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

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Message-ID: <20101118110301.GA16625@google.com>
Date:	Thu, 18 Nov 2010 03:03:01 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Ted Ts'o" <tytso@....edu>, Dave Chinner <david@...morbit.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, Nov 17, 2010 at 04:53:09PM -0800, Andrew Morton wrote:
> 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?

This is actually what the patch does - by having mlock() use a read fault,
pages are loaded in memory and mlocked, but the ptes are not marked as
writable so that a later write access will be caught as a write fault at
that time (with all the usual dirtying and page_mkwrite() callbacks).

> 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!

Yes, handle_mm_fault() is careful to never create writable ptes pointing
to clean file pages, so that a later write fault will correctly dirty
the corresponding page.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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