linux-kernel - Re: [PATCH]mmap: avoid unnecessary anon

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Message-ID: <20110330032504.GD21838@one.firstfloor.org>
Date:	Wed, 30 Mar 2011 05:25:04 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Shaohua Li <shaohua.li@...el.com>,
	linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH]mmap: avoid unnecessary anon_vma lock

On Tue, Mar 29, 2011 at 03:35:17PM -0700, Andrew Morton wrote:
> On Mon, 28 Mar 2011 09:57:39 -0700
> Andi Kleen <andi@...stfloor.org> wrote:
> 
> > Shaohua Li <shaohua.li@...el.com> writes:
> > 
> > > If we only change vma->vm_end, we can avoid taking anon_vma lock even 'insert'
> > > isn't NULL, which is the case of split_vma.
> > > From my understanding, we need the lock before because rmap must get the
> > > 'insert' VMA when we adjust old VMA's vm_end (the 'insert' VMA is linked to
> > > anon_vma list in __insert_vm_struct before).
> > > But now this isn't true any more. The 'insert' VMA is already linked to
> > > anon_vma list in __split_vma(with anon_vma_clone()) instead of
> > > __insert_vm_struct. There is no race rmap can't get required VMAs.
> > > So the anon_vma lock is unnecessary, and this can reduce one locking in brk
> > > case and improve scalability.
> > 
> > Looks good to me.
> 
> Looks way too tricky to me.
> 
> Please review this code for maintainability.  Have we documented what
> we're doing as completely and as clearly as we are able?

I agree the comments could be improved, but the code change looked good
to me. I don't think it impacts maintainability by itself because
we already do similar magic.

-Andi
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