From: Andrew Morton <akpm@linux-foundation.org>
To: Michel Lespinasse <walken@google.com>
Cc: linux-mm@kvack.org, Rik van Riel <riel@redhat.com>,
Hugh Dickins <hughd@google.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: protect against concurrent vma expansion
Date: Mon, 3 Dec 2012 15:01:10 -0800 [thread overview]
Message-ID: <20121203150110.39c204ff.akpm@linux-foundation.org> (raw)
In-Reply-To: <1354344987-28203-1-git-send-email-walken@google.com>
On Fri, 30 Nov 2012 22:56:27 -0800
Michel Lespinasse <walken@google.com> wrote:
> expand_stack() runs with a shared mmap_sem lock. Because of this, there
> could be multiple concurrent stack expansions in the same mm, which may
> cause problems in the vma gap update code.
>
> I propose to solve this by taking the mm->page_table_lock around such vma
> expansions, in order to avoid the concurrency issue. We only have to worry
> about concurrent expand_stack() calls here, since we hold a shared mmap_sem
> lock and all vma modificaitons other than expand_stack() are done under
> an exclusive mmap_sem lock.
>
> I previously tried to achieve the same effect by making sure all
> growable vmas in a given mm would share the same anon_vma, which we
> already lock here. However this turned out to be difficult - all of the
> schemes I tried for refcounting the growable anon_vma and clearing
> turned out ugly. So, I'm now proposing only the minimal fix.
>
I think I don't understand the problem fully. Let me demonstrate:
a) vma_lock_anon_vma() doesn't take a lock which is specific to
"this" anon_vma. It takes anon_vma->root->mutex. That mutex is
shared with vma->vm_next, yes? If so, we have no problem here?
(which makes me suspect that the races lies other than where I think
it lies).
b) I can see why a broader lock is needed in expand_upwards(): it
plays with a different vma: vma->vm_next. But expand_downwards()
doesn't do that - it only alters "this" vma. So I'd have thought
that vma_lock_anon_vma("this" vma) would be sufficient.
What are the performance costs of this change?
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next prev parent reply other threads:[~2012-12-03 23:01 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-12-01 6:56 [PATCH] mm: protect against concurrent vma expansion Michel Lespinasse
2012-12-03 23:01 ` Andrew Morton [this message]
2012-12-04 0:35 ` Michel Lespinasse
2012-12-04 0:43 ` Andrew Morton
2012-12-04 14:48 ` Michel Lespinasse
2012-12-20 1:56 ` Simon Jeons
2012-12-20 3:01 ` Michel Lespinasse
2013-01-04 0:40 ` Simon Jeons
2013-01-04 0:50 ` Michel Lespinasse
2013-01-04 1:18 ` Simon Jeons
2013-01-04 2:49 ` Al Viro
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