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

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Message-Id: <20101117165309.fa859fd3.akpm@linux-foundation.org>
Date:	Wed, 17 Nov 2010 16:53:09 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Dave Chinner <david@...morbit.com>,
	Michel Lespinasse <walken@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.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, 17 Nov 2010 18:52:30 -0500
"Ted Ts'o" <tytso@....edu> wrote:

> On Thu, Nov 18, 2010 at 10:11:43AM +1100, Dave Chinner wrote:
> > I don't think ->page_mkwrite can be worked around - we need that to
> > be called on the first write fault of any mmap()d page to ensure it
> > is set up correctly for writeback.  If we don't get write faults
> > after the page is mlock()d, then we need the ->page_mkwrite() call
> > during the mlock() call.
> 
> OK, so I'm not an mm hacker, so maybe I'm missing something.  Could
> part of this be fixed by simply sending the write faults for
> mlock()'ed pages, so page_mkwrite() gets called when the page is
> dirtied.  Seems like a real waste to have the file system pre-allocate
> all of the blocks for a mlock()'ed region.  Why does mlock() have to
> result in the write faults getting suppressed when the page is
> actually dirtied?

Yup, I don't think it would be too bad to take a minor fault each time
an mlocked page transitions from clean->dirty.

In fact we should already be doing that, after the mlocked page gets
written back by kupdate?  Hope so!

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