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Date: Sun, 16 Jun 2002 17:35:24 +0100 (BST)
From: Hugh Dickins <hugh@veritas.com>
To: Kevin Easton <s3159795@student.anu.edu.au>
cc: linux-kernel@vger.kernel.org, rmk@arm.linux.org.uk
Subject: Re: 2.4.18 no timestamp update on modified mmapped files
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On Sun, 16 Jun 2002, Kevin Easton wrote:
> 
> So... the difference on i386 is just the definitions of the protection_map
> entries that are used.. specifically that PAGE_SHARED in asm-i386/pgtable.h
> includes _PAGE_RW? Changing this definition to be the same as the PAGE_COPY
> definition would be one fix?

It _could_ be _part_ of a fix (to the "problem" of dirty unbacked
pages arriving unheralded at the filesystem, too late to find
space for them; but our reluctance to have read faults allocate).

I say "could" because it would tend to cause double (read then write)
faulting more widely than necessary; I say "part" because at present
do_wp_page expects to be handling private Copy-On-Write faults rather
than shared mappings (please correct me if I'm wrong, Russell); and
we would still need to implement a callout down to the filesystem
(e.g. "wppage" method I suggested) to allocate the space (though,
doing it on the cheap, that method could be "nopage" revisited).

And I put quotes around "problem" because I'm uncertain how seriously
to take it, and we've had no chorus of anxious developers and users.

Hugh

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