linux-kernel - Re: [PATCH] mm: numa: Recheck for transhuge pages under lock during protection changes

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Message-ID: <53205014.2050602@redhat.com>
Date:	Wed, 12 Mar 2014 08:16:20 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Sasha Levin <sasha.levin@...cle.com>,
	David Rientjes <rientjes@...gle.com>, hhuang@...hat.com,
	knoel@...hat.com, aarcange@...hat.com,
	Davidlohr Bueso <davidlohr@...com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: numa: Recheck for transhuge pages under lock during
 protection changes

On 03/12/2014 06:36 AM, Mel Gorman wrote:
> Andrew, this should go with the patches 
> mmnuma-reorganize-change_pmd_range.patch
> mmnuma-reorganize-change_pmd_range-fix.patch
> move-mmu-notifier-call-from-change_protection-to-change_pmd_range.patch
> in mmotm please.
> 
> Thanks.

That would be nice indeed :)

I am still not entirely sure why the kernel did not hit this race
before my reorganize change_pmd_range patch. Maybe gcc used to do
one load and now it does two?

> The problem is that a transhuge check is made without holding the PTL. It's
> possible at the time of the check that a parallel fault clears the pmd
> and inserts a new one which then triggers the VM_BUG_ON check.  This patch
> removes the VM_BUG_ON but fixes the race by rechecking transhuge under the
> PTL when marking page tables for NUMA hinting and bailing if a race occurred.
> It is not a problem for calls to mprotect() as they hold mmap_sem for write.
> 
> Reported-by: Sasha Levin <sasha.levin@...cle.com>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Rik van Riel <riel@...hat.com>

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