linux-kernel - Re: [PATCH -mm] mm,numa,mprotect: always continue after finding a stable thp page

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Date:	Thu, 06 Mar 2014 17:52:47 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Sasha Levin <sasha.levin@...cle.com>
CC:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, mgorman@...e.de,
	hhuang@...hat.com, knoel@...hat.com, aarcange@...hat.com
Subject: Re: [PATCH -mm] mm,numa,mprotect: always continue after finding a
 stable thp page

On 03/06/2014 05:31 PM, Rik van Riel wrote:
> On Thu, 06 Mar 2014 16:12:28 -0500
> Sasha Levin <sasha.levin@...cle.com> wrote:
>
>> While fuzzing with trinity inside a KVM tools guest running latest -next kernel I've hit the
>> following spew. This seems to be introduced by your patch "mm,numa: reorganize change_pmd_range()".
>
> That patch should not introduce any functional changes, except for
> the VM_BUG_ON that catches the fact that we fell through to the 4kB
> pte handling code, despite having just handled a THP pmd...
>
> Does this patch fix the issue?
>
> Mel, am I overlooking anything obvious? :)
>
> ---8<---
>
> Subject: mm,numa,mprotect: always continue after finding a stable thp page
>
> When turning a thp pmds into a NUMA one, change_huge_pmd will
> return 0 when the pmd already is a NUMA pmd.

I did miss something obvious.  In this case, the code returns 1.

> However, change_pmd_range would fall through to the code that
> handles 4kB pages, instead of continuing on to the next pmd.

Maybe the case that I missed is when khugepaged is in the
process of collapsing pages into a transparent huge page?

If the virtual CPU gets de-scheduled by the host for long
enough, it would be possible for khugepaged to run on
another virtual CPU, and turn the pmd into a THP pmd,
before that VM_BUG_ON test.

I see that khugepaged takes the mmap_sem for writing in the
collapse code, and it looks like task_numa_work takes the
mmap_sem for reading, so I guess that may not be possible?

Andrea, would you happen to know what case am I missing?
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