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

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Message-ID: <20101117125756.GA5576@amd>
Date:	Wed, 17 Nov 2010 23:57:56 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	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>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>,
	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, 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.

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).

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