linux-kernel - Re: [PATCH] ksm: make rmap walks more scalable

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Date:	Thu, 20 Dec 2012 14:26:43 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Petr Holasek <pholasek@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] ksm: make rmap walks more scalable

On Thu, Dec 20, 2012 at 1:49 PM, Sasha Levin <sasha.levin@...cle.com> wrote:
> On 12/19/2012 08:44 PM, Hugh Dickins wrote:
>> The rmap walks in ksm.c are like those in rmap.c:
>> they can safely be done with anon_vma_lock_read().
>>
>> Signed-off-by: Hugh Dickins <hughd@...gle.com>
>> ---
>
> Hi Hugh,
>
> This patch didn't fix the ksm oopses I'm seeing.
>
> This is with both patches applied:

Looks like another NULL mm pointer in ksmd.. Hugh fixed one in
2832bc19f666 ("sched: numa: ksm: fix oops in task_numa_placment()"),
this looks like more of the same.

At a guess, it looks like get_mergeable_page() has a rmap_item with no
mm. No idea how that happened. Hugh? Some race due to something that
depended on the mmap_sem being exclusive, rather than for
read-ownership?

              Linus
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