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Subject: Re: Switch per cpu partial page support off for debugging
From: Pekka Enberg <penberg@kernel.org>
To: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>, Eric Dumazet <eric.dumazet@gmail.com>,
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Date: Sun, 08 Jan 2012 12:54:01 +0200
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On Fri, 2012-01-06 at 09:37 -0800, David Rientjes wrote:
> On Wed, 14 Dec 2011, David Rientjes wrote:
> 
> > > > Eric saw an issue with accounting of slabs during validation. Its not
> > > > possible to determine accurately how many per cpu partial slabs exist at
> > > > any time so this switches off per cpu partial pages during debug.
> > > > 
> > > > Subject: Switch per cpu partial page support off for debugging
> > > > 
> > > > Otherwise we have accounting issues.
> > > > 
> > > > Signed-off-by: Christoph Lameter <cl@linux.com>
> > > > 
> > > > 
> > > > ---
> > > > mm/slub.c |    4 +++-
> > > > 1 file changed, 3 insertions(+), 1 deletion(-)
> > > > 
> > > > Index: linux-2.6/mm/slub.c
> > > > ===================================================================
> > > > --- linux-2.6.orig/mm/slub.c	2011-11-22 11:57:21.000000000 -0600
> > > > +++ linux-2.6/mm/slub.c	2011-11-22 11:57:55.000000000 -0600
> > > > @@ -3027,7 +3027,9 @@ static int kmem_cache_open(struct kmem_c
> > > > 	 *    per node list when we run out of per cpu objects. We only fetch
> > > > 50%
> > > > 	 *    to keep some capacity around for frees.
> > > > 	 */
> > > > -	if (s->size >= PAGE_SIZE)
> > > > +	if (kmem_cache_debug(s))
> > > > +		s->cpu_partial = 0;
> > > > +	else if (s->size >= PAGE_SIZE)
> > > > 		s->cpu_partial = 2;
> > > > 	else if (s->size >= 1024)
> > > > 		s->cpu_partial = 6;
> > 
> > I think this deserves a comment since it's not clear why it's being done 
> > neither in the code or by the changelog.  Also, you'd want a check for 
> > kmem_cache_debug() when storing to /sys/kernel/slab/cache/cpu_partial as 
> > well, otherwise this could easily be broken from userspace again.  (It 
> > also seems like Documentation/ABI/testing/sysfs-kernel-slab got left out 
> > of an update when that was added, too.)
> > 
> 
> This patch is merged in slab/urgent without any changes from my comment 
> above; since userspace can alter cpu_partial directly via 
> /sys/kernel/slab/cache/cpu_partial which would reintroduce the 
> "accounting issues", shouldn't that return -EINVAL when debug is enabled?

I think I applied the patch before your comments. EINVAL sounds
reasonable to me. Christoph?

			Pekka