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

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Date:	Wed, 2 Jan 2013 10:48:42 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Christoph Lameter <cl@...ux.com>,
	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 Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] tmpfs mempolicy: fix /proc/mounts corrupting
 memory

On Wed, 2 Jan 2013, Linus Torvalds wrote:
> On Wed, Jan 2, 2013 at 7:57 AM, Christoph Lameter <cl@...ux.com> wrote:
> > On Wed, 2 Jan 2013, Hugh Dickins wrote:
> >
> >> @@ -2796,10 +2787,7 @@ int mpol_to_str(char *buffer, int maxlen
> >>       case MPOL_BIND:
> >>               /* Fall through */
> >>       case MPOL_INTERLEAVE:
> >> -             if (no_context)
> >> -                     nodes = pol->w.user_nodemask;
> >> -             else
> >> -                     nodes = pol->v.nodes;
> >> +             nodes = pol->v.nodes;
> >>               break;
> >>
> >
> > no_context was always true. Why is the code from the false branch kept?
> 
> no_context is zero in the caller in fs/proc/task_mmu.c, and one in the
> mm/shmem.c caller. So it's not always true (for mpol_parse_str() there
> is only one caller, and it's always true as Hugh said).

Yes, I think Christoph was remembering the old days when mpol_to_str()
started out just for tmpfs; later /proc/pid/numa_maps extended it for
use on vmas (the "contextualized" !no_context case).

> 
> Anyway, I do not know why Hugh took the true case, but I don't really
> imagine that it matters. So I'll take these two patches, but it would
> be good if you double-checked this, Hugh.

Thanks, yes, I played with a number of ways of fixing it (and sat on my
original fix for several days, rightly guessing this an area where more
problems would emerge - only later realizing mpol=prefer:Node wrong too).

I could probably have kept mpol_to_str()'s no_context arg, and done
something with it in the MPOL_PREFERRED case; perhaps would have chosen
that if the arg had been more understandably named than "no_context";
but in the end thought removing the need for the arg was simplest.

Hugh
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