linux-kernel - Re: [PATCH 08 of 11] anon-vma-rwsem

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Date:	Thu, 8 May 2008 04:56:52 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, steiner@....com,
	holt@....com, npiggin@...e.de, a.p.zijlstra@...llo.nl,
	kvm-devel@...ts.sourceforge.net, kanojsarcar@...oo.com,
	rdreier@...co.com, swise@...ngridcomputing.com,
	linux-kernel@...r.kernel.org, avi@...ranet.com, linux-mm@...ck.org,
	general@...ts.openfabrics.org, hugh@...itas.com,
	rusty@...tcorp.com.au, aliguori@...ibm.com, chrisw@...hat.com,
	marcelo@...ck.org, dada1@...mosbay.com, paulmck@...ibm.com
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem

On Wed, May 07, 2008 at 06:12:32PM -0700, Christoph Lameter wrote:
> Andrea's mm_lock could have wider impact. It is the first effective 
> way that I have seen of temporarily holding off reclaim from an address 
> space. It sure is a brute force approach.

The only improvement I can imagine on mm_lock, is after changing the
name to global_mm_lock() to reestablish the signal_pending check in
the loop that takes the spinlock and to backoff and put the cap to 512
vmas so the ram wasted on anon-vmas wouldn't save more than 10-100usec
at most (plus the vfree that may be a bigger cost but we're ok to pay
it and it surely isn't security related).

Then on the long term we need to talk to Matt on returning a parameter
to the sort function to break the loop. After that we remove the 512
vma cap and mm_lock is free to run as long as it wants like
/dev/urandom, nobody can care less how long it will run before
returning as long as it reacts to signals.

This is the right way if we want to support XPMEM/GRU efficiently and
without introducing unnecessary regressions in the VM fastpaths and VM
footprint.
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