linux-kernel - Re: mmap vs mtime in 2.6.26 and up

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Date:	Fri, 15 May 2009 18:40:29 +0200
From:	Ferenc Wagner <wferi@...f.hu>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Ray Lee <ray-lk@...rabbit.org>,
	Andrew Morton <akpm@...ux-foundation.org>, xfs@....sgi.com,
	linux-kernel@...r.kernel.org,
	Anton Salikhmetov <salikhmetov@...il.com>
Subject: Re: mmap vs mtime in 2.6.26 and up

Christoph Hellwig <hch@...radead.org> writes:

> On Tue, May 12, 2009 at 08:37:43AM -0700, Ray Lee wrote:
>>> I've noticed that the last modification times of our RRD files got
>>> stuck after upgrading from 2.6.24 to 2.6.26 (Debian Etch -> Lenny; I
>>> also tested with 2.6.30-rc5, they are still stuck). ??It has some
>>> literature, most notably kernel bug #2645, but that's closed long ago
>>> and the resulting patch http://lkml.org/lkml/2008/1/22/370 is present
>>> in my kernels. ??Still, the test program (version 3 from the bug report)
>>> gives failures:
>
> The problem is pretty simple.  do_wp_page and __do_fault use
> file_update_time to update ctime and mtime.  But this function is only
> a helper for simply filesystems that have a binary inode dirty/non dirty
> state and keep the m/ctime purely in the Linux inode.  It must not be
> called from generic code as more complex filesystems need a notification
> through ->setattr to update the timestamps.  This will also affect other
> filesystems like ubifs.  I'm not entirely sure why it ever worked
> before, we must have picked up those c/mtime updates by accident
> somehow.

Thanks for the analysis.  Unfortunately I don't nearly know enough to
work on this issue, but would like to track it as it affects our
backup system.  So, shouldn't #2645 be reopened again?
-- 
Feri.
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