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Re: [PATCHv3 4/5] mm: make compound_head() robust

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  Re: [PATCHv3 4/5] mm: make compound_head() robust Andrew Morton <akpm@linux-foundation.org> - 2015-08-21 01:40 +0200
    Re: [PATCHv3 4/5] mm: make compound_head() robust "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-21 14:20 +0200
      Re: [PATCHv3 4/5] mm: make compound_head() robust Christoph Lameter <cl@linux.com> - 2015-08-21 18:20 +0200
        Re: [PATCHv3 4/5] mm: make compound_head() robust "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-21 21:40 +0200
          Re: [PATCHv3 4/5] mm: make compound_head() robust Andrew Morton <akpm@linux-foundation.org> - 2015-08-21 21:40 +0200
            Re: [PATCHv3 4/5] mm: make compound_head() robust Christoph Lameter <cl@linux.com> - 2015-08-21 23:20 +0200
            Re: [PATCHv3 4/5] mm: make compound_head() robust Vlastimil Babka <vbabka@suse.cz> - 2015-08-24 17:50 +0200
      Re: [PATCHv3 4/5] mm: make compound_head() robust Vlastimil Babka <vbabka@suse.cz> - 2015-08-25 13:50 +0200
        Re: [PATCHv3 4/5] mm: make compound_head() robust "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-25 20:40 +0200
          Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-25 22:20 +0200
            Re: [PATCHv3 4/5] mm: make compound_head() robust Vlastimil Babka <vbabka@suse.cz> - 2015-08-25 22:50 +0200
              Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-25 23:30 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-08-26 17:10 +0200
                  Re: [PATCHv3 4/5] mm: make compound_head() robust Vlastimil Babka <vbabka@suse.cz> - 2015-08-26 17:40 +0200
                  Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-26 18:40 +0200
                Re: [PATCHv3 4/5] mm: make compound_head() robust Hugh Dickins <hughd@google.com> - 2015-08-26 20:20 +0200
                  Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-26 23:30 +0200
                    Re: [PATCHv3 4/5] mm: make compound_head() robust Hugh Dickins <hughd@google.com> - 2015-08-27 00:30 +0200
                      Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-27 01:40 +0200
                    Re: [PATCHv3 4/5] mm: make compound_head() robust Michal Hocko <mhocko@kernel.org> - 2015-08-27 17:10 +0200
                      Re: [PATCHv3 4/5] mm: make compound_head() robust Michal Hocko <mhocko@kernel.org> - 2015-08-27 18:10 +0200
                        Re: [PATCHv3 4/5] mm: make compound_head() robust Hugh Dickins <hughd@google.com> - 2015-08-27 19:30 +0200
                          Re: [PATCHv3 4/5] mm: make compound_head() robust Michal Hocko <mhocko@kernel.org> - 2015-08-27 20:10 +0200
                      Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-27 18:40 +0200
                        Re: [PATCHv3 4/5] mm: make compound_head() robust Michal Hocko <mhocko@kernel.org> - 2015-08-27 20:20 +0200
                          Re: [PATCHv3 4/5] mm: make compound_head() robust "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-27 21:20 +0200

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#1214671

On Thu 27-08-15 17:09:17, Michal Hocko wrote:
[...]
> Btw. Do we need the same think for page::mapping and KSM?

I guess we are safe here because the address for mappings comes from
kmalloc and that aligned properly, right?
-- 
Michal Hocko
SUSE Labs
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#1214733

On Thu, 27 Aug 2015, Michal Hocko wrote:
> On Thu 27-08-15 17:09:17, Michal Hocko wrote:
> [...]
> > Btw. Do we need the same think for page::mapping and KSM?
> 
> I guess we are safe here because the address for mappings comes from
> kmalloc and that aligned properly, right?

Not quite right, in fact.  Because usually the struct address_space
is embedded within the struct inode (at i_data), and the struct inode
embedded within the fs-dependent inode, and that's what's kmalloc'ed.

What makes the mapping pointer low bits safe is include/linux/fs.h:
struct address_space {
	...
} __attribute__((aligned(sizeof(long))));

Which we first had to add in for the cris architecture, which stumbled
not on a genuine allocated address_space, but on that funny statically
declared swapper_space in mm/swap_state.c.

But struct anon_vma and KSM's struct stable_node (which depend on
the same scheme for low bits of page->mapping) have no such alignment
attribute specified: those ones are indeed relying on the kmalloc
guarantee as you suppose.

Does struct rcu_head have no __attribute__((aligned(whatever)))?
Perhaps that attribute should be added when it's needed.

