Re: Switch per cpu partial page support off for debugging

From: David Rientjes
Date: Fri Jan 06 2012 - 12:37:42 EST


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@xxxxxxxxx>
> > >
> > >
> > > ---
> > > 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?
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