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Re: [PATCH] fs/select: add vmalloc fallback for select(2)

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  Re: [PATCH] fs/select: add vmalloc fallback for select(2) Jason Baron <jbaron@akamai.com> - 2016-09-23 18:50 +0200
    Re: [PATCH] fs/select: add vmalloc fallback for select(2) Vlastimil Babka <vbabka@suse.cz> - 2016-09-27 10:50 +0200
      Re: [PATCH] fs/select: add vmalloc fallback for select(2) Nicholas Piggin <npiggin@gmail.com> - 2016-09-27 13:30 +0200
        RE: [PATCH] fs/select: add vmalloc fallback for select(2) David Laight <David.Laight@ACULAB.COM> - 2016-09-27 13:50 +0200
          Re: [PATCH] fs/select: add vmalloc fallback for select(2) Nicholas Piggin <npiggin@gmail.com> - 2016-09-27 13:50 +0200
            Re: [PATCH] fs/select: add vmalloc fallback for select(2) Vlastimil Babka <vbabka@suse.cz> - 2016-09-27 14:00 +0200
              RE: [PATCH] fs/select: add vmalloc fallback for select(2) David Laight <David.Laight@ACULAB.COM> - 2016-09-28 18:40 +0200
                Re: [PATCH] fs/select: add vmalloc fallback for select(2) Vlastimil Babka <vbabka@suse.cz> - 2016-09-28 22:10 +0200

#1490322 — Re: [PATCH] fs/select: add vmalloc fallback for select(2)

Hi,

On 09/23/2016 03:24 AM, Nicholas Piggin wrote:
> On Fri, 23 Sep 2016 14:42:53 +0800
> "Hillf Danton" <hillf.zj@alibaba-inc.com> wrote:
>
>>>
>>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
>>> with the number of fds passed. We had a customer report page allocation
>>> failures of order-4 for this allocation. This is a costly order, so it might
>>> easily fail, as the VM expects such allocation to have a lower-order fallback.
>>>
>>> Such trivial fallback is vmalloc(), as the memory doesn't have to be
>>> physically contiguous. Also the allocation is temporary for the duration of the
>>> syscall, so it's unlikely to stress vmalloc too much.
>>>
>>> Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
>>> it doesn't need this kind of fallback.
>
> How about something like this? (untested)
>
> Eric isn't wrong about vmalloc sucking :)
>
> Thanks,
> Nick
>
>
> ---
>   fs/select.c | 57 +++++++++++++++++++++++++++++++++++++++++++--------------
>   1 file changed, 43 insertions(+), 14 deletions(-)
>
> diff --git a/fs/select.c b/fs/select.c
> index 8ed9da5..3b4834c 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -555,6 +555,7 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
>   	void *bits;
>   	int ret, max_fds;
>   	unsigned int size;
> +	size_t nr_bytes;
>   	struct fdtable *fdt;
>   	/* Allocate small arguments on the stack to save memory and be faster */
>   	long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
> @@ -576,21 +577,39 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
>   	 * since we used fdset we need to allocate memory in units of
>   	 * long-words.
>   	 */
> -	size = FDS_BYTES(n);
> +	ret = -ENOMEM;
>   	bits = stack_fds;
> -	if (size > sizeof(stack_fds) / 6) {
> -		/* Not enough space in on-stack array; must use kmalloc */
> +	size = FDS_BYTES(n);
> +	nr_bytes = 6 * size;
> +
> +	if (unlikely(nr_bytes > PAGE_SIZE)) {
> +		/* Avoid multi-page allocation if possible */
>   		ret = -ENOMEM;
> -		bits = kmalloc(6 * size, GFP_KERNEL);
> -		if (!bits)
> -			goto out_nofds;
> +		fds.in = kmalloc(size, GFP_KERNEL);
> +		fds.out = kmalloc(size, GFP_KERNEL);
> +		fds.ex = kmalloc(size, GFP_KERNEL);
> +		fds.res_in = kmalloc(size, GFP_KERNEL);
> +		fds.res_out = kmalloc(size, GFP_KERNEL);
> +		fds.res_ex = kmalloc(size, GFP_KERNEL);
> +
> +		if (!(fds.in && fds.out && fds.ex &&
> +				fds.res_in && fds.res_out && fds.res_ex))
> +			goto out;
> +	} else {
> +		if (nr_bytes > sizeof(stack_fds)) {
> +			/* Not enough space in on-stack array */
> +			if (nr_bytes > PAGE_SIZE * 2)

The 'if' looks extraneous?

