Re: [PATCH v2] mm: slab: make page_cgroup_ino() to recognize non-compound slab pages properly

[Roman Gushchin]

On Thu, Oct 31, 2019 at 09:38:40AM +0000, Naoya Horiguchi wrote:
> On Wed, Oct 30, 2019 at 09:16:08PM -0700, Andrew Morton wrote:
> > On Wed, 30 Oct 2019 18:21:51 -0700 Roman Gushchin <guro@xxxxxx> wrote:
> > 
> > > page_cgroup_ino() doesn't return a valid memcg pointer for non-compound
> > > slab pages, because it depends on PgHead AND PgSlab flags to be set
> > > to determine the memory cgroup from the kmem_cache.
> > > It's correct for compound pages, but not for generic small pages. Those
> > > don't have PgHead set, so it ends up returning zero.
> > > 
> > > Fix this by replacing the condition to PageSlab() && !PageTail().
> > > 
> > > Before this patch:
> > > [root@localhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/user@xxxxxxxxx/ | grep slab
> > > 0x0000000000000080	        38        0  _______S___________________________________	slab
> > > 
> > > After this patch:
> > > [root@localhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/user@xxxxxxxxx/ | grep slab
> > > 0x0000000000000080	       147        0  _______S___________________________________	slab
> > > 
> > > Fixes: 4d96ba353075 ("mm: memcg/slab: stop setting page->mem_cgroup pointer for slab pages")
> > > Signed-off-by: Roman Gushchin <guro@xxxxxx>
> > > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > > Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> > > Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> > 
> > Affects /proc/kpagecgroup, but page_cgroup_ino() is also used in the
> > memory-failure code - I wonder what effect this bug has there?
> 
> hwpoison_filter_task() uses output of page_cgroup_ino() in order to
> filter error injection events based on memcg.
> So if page_cgroup_ino() fails to return memcg pointer, we just fail
> to inject memory error.  Considering that hwpoison filter is for testing,
> affected users are limited and the impact should be marginal.
> 
> > 
> > IOW, should we backport this into -stable?
> 
> I think yes, because the patch is small enough and clearly fixes a bug.

I agree.

Thanks!