linux-kernel - Re: [PATCH 1/2] tmpfs mempolicy: fix /proc/mounts corrupting memory

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Date:	Wed, 2 Jan 2013 14:32:16 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Hugh Dickins <hughd@...gle.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] tmpfs mempolicy: fix /proc/mounts corrupting
 memory

On Wed, 2 Jan 2013, Hugh Dickins wrote:

> Recent NUMA enhancements are not to blame: this dates back to 2.6.35,
> when commit e17f74af351c "mempolicy: don't call mpol_set_nodemask()
> when no_context" skipped mpol_parse_str()'s call to mpol_set_nodemask(),
> which used to initialize v.preferred_node, or set MPOL_F_LOCAL in flags.
> With slab poisoning, you can then rely on mpol_to_str() to set the bit
> for node 0x6b6b, probably in the next page above the caller's stack.

Ugly. But 2.6.35 means that the patch was not included in several
enterprise linux releases.

> I don't understand why MPOL_LOCAL is described as a pseudo-policy:
> it's a reasonable policy which suffers from a confusing implementation
> in terms of MPOL_PREFERRED with MPOL_F_LOCAL.  I believe this would be
> much more robust if MPOL_LOCAL were recognized in switch statements
> throughout, MPOL_F_LOCAL deleted, and MPOL_PREFERRED use the (possibly
> empty) nodes mask like everyone else, instead of its preferred_node
> variant (I presume an optimization from the days before MPOL_LOCAL).
> But that would take me too long to get right and fully tested.

The current approaches to implementing NUMA scheduling are making
MPOL_LOCAL an explicit policy. See
https://patchwork.kernel.org/patch/1703641/.

Does that address the concerns?
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