> a lot about these structures. So keeping one more bit isn't a rocket science.
> 

Fair enough, however, my current implementation does not require that
userspace keep track of any extra information.  With the VM_LOCKONFAULT
flag mremap() keeps the properties that were set with mlock() or
equivalent across remaps.

> >
> >>
> >> Much simpler for users-pace solution is a mm-wide flag which turns all further
> >> mlocks and MAP_LOCKED into lock-on-fault. Something like
> >> mlockall(MCL_NOPOPULATE_LOCKED).
> >
> > This set certainly adds the foundation for such a change if you think it
> > would be useful.  That particular behavior was not part of my inital use
> > case though.
> >
> 
> This looks like much easier solution: you don't need new syscall and after
> enabling that lock-on-fault mode userspace still can get old behaviour simply
> by touching newly locked area.

Again, this suggestion requires that userspace know more about VM than
with my implementation and will require it to walk an entire mapping
before use to fault it in if required.  With the current implementation,
mlock continues to function as it has, with the additional flexibility
of being able to request that areas not be prepopulated.

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#1213046

On Fri 21-08-15 14:31:32, Eric B Munson wrote:
[...]
> I am in the middle of implementing lock on fault this way, but I cannot
> see how we will hanlde mremap of a lock on fault region.  Say we have
> the following:
> 
>     addr = mmap(len, MAP_ANONYMOUS, ...);
>     mlock(addr, len, MLOCK_ONFAULT);
>     ...
>     mremap(addr, len, 2 * len, ...)
> 
> There is no way for mremap to know that the area being remapped was lock
> on fault so it will be locked and prefaulted by remap.  How can we avoid
> this without tracking per vma if it was locked with lock or lock on
> fault?

Yes mremap is a problem and it is very much similar to mmap(MAP_LOCKED).
It doesn't guarantee the full mlock semantic because it leaves partially
populated ranges behind without reporting any error.

Considering the current behavior I do not thing it would be terrible
thing to do what Konstantin was suggesting and populate only the full
ranges in a best effort mode (it is done so anyway) and document the
behavior properly.
"
       If the memory segment specified by old_address and old_size is
       locked (using mlock(2) or similar), then this lock is maintained
       when the segment is resized and/or relocated. As a consequence,
       the amount of memory locked by the process may change.

       If the range is already fully populated and the range is
       enlarged the new range is attempted to be fully populated
       as well to preserve the full mlock semantic but there is no
       guarantee this will succeed. Partially populated (e.g. created by
       mlock(MLOCK_ONFAULT)) ranges do not have the full mlock semantic
       so they are not populated on resize.
"

So what we have as a result is that partially populated ranges are
preserved and fully populated ones work in the best effort mode the same
way as they are now.

Does that sound at least remotely reasonably?


-- 
Michal Hocko
SUSE Labs
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#1213048

On 08/25/2015 03:41 PM, Michal Hocko wrote:
> On Fri 21-08-15 14:31:32, Eric B Munson wrote:
> [...]
>> I am in the middle of implementing lock on fault this way, but I cannot
>> see how we will hanlde mremap of a lock on fault region.  Say we have
>> the following:
>>
>>      addr = mmap(len, MAP_ANONYMOUS, ...);
>>      mlock(addr, len, MLOCK_ONFAULT);
>>      ...
>>      mremap(addr, len, 2 * len, ...)
>>
>> There is no way for mremap to know that the area being remapped was lock
>> on fault so it will be locked and prefaulted by remap.  How can we avoid
>> this without tracking per vma if it was locked with lock or lock on
>> fault?
>
> Yes mremap is a problem and it is very much similar to mmap(MAP_LOCKED).
> It doesn't guarantee the full mlock semantic because it leaves partially
> populated ranges behind without reporting any error.

Hm, that's right.

> Considering the current behavior I do not thing it would be terrible
> thing to do what Konstantin was suggesting and populate only the full
> ranges in a best effort mode (it is done so anyway) and document the
> behavior properly.
> "
>         If the memory segment specified by old_address and old_size is
>         locked (using mlock(2) or similar), then this lock is maintained
>         when the segment is resized and/or relocated. As a consequence,
>         the amount of memory locked by the process may change.
>
>         If the range is already fully populated and the range is
>         enlarged the new range is attempted to be fully populated
>         as well to preserve the full mlock semantic but there is no
>         guarantee this will succeed. Partially populated (e.g. created by
>         mlock(MLOCK_ONFAULT)) ranges do not have the full mlock semantic
>         so they are not populated on resize.
> "
>
> So what we have as a result is that partially populated ranges are
> preserved and fully populated ones work in the best effort mode the same
> way as they are now.
>
> Does that sound at least remotely reasonably?

