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

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Date:	Thu, 20 Dec 2012 14:40:59 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Sasha Levin <sasha.levin@...cle.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, 20 Dec 2012, Linus Torvalds wrote:
> 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?

No, it's just a misunderstanding: Sasha's problem is with a linux-next
that has Petr's NUMA KSM patch in, and we're still ironing known issues
out of that one.  Not a problem for 3.8-rc1.

Hugh
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