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Subject: Re: [PATCH] mm/filemap.c: unconditionally call mark_page_accessed
From: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
To: Andrea Arcangeli <andrea@suse.de>
Cc: Hugh Dickins <hugh@veritas.com>, Nick Piggin <npiggin@suse.de>,
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Date: Thu, 15 Mar 2007 18:15:45 -0500
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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