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Re: mmap vs mtime in 2.6.26 and up

To: Ray Lee <ray-lk@xxxxxxxxxxxxx>
Subject: Re: mmap vs mtime in 2.6.26 and up
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Tue, 12 May 2009 11:53:01 -0400
Cc: Ferenc Wagner <wferi@xxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Anton Salikhmetov <salikhmetov@xxxxxxxxx>
In-reply-to: <2c0942db0905120837g702b457bm52a412293925021a@xxxxxxxxxxxxxx>
References: <874ovws94s.fsf@xxxxxxxxxxxxx> <87ljp2v6vt.fsf@xxxxxxxxxxxxx> <2c0942db0905120837g702b457bm52a412293925021a@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
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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