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

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Date:	Thu, 8 May 2008 07:20:19 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christoph Lameter <clameter@....com>,
	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 09:14:45PM -0700, Linus Torvalds wrote:
> IOW, you didn't even look at it, did you?

Actually I looked both at the struct and at the slab alignment just in
case it was changed recently. Now after reading your mail I also
compiled it just in case.

2.6.26-rc1

# name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables <limit> <batchcount> <sharedfactor> : slabdata <active_slabs> <num_slabs> <sharedavail>
anon_vma             260    576     24  144    1 : tunables  120   60    8 : slabdata      4      4      0
                                    ^^  ^^^

2.6.26-rc1 + below patch

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -27,6 +27,7 @@ struct anon_vma {
 struct anon_vma {
 	spinlock_t lock;	/* Serialize access to vma list */
 	struct list_head head;	/* List of private "related" vmas */
+	int flag:1;
 };
 
 #ifdef CONFIG_MMU

# name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables <limit> <batchcount> <sharedfactor> : slabdata <active_slabs> <num_slabs> <sharedavail>
anon_vma             250    560     32  112    1 : tunables  120   60    8 : slabdata      5      5      0
                                    ^^  ^^^

Not a big deal sure to grow it 33%, it's so small anyway, but I don't
see the point in growing it. sort() can be interrupted by signals, and
until it can we can cap it to 512 vmas making the worst case taking
dozen usecs, I fail to see what you have against sort().

Again: if a vma bitflag + global lock could have avoided sort and run
O(N) instead of current O(N*log(N)) I would have done that
immediately, infact I was in the process of doing it when you posted
the followup. Nothing personal here, just staying technical. Hope you
too.
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