I'll basically repeat what I said earlier:

- mremap scanning existing pte's to figure out the population would slow 
it down for no good reason
- it would be unreliable anyway:
   - example: was the area completely populated because MLOCK_ONFAULT 
was not used or because the  process faulted it already
   - example: was the area not completely populated because 
MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to populate 
it fully?

I think the first point is a pointless regression for workloads that use 
just plain mlock() and don't want the onfault semantics. Unless there's 
some shortcut? Does vma have a counter of how much is populated? (I 
don't think so?)
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#1213077

On Tue 25-08-15 15:55:46, Vlastimil Babka wrote:
> On 08/25/2015 03:41 PM, Michal Hocko wrote:
[...]
> >So what we have as a result is that partially populated ranges are
> >preserved and fully populated ones work in the best effort mode the same
> >way as they are now.
> >
> >Does that sound at least remotely reasonably?
> 
> I'll basically repeat what I said earlier:
> 
> - mremap scanning existing pte's to figure out the population would slow it
> down for no good reason

So do we really need to populate the enlarged range? All the man page is
saying is that the lock is maintained. Which will be still the case. It
is true that the failure is unlikely (unless you are running in the
memcg) but you cannot rely on the full mlock semantic so what would be a
problem?

> - it would be unreliable anyway:
>   - example: was the area completely populated because MLOCK_ONFAULT was not
> used or because the  process faulted it already

OK, I see this as being a problem. Especially if the buffer is increase
2*original_len

>   - example: was the area not completely populated because MLOCK_ONFAULT was
> used, or because mmap(MAP_LOCKED) failed to populate it fully?

What would be the difference? Both are ONFAULT now.

> I think the first point is a pointless regression for workloads that use
> just plain mlock() and don't want the onfault semantics. Unless there's some
> shortcut? Does vma have a counter of how much is populated? (I don't think
> so?)

-- 
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SUSE Labs
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#1213049

On Tue, Aug 25, 2015 at 4:41 PM, Michal Hocko <mhocko@kernel.org> wrote:
> On Fri 21-08-15 14:31:32, Eric B Munson wrote:
> [...]
>> I am in the middle of implementing lock on fault this way, but I cannot
>> see how we will hanlde mremap of a lock on fault region.  Say we have
>> the following:
>>
>>     addr = mmap(len, MAP_ANONYMOUS, ...);
>>     mlock(addr, len, MLOCK_ONFAULT);
>>     ...
>>     mremap(addr, len, 2 * len, ...)
>>
>> There is no way for mremap to know that the area being remapped was lock
>> on fault so it will be locked and prefaulted by remap.  How can we avoid
>> this without tracking per vma if it was locked with lock or lock on
>> fault?
>
> Yes mremap is a problem and it is very much similar to mmap(MAP_LOCKED).
> It doesn't guarantee the full mlock semantic because it leaves partially
> populated ranges behind without reporting any error.
>
> Considering the current behavior I do not thing it would be terrible
> thing to do what Konstantin was suggesting and populate only the full
> ranges in a best effort mode (it is done so anyway) and document the
> behavior properly.
> "
>        If the memory segment specified by old_address and old_size is
>        locked (using mlock(2) or similar), then this lock is maintained
>        when the segment is resized and/or relocated. As a consequence,
>        the amount of memory locked by the process may change.
>
>        If the range is already fully populated and the range is
>        enlarged the new range is attempted to be fully populated
>        as well to preserve the full mlock semantic but there is no
>        guarantee this will succeed. Partially populated (e.g. created by
>        mlock(MLOCK_ONFAULT)) ranges do not have the full mlock semantic
>        so they are not populated on resize.
> "
>
> So what we have as a result is that partially populated ranges are
> preserved and fully populated ones work in the best effort mode the same
> way as they are now.
>
> Does that sound at least remotely reasonably?

The problem is that mremap have to scan ptes to detect that and old behaviour
becomes very fragile: one fail and mremap will never populate that vma again.
For now I think new flag "MREMAP_NOPOPULATE" is a better option.

>
>
> --
> Michal Hocko
> SUSE Labs
>
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