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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 1 Dec 2012 10:38:16 -0800
Message-ID: <CA+55aFx8QtP0hg8qxn__4vHQuzH7QkhTN-4fwgOpM-A=KuBBjA@mail.gmail.com>
Subject: Re: [RFC PATCH] mm/migration: Remove anon vma locking from
 try_to_unmap() use
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To: Ingo Molnar <mingo@kernel.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Paul Turner <pjt@google.com>, Lee Schermerhorn <Lee.Schermerhorn@hp.com>, Christoph Lameter <cl@linux.com>, Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Thomas Gleixner <tglx@linutronix.de>, Johannes Weiner <hannes@cmpxchg.org>, Hugh Dickins <hughd@google.com>

On Sat, Dec 1, 2012 at 4:26 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
>
> So as a quick concept hack I wrote the patch attached below.
> (It's not signed off, see the patch description text for the
> reason.)

Well, it confirms that anon_vma locking is a big problem, but as
outlined in my other email it's completely incorrect from an actual
behavior standpoint.

Btw, I think the anon_vma lock could be made a spinlock instead of a
mutex or rwsem, but that would probably take more work. We *shouldn't*
be doing anything that needs IO inside the anon_vma lock, though, so
it *should* be doable. But there are probably quite a bit of
allocations inside the lock, and I know it covers huge areas, so a
spinlock might not only be hard to convert to, it quite likely has
latency issues too.

Oh, btw, MUTEX_SPIN_ON_OWNER may well improve performance too, but it
gets disabled by DEBUG_MUTEXES. So some of the performance impact of
the vma locking may be *very* kernel-config dependent.

                  Linus

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