Hugh
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#1214760

On Thu 27-08-15 10:28:48, Hugh Dickins wrote:
> On Thu, 27 Aug 2015, Michal Hocko wrote:
> > On Thu 27-08-15 17:09:17, Michal Hocko wrote:
> > [...]
> > > Btw. Do we need the same think for page::mapping and KSM?
> > 
> > I guess we are safe here because the address for mappings comes from
> > kmalloc and that aligned properly, right?
> 
> Not quite right, in fact.  Because usually the struct address_space
> is embedded within the struct inode (at i_data), and the struct inode
> embedded within the fs-dependent inode, and that's what's kmalloc'ed.
> 
> What makes the mapping pointer low bits safe is include/linux/fs.h:
> struct address_space {
> 	...
> } __attribute__((aligned(sizeof(long))));

Oh, right you are.

> Which we first had to add in for the cris architecture, which stumbled
> not on a genuine allocated address_space, but on that funny statically
> declared swapper_space in mm/swap_state.c.

Thanks for the clarification.

> But struct anon_vma and KSM's struct stable_node (which depend on
> the same scheme for low bits of page->mapping) have no such alignment
> attribute specified: those ones are indeed relying on the kmalloc
> guarantee as you suppose.
> 
> Does struct rcu_head have no __attribute__((aligned(whatever)))?
> Perhaps that attribute should be added when it's needed.

That's basically what I meant in the previous email.
-- 
Michal Hocko
SUSE Labs
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#1214690

On Thu, Aug 27, 2015 at 05:09:17PM +0200, Michal Hocko wrote:
> On Wed 26-08-15 14:29:16, Paul E. McKenney wrote:
> > On Wed, Aug 26, 2015 at 11:18:45AM -0700, Hugh Dickins wrote:
> [...]
> > > But if you do one day implement that, wouldn't sl?b.c have to use
> > > call_rcu_with_added_meaning() instead of call_rcu(), to be in danger
> > > of getting that bit set?  (No rcu_head is placed in a PageTail page.)
> > 
> > Good point, call_rcu_lazy(), but yes.
> > 
> > > So although it might be a little strange not to use a variant intended
> > > for freeing memory when indeed that's what it's doing, it would not be
> > > the end of the world for SLAB_DESTROY_BY_RCU to carry on using straight
> > > call_rcu(), in defence of the struct page safety Kirill is proposing.
> > 
> > As long as you are OK with the bottom bit being zero throughout the RCU
> > processing, yes.
> 
> I am really not sure I udnerstand. What will prevent
> call_rcu(&page->rcu_head, free_page_rcu) done in a random driver?

As long as it uses call_rcu(), call_rcu_bh(), call_rcu_sched(),
or call_srcu() and not some future call_rcu_lazy(), no problem.

But yes, if you are going to assume that RCU leaves the bottom
bit of the rcu_head structure's ->next field zero, then everything
everywhere in the kernel might in the future need to be careful of
exactly what variant of call_rcu() is used.

> Cannot the RCU simply claim bit1? I can see 1146edcbef37 ("rcu: Loosen
> __call_rcu()'s rcu_head alignment constraint") but AFAIU all it would
> take to fix this would be to require struct rcu_head to be aligned to
> 32b no?

There are some architectures that guarantee only 16-bit alignment.
If those architectures are fixed to do 32-bit alignment, or if support
for them is dropped, then the future restrictions mentioned above could
be dropped.

							Thanx, Paul

> Btw. Do we need the same think for page::mapping and KSM?
> -- 
> Michal Hocko
> SUSE Labs
> 

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#1214761

On Thu 27-08-15 09:36:34, Paul E. McKenney wrote:
> On Thu, Aug 27, 2015 at 05:09:17PM +0200, Michal Hocko wrote:
> > On Wed 26-08-15 14:29:16, Paul E. McKenney wrote:
> > > On Wed, Aug 26, 2015 at 11:18:45AM -0700, Hugh Dickins wrote:
> > [...]
> > > > But if you do one day implement that, wouldn't sl?b.c have to use
> > > > call_rcu_with_added_meaning() instead of call_rcu(), to be in danger
> > > > of getting that bit set?  (No rcu_head is placed in a PageTail page.)
> > > 
> > > Good point, call_rcu_lazy(), but yes.
> > > 
> > > > So although it might be a little strange not to use a variant intended
> > > > for freeing memory when indeed that's what it's doing, it would not be
> > > > the end of the world for SLAB_DESTROY_BY_RCU to carry on using straight
> > > > call_rcu(), in defence of the struct page safety Kirill is proposing.
> > > 
> > > As long as you are OK with the bottom bit being zero throughout the RCU
> > > processing, yes.
> > 
> > I am really not sure I udnerstand. What will prevent
> > call_rcu(&page->rcu_head, free_page_rcu) done in a random driver?
> 
> As long as it uses call_rcu(), call_rcu_bh(), call_rcu_sched(),
> or call_srcu() and not some future call_rcu_lazy(), no problem.
> 
> But yes, if you are going to assume that RCU leaves the bottom
> bit of the rcu_head structure's ->next field zero, then everything
> everywhere in the kernel might in the future need to be careful of
> exactly what variant of call_rcu() is used.

