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

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Message-ID: <20090512155301.GA23160@infradead.org>
Date:	Tue, 12 May 2009 11:53:01 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Ray Lee <ray-lk@...rabbit.org>
Cc:	Ferenc Wagner <wferi@...f.hu>,
	Christoph Hellwig <hch@...radead.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

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.
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