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From: Michel Lespinasse <walken@google.com>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: linux-mm@kvack.org, riel@redhat.com, peterz@infradead.org,
	hughd@google.com, daniel.santos@pobox.com,
	linux-kernel@vger.kernel.org, akpm@linux-foundation.org
Subject: Re: [PATCH 2/7] mm: fix potential anon_vma locking issue in mprotect()
Date: Tue, 4 Sep 2012 14:53:47 -0700	[thread overview]
Message-ID: <20120904215347.GA6769@google.com> (raw)
In-Reply-To: <20120904142745.GE3334@redhat.com>

On Tue, Sep 04, 2012 at 04:27:45PM +0200, Andrea Arcangeli wrote:
> Hi Michel,
> 
> On Tue, Sep 04, 2012 at 02:20:52AM -0700, Michel Lespinasse wrote:
> > This change fixes an anon_vma locking issue in the following situation:
> > - vma has no anon_vma
> > - next has an anon_vma
> > - vma is being shrunk / next is being expanded, due to an mprotect call
> > 
> > We need to take next's anon_vma lock to avoid races with rmap users
> > (such as page migration) while next is being expanded.
> > 
> > This change also removes an optimization which avoided taking anon_vma
> > lock during brk adjustments. We could probably make that optimization
> > work again, but the following anon rmap change would break it,
> > so I kept things as simple as possible here.
> 
> Agreed, definitely a bug not to take the lock whenever any
> vm_start/vm_pgoff are moved, regardless if they're the next or current
> vma. Only vm_end can be moved without taking the lock.
> 
> I'd prefer to fix it like this though:
> 
> -	if (vma->anon_vma && (importer || start != vma->vm_start)) {
> +	if ((vma->anon_vma && (importer || start != vma->vm_start) ||
> +           (adjust_next && next->anon_vma)) {

I think the minimal fix would actually be:

 	if (vma->anon_vma && (importer || start != vma->vm_start)) {
 		anon_vma = vma->anon_vma;
+	else if (next->anon_vma && adjust_next)
+		anon_vma = next->anon_vma;

I suppose if we were to consider adding this fix to the stable series,
we should probably do it in such a minimal way. I hadn't actually
considered it, because I was only thinking about this patch series,
and at patch 4/7 it becomes necessary to lock the anon_vma even if
only the vm_end side gets modified (so we'd still end up with what I
proposed in the end)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

  reply	other threads:[~2012-09-04 21:53 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-09-04  9:20 [PATCH 0/7] use interval trees for anon rmap Michel Lespinasse
2012-09-04  9:20 ` [PATCH 1/7] mm: interval tree updates Michel Lespinasse
2012-09-07 22:13   ` Andrew Morton
2012-09-07 22:29     ` Michel Lespinasse
2012-09-07 22:55       ` Andrew Morton
2012-09-07 23:26         ` Michel Lespinasse
2012-09-08  4:45           ` Hillf Danton
2012-09-07 23:26         ` Michel Lespinasse
2012-09-04  9:20 ` [PATCH 2/7] mm: fix potential anon_vma locking issue in mprotect() Michel Lespinasse
2012-09-04 14:27   ` Andrea Arcangeli
2012-09-04 21:53     ` Michel Lespinasse [this message]
2012-09-04 22:16       ` Andrea Arcangeli
2012-09-05  0:45         ` Michel Lespinasse
2012-09-04  9:20 ` [PATCH 3/7] mm anon rmap: remove anon_vma_moveto_tail Michel Lespinasse
2012-09-04  9:20 ` [PATCH 4/7] mm anon rmap: replace same_anon_vma linked list with an interval tree Michel Lespinasse
2012-09-05  0:51   ` Michel Lespinasse
2012-09-04  9:20 ` [PATCH 5/7] mm rmap: remove vma_address check for address inside vma Michel Lespinasse
2012-09-04  9:20 ` [PATCH 6/7] mm: add CONFIG_DEBUG_VM_RB build option Michel Lespinasse
2012-09-14 22:14   ` Sasha Levin
2012-09-14 22:40     ` Sasha Levin
2012-09-14 22:46     ` Michel Lespinasse
2012-09-15  0:00       ` Michel Lespinasse
2012-09-15  7:52         ` Jiri Slaby
2012-09-16 19:07           ` Hugh Dickins
2012-09-22  7:19             ` Jiri Slaby
2012-09-15  9:26         ` Sasha Levin
2012-09-20 21:39           ` Fengguang Wu
2012-09-20 22:27             ` Hugh Dickins
2012-09-20 22:31               ` Fengguang Wu
2012-09-04  9:20 ` [PATCH 7/7] mm: avoid taking rmap locks in move_ptes() Michel Lespinasse

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