OK, so it would be call_rcu_$special to use the bit. This wasn't entirely
clear to me. I thought it would be opposite.

> > Cannot the RCU simply claim bit1? I can see 1146edcbef37 ("rcu: Loosen
> > __call_rcu()'s rcu_head alignment constraint") but AFAIU all it would
> > take to fix this would be to require struct rcu_head to be aligned to
> > 32b no?
> 
> There are some architectures that guarantee only 16-bit alignment.
> If those architectures are fixed to do 32-bit alignment, or if support
> for them is dropped, then the future restrictions mentioned above could
> be dropped.

My understanding of the discussion which led to the above patch is that
m68k allows for 32b alignment you just have to be explicit about that
(http://thread.gmane.org/gmane.linux.ports.m68k/5932/focus=5960). Which
other archs would be affected?

I mean, this patch allows for quite some simplification in the mm code.
And I think that RCU can live with mm of the low bits without any
issues. You've said that one bit should be sufficient for the RCU use
case. So having 2 bits sounds like a good thing.
-- 
Michal Hocko
SUSE Labs
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#1214795

On Thu, Aug 27, 2015 at 08:14:35PM +0200, Michal Hocko wrote:
> On Thu 27-08-15 09:36:34, Paul E. McKenney wrote:
> > On Thu, Aug 27, 2015 at 05:09:17PM +0200, Michal Hocko wrote:
> > > On Wed 26-08-15 14:29:16, Paul E. McKenney wrote:
> > > > On Wed, Aug 26, 2015 at 11:18:45AM -0700, Hugh Dickins wrote:
> > > [...]
> > > > > But if you do one day implement that, wouldn't sl?b.c have to use
> > > > > call_rcu_with_added_meaning() instead of call_rcu(), to be in danger
> > > > > of getting that bit set?  (No rcu_head is placed in a PageTail page.)
> > > > 
> > > > Good point, call_rcu_lazy(), but yes.
> > > > 
> > > > > So although it might be a little strange not to use a variant intended
> > > > > for freeing memory when indeed that's what it's doing, it would not be
> > > > > the end of the world for SLAB_DESTROY_BY_RCU to carry on using straight
> > > > > call_rcu(), in defence of the struct page safety Kirill is proposing.
> > > > 
> > > > As long as you are OK with the bottom bit being zero throughout the RCU
> > > > processing, yes.
> > > 
> > > I am really not sure I udnerstand. What will prevent
> > > call_rcu(&page->rcu_head, free_page_rcu) done in a random driver?
> > 
> > As long as it uses call_rcu(), call_rcu_bh(), call_rcu_sched(),
> > or call_srcu() and not some future call_rcu_lazy(), no problem.
> > 
> > But yes, if you are going to assume that RCU leaves the bottom
> > bit of the rcu_head structure's ->next field zero, then everything
> > everywhere in the kernel might in the future need to be careful of
> > exactly what variant of call_rcu() is used.
> 
> OK, so it would be call_rcu_$special to use the bit. This wasn't entirely
> clear to me. I thought it would be opposite.

Yes.  And I cannot resist adding that the need to avoid
call_rcu_$special() would be with respect to a given rcu_head structure,
not global.  Though I believe that you already figured that out.  ;-)

> > > Cannot the RCU simply claim bit1? I can see 1146edcbef37 ("rcu: Loosen
> > > __call_rcu()'s rcu_head alignment constraint") but AFAIU all it would
> > > take to fix this would be to require struct rcu_head to be aligned to
> > > 32b no?
> > 
> > There are some architectures that guarantee only 16-bit alignment.
> > If those architectures are fixed to do 32-bit alignment, or if support
> > for them is dropped, then the future restrictions mentioned above could
> > be dropped.
> 
> My understanding of the discussion which led to the above patch is that
> m68k allows for 32b alignment you just have to be explicit about that
> (http://thread.gmane.org/gmane.linux.ports.m68k/5932/focus=5960). Which
> other archs would be affected?
> 
> I mean, this patch allows for quite some simplification in the mm code.
> And I think that RCU can live with mm of the low bits without any
> issues. You've said that one bit should be sufficient for the RCU use
> case. So having 2 bits sounds like a good thing.

As long as MM doesn't use call_rcu_$special() for the rcu_head structure
in question, as long as MM is OK with the bottom bit of ->next always
being zero during a grace period, and as long as MM avoids writing
to ->next during a grace period, we should be good as is, even if a
call_rcu_$special() becomes necessary.

							Thanx, Paul

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