Also, I wonder if we can just avoid some allocations altogether by 
checking by if the user fd_set pointers are NULL? That can avoid failures :)

Thanks,

-Jason

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#1491765

On 09/23/2016 06:47 PM, Jason Baron wrote:
> Hi,
>
> On 09/23/2016 03:24 AM, Nicholas Piggin wrote:
>> On Fri, 23 Sep 2016 14:42:53 +0800
>> "Hillf Danton" <hillf.zj@alibaba-inc.com> wrote:
>>
>>>>
>>>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
>>>> with the number of fds passed. We had a customer report page allocation
>>>> failures of order-4 for this allocation. This is a costly order, so it might
>>>> easily fail, as the VM expects such allocation to have a lower-order fallback.
>>>>
>>>> Such trivial fallback is vmalloc(), as the memory doesn't have to be
>>>> physically contiguous. Also the allocation is temporary for the duration of the
>>>> syscall, so it's unlikely to stress vmalloc too much.
>>>>
>>>> Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
>>>> it doesn't need this kind of fallback.
>>
>> How about something like this? (untested)

This pushes the limit further, but might just delay the problem. Could be an 
optimization on top if there's enough interest, though.

[...]

>> +
>> +		if (!(fds.in && fds.out && fds.ex &&
>> +				fds.res_in && fds.res_out && fds.res_ex))
>> +			goto out;
>> +	} else {
>> +		if (nr_bytes > sizeof(stack_fds)) {
>> +			/* Not enough space in on-stack array */
>> +			if (nr_bytes > PAGE_SIZE * 2)
>
> The 'if' looks extraneous?
>
> Also, I wonder if we can just avoid some allocations altogether by
> checking by if the user fd_set pointers are NULL? That can avoid failures :)

That would be a more major rewrite, as the core algorithm doesn't expect NULLs.

> Thanks,
>
> -Jason
>

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#1491826

On Tue, 27 Sep 2016 10:44:04 +0200
Vlastimil Babka <vbabka@suse.cz> wrote:

> On 09/23/2016 06:47 PM, Jason Baron wrote:
> > Hi,
> >
> > On 09/23/2016 03:24 AM, Nicholas Piggin wrote:  
> >> On Fri, 23 Sep 2016 14:42:53 +0800
> >> "Hillf Danton" <hillf.zj@alibaba-inc.com> wrote:
> >>  
> >>>>
> >>>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
> >>>> with the number of fds passed. We had a customer report page allocation
> >>>> failures of order-4 for this allocation. This is a costly order, so it might
> >>>> easily fail, as the VM expects such allocation to have a lower-order fallback.
> >>>>
> >>>> Such trivial fallback is vmalloc(), as the memory doesn't have to be
> >>>> physically contiguous. Also the allocation is temporary for the duration of the
> >>>> syscall, so it's unlikely to stress vmalloc too much.
> >>>>
> >>>> Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
> >>>> it doesn't need this kind of fallback.  
> >>
> >> How about something like this? (untested)  
> 
> This pushes the limit further, but might just delay the problem. Could be an 
> optimization on top if there's enough interest, though.

What's your customer doing with those selects? If they care at all about
performance, I doubt they want select to attempt order-4 allocations, fail,
then use vmalloc :)

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#1491844

From: Nicholas Piggin
> Sent: 27 September 2016 12:25
> On Tue, 27 Sep 2016 10:44:04 +0200
> Vlastimil Babka <vbabka@suse.cz> wrote:
> 
> > On 09/23/2016 06:47 PM, Jason Baron wrote:
> > > Hi,
> > >
> > > On 09/23/2016 03:24 AM, Nicholas Piggin wrote:
> > >> On Fri, 23 Sep 2016 14:42:53 +0800
> > >> "Hillf Danton" <hillf.zj@alibaba-inc.com> wrote:
> > >>
> > >>>>
> > >>>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
> > >>>> with the number of fds passed. We had a customer report page allocation
> > >>>> failures of order-4 for this allocation. This is a costly order, so it might
> > >>>> easily fail, as the VM expects such allocation to have a lower-order fallback.
> > >>>>
> > >>>> Such trivial fallback is vmalloc(), as the memory doesn't have to be
> > >>>> physically contiguous. Also the allocation is temporary for the duration of the
> > >>>> syscall, so it's unlikely to stress vmalloc too much.
> > >>>>
> > >>>> Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
> > >>>> it doesn't need this kind of fallback.
> > >>
> > >> How about something like this? (untested)
> >
> > This pushes the limit further, but might just delay the problem. Could be an
> > optimization on top if there's enough interest, though.
> 
> What's your customer doing with those selects? If they care at all about
> performance, I doubt they want select to attempt order-4 allocations, fail,
> then use vmalloc :)

If they care about performance they shouldn't be passing select() lists that
are anywhere near that large.
If the number of actual fd is small - use poll().

