linux-kernel - Re: [PATCH 14/15] mm: numa: Flush TLB if NUMA hinting faults race with PTE scan update

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Message-ID: <52A0DC7F.7050403@redhat.com>
Date:	Thu, 05 Dec 2013 15:05:19 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Alex Thorlton <athorlton@....com>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, hhuang@...hat.com
Subject: Re: [PATCH 14/15] mm: numa: Flush TLB if NUMA hinting faults race
 with PTE scan update

On 12/05/2013 02:54 PM, Mel Gorman wrote:

> I think that's a better fit and a neater fix. Thanks! I think it barriers
> more than it needs to (definite cost vs maybe cost), the flush can be
> deferred until we are definitely trying to migrate and the pte case is
> not guaranteed to be flushed before migration due to pte_mknonnuma causing
> a flush in ptep_clear_flush to be avoided later. Mashing the two patches
> together yields this.

I think this would fix the numa migrate case.

However, I believe the same issue is also present in
mprotect(..., PROT_NONE) vs. compaction, for programs
that trap SIGSEGV for garbage collection purposes.

They could lose modifications done in-between when
the pte was set to PROT_NONE, and the actual TLB
flush, if compaction moves the page around in-between
those two events.

I don't know if this is a case we need to worry about
at all, but I think the same fix would apply to that
code path, so I guess we might as well make it...
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