linux-kernel - Re: [BUG] Bad page states

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Date:	Mon, 8 Aug 2016 21:37:54 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>
Subject: Re: [BUG] Bad page states

On Mon, Aug 08, 2016 at 10:48:45AM -0700, Linus Torvalds wrote:
...
> > [   43.477693] BUG: Bad page state in process S05containers  pfn:1ff02a3
> > [   43.484417] page:ffffea007fc0a8c0 count:0 mapcount:-511 mapping:          (null) index:0x0
> > [   43.492737] flags: 0x1000000000000000()
> > [   43.496602] page dumped because: nonzero mapcount
> 
> Hmm. The _mapcount field is a union with other fields, but that number
> doesn't make sense for any of the other fields.
> 
> So it's almost certainly related to "PAGE_KMEMCG_MAPCOUNT_VALUE". So

Yes, it is - my bad. The thing is I set/clear PAGE_KMEMCG_MAPCOUNT_VALUE
for pages allocated with __GFP_ACCOUNT iff memcg_kmem_enabled() is true
(see __alloc_pages_nodemask and free_pages_prepare), while the latter
gets disabled when the last cgroup gets destroyed. So if you do

 mkdir /sys/fs/cgroup/memory/test
 # run something in the root cgroup that allocates pages with
 # __GFP_ACCOUNT, e.g. a program using pipe
 rmdir /sys/fs/cgroup/memory/test

Then, if there are no other memory cgroups, you'll see the bug.

Sorry about that :-(

Obviously, the PageKmemcg flag should only be set for pages that are
actually accounted to a non-root kmemcg and hence pin memcg_kmem_enabled
static key. I'll fix that.

Thanks,
Vladimir