Otherwise you want one of the 'event' mechanisms in order to avoid setting
the markers on every fd after every event (can't remember how you do that
in Linux).

At least this isn't SYSV - poll() was O(n^2) in the number of fd
(because the fd were on a linked list).

	David

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#1491846

On Tue, 27 Sep 2016 11:37:24 +0000
David Laight <David.Laight@ACULAB.COM> wrote:

> From: Nicholas Piggin
> > Sent: 27 September 2016 12:25
> > On Tue, 27 Sep 2016 10:44:04 +0200
> > Vlastimil Babka <vbabka@suse.cz> wrote:
> >   
> > > On 09/23/2016 06:47 PM, Jason Baron wrote:  
> > > > Hi,
> > > >
> > > > On 09/23/2016 03:24 AM, Nicholas Piggin wrote:  
> > > >> On Fri, 23 Sep 2016 14:42:53 +0800
> > > >> "Hillf Danton" <hillf.zj@alibaba-inc.com> wrote:
> > > >>  
> > > >>>>
> > > >>>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
> > > >>>> with the number of fds passed. We had a customer report page allocation
> > > >>>> failures of order-4 for this allocation. This is a costly order, so it might
> > > >>>> easily fail, as the VM expects such allocation to have a lower-order fallback.
> > > >>>>
> > > >>>> Such trivial fallback is vmalloc(), as the memory doesn't have to be
> > > >>>> physically contiguous. Also the allocation is temporary for the duration of the
> > > >>>> syscall, so it's unlikely to stress vmalloc too much.
> > > >>>>
> > > >>>> Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
> > > >>>> it doesn't need this kind of fallback.  
> > > >>
> > > >> How about something like this? (untested)  
> > >
> > > This pushes the limit further, but might just delay the problem. Could be an
> > > optimization on top if there's enough interest, though.  
> > 
> > What's your customer doing with those selects? If they care at all about
> > performance, I doubt they want select to attempt order-4 allocations, fail,
> > then use vmalloc :)  
> 
> If they care about performance they shouldn't be passing select() lists that
> are anywhere near that large.
> If the number of actual fd is small - use poll().

Right. Presumably it's some old app they're still using, no?

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#1491860

On 09/27/2016 01:42 PM, Nicholas Piggin wrote:
> On Tue, 27 Sep 2016 11:37:24 +0000
> David Laight <David.Laight@ACULAB.COM> wrote:
>
>> From: Nicholas Piggin
>> > Sent: 27 September 2016 12:25
>> > On Tue, 27 Sep 2016 10:44:04 +0200
>> > Vlastimil Babka <vbabka@suse.cz> wrote:
>> >
>> >
>> > What's your customer doing with those selects? If they care at all about
>> > performance, I doubt they want select to attempt order-4 allocations, fail,
>> > then use vmalloc :)
>>
>> If they care about performance they shouldn't be passing select() lists that
>> are anywhere near that large.
>> If the number of actual fd is small - use poll().
>
> Right. Presumably it's some old app they're still using, no?

Process name suggests it's part of db2 database. It seems it has to implement 
its own interface to select() syscall, because glibc itself seems to have a 
FD_SETSIZE limit of 1024, which is probably why this wasn't an issue for all the 
years...

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#1492687

From: Vlastimil Babka
> Sent: 27 September 2016 12:51
...
> Process name suggests it's part of db2 database. It seems it has to implement
> its own interface to select() syscall, because glibc itself seems to have a
> FD_SETSIZE limit of 1024, which is probably why this wasn't an issue for all the
> years...

ISTR the canonical way to increase the size being to set FD_SETSIZE
to a larger value before including any of the headers.

Or doesn't that work with linux and glibc ??

	David

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#1492943

On 09/28/2016 06:30 PM, David Laight wrote:
> From: Vlastimil Babka
>> Sent: 27 September 2016 12:51
> ...
>> Process name suggests it's part of db2 database. It seems it has to implement
>> its own interface to select() syscall, because glibc itself seems to have a
>> FD_SETSIZE limit of 1024, which is probably why this wasn't an issue for all the
>> years...
> 
> ISTR the canonical way to increase the size being to set FD_SETSIZE
> to a larger value before including any of the headers.
> 
> Or doesn't that work with linux and glibc ??

Doesn't seem so.

> 
> 	David
> 

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