linux-kernel - Re: [PATCH] writeback: guard against jiffies wraparound on inode->dirtied

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Date:	Wed, 1 Apr 2009 17:26:30 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, fengguang.wu@...el.com
Subject: Re: [PATCH] writeback: guard against jiffies wraparound on
 inode->dirtied_when checks (try #3)

On Wed, 01 Apr 2009 22:22:06 +0200
Andi Kleen <andi@...stfloor.org> wrote:

> Jeff Layton <jlayton@...hat.com> writes:
> >
> > The problem is that these checks assume that dirtied_when is updated
> > periodically. If an inode is continuously being used for I/O it can be
> > persistently marked as dirty and will continue to age. Once the time
> > difference between dirtied_when and the jiffies value it is being
> > compared to is greater than or equal to half the maximum of the jiffies
> > type, the logic of the time_*() macros inverts and the opposite of what
> > is needed is returned. On 32-bit architectures that's just under 25 days
> > (assuming HZ == 1000).
> 
> I wonder if this can happen in other places using jiffies time stamp
> too. Why not? Perhaps that check macro should be in timer.h and some auditing done
> over the whiole code base?
> 

It certainly can happen in other places. We've seen very similar
problems in NFS, and they were fixed in similar ways. That's where the
time_in_range macro came from. I agree that a thorough audit of jiffies
usage would be a fine thing...

One possibility might be a new debugging option. We could add
replacement time_after() and time_before() macros that also check
whether the difference in times is beyond a certain threshold
(maybe a day or week or so), and pop a printk or otherwise record
info about it when one is detected?

That wouldn't find all of the problem cases, but it might help ID some
of them.

-- 
Jeff Layton <jlayton@...hat.com>
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