linux-kernel - Re: [PATCH] mm/filemap.c: unconditionally call mark_page

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Message-Id: <1174000545.14380.22.camel@kleikamp.austin.ibm.com>
Date:	Thu, 15 Mar 2007 18:15:45 -0500
From:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
To:	Andrea Arcangeli <andrea@...e.de>
Cc:	Hugh Dickins <hugh@...itas.com>, Nick Piggin <npiggin@...e.de>,
	Chuck Ebbert <cebbert@...hat.com>,
	Ashif Harji <asharji@...uwaterloo.ca>,
	Miquel van Smoorenburg <miquels@...tron.nl>,
	linux-mm@...ck.org, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/filemap.c: unconditionally call mark_page_accessed

On Thu, 2007-03-15 at 23:59 +0100, Andrea Arcangeli wrote:
> On Thu, Mar 15, 2007 at 05:44:01PM +0000, Hugh Dickins wrote:
> > who removed the !offset condition, he should be consulted on its
> > reintroduction.
> 
> the !offset check looks a pretty broken heuristic indeed, it would
> break random I/O.

I wouldn't call it broken.  At worst, I'd say it's imperfect.  But
that's the nature of a heuristic.  It most likely works in a huge
majority of cases.

> The real fix is to add a ra.prev_offset along with
> ra.prev_page, and if who implements it wants to be stylish he can as
> well use a ra.last_contiguous_read structure that has a page and
> offset fields (and then of course remove ra.prev_page).

I suggested something along these lines, but I wonder if it's overkill.
The !offset check is simple and appears to be a decent improvement over
the current code.
-- 
David Kleikamp
IBM Linux Technology Center

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