linux-kernel - Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

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Date:	Wed, 17 Nov 2010 16:28:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	Michel Lespinasse <walken@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Kosaki Motohiro <kosaki.motohiro@...fujitsu.com>,
	Theodore Tso <tytso@...gle.com>,
	Michael Rubin <mrubin@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering
 writeback

On Wed, 2010-11-17 at 23:57 +1100, Nick Piggin wrote:
> On Wed, Nov 17, 2010 at 04:23:58AM -0800, Michel Lespinasse wrote:
> > When faulting in pages for mlock(), we want to break COW for anonymous
> > or file pages within VM_WRITABLE, non-VM_SHARED vmas. However, there is
> > no need to write-fault into VM_SHARED vmas since shared file pages can
> > be mlocked first and dirtied later, when/if they actually get written to.
> > Skipping the write fault is desirable, as we don't want to unnecessarily
> > cause these pages to be dirtied and queued for writeback.
> 
> It's not just to break COW, but to do block allocation and such
> (filesystem's page_mkwrite op). That needs to at least be explained
> in the changelog.

Agreed, the 0/3 description actually does mention this.

> Filesystem doesn't have a good way to fully pin required things
> according to mlock, but page_mkwrite provides some reasonable things
> (like block allocation / reservation).

Right, but marking all pages dirty isn't really sane. I can imagine
making the reservation but not marking things dirty solution, although
it might be lots harder to implement, esp since some filesystems don't
actually have a page_mkwrite() implementation.


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