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

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Message-ID: <alpine.LFD.1.10.0805071853500.3024@woody.linux-foundation.org>
Date:	Wed, 7 May 2008 18:57:05 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrea Arcangeli <andrea@...ranet.com>
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 Thu, 8 May 2008, Andrea Arcangeli wrote:
> 
> So because the bitflag can't prevent taking the same lock twice on two
> different vmas in the same mm, we still can't remove the sorting

Andrea. 

Take five minutes. Take a deep breadth. And *think* about actually reading 
what I wrote.

The bitflag *can* prevent taking the same lock twice. It just needs to be 
in the right place.

Let me quote it for you:

> So the flag wouldn't be one of the VM_xyzzy flags, and would require 
> adding a new field to "struct anon_vma()"

IOW, just make it be in that anon_vma (and the address_space). No sorting 
required.

> I think it's more interesting to put a cap on the number of vmas to
> min(1024,max_map_count). The sort time on an 8k array runs in constant
> time.

Shut up already. It's not constant time just because you can cap the 
overhead. We're not in a university, and we care about performance, not 
your made-up big-O notation.

			Linus
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