linux-kernel - Re: [RFC PATCH] mm/migration: Remove anon vma locking from try_to

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

On Sat, Dec 1, 2012 at 4:26 AM, Ingo Molnar <mingo@...nel.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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