Re: Dirty pages underflow on 3.14.23

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jan 05, 2015 at 06:05:59PM -0500, Mikulas Patocka wrote:
> Hi
> 
> I would like to report a memory management bug where the dirty pages count 
> underflowed.
> 
> It happened after some time that the Dirty pages count underflowed, as can 
> be seen in /proc/meminfo. The underflow condition was persistent, 
> /proc/meminfo was showing the big value even when the system was 
> completely idle. The counter never returned to zero.
> 
> The system didn't crash, but it became very slow - because of the big 
> value in the "Dirty" field, lazy writing was not working anymore, any 
> process that created a dirty page triggered immediate writeback, which 
> slowed down the system very much. The only fix was to reboot the machine.
> 
> The kernel version where this happened is 3.14.23. The kernel is compiled 
> without SMP and with peemption. The system is single-core 32-bit x86.
> 
> The bug probably happened during git pull or apt-get update, though one 
> can't be sure that these commands caused it.
> 
> I see that 3.14.24 containes some fix for underflow (commit 
> 6619741f17f541113a02c30f22a9ca22e32c9546, upstream commit 
> abe5f972912d086c080be4bde67750630b6fb38b), but it doesn't seem that that 
> commit fixes this condition. If you have a commit that could fix this, say 
> it.

That's an unrelated counter, but there is a known dirty underflow
problem that was addressed in 87a7e00b206a ("mm: protect
set_page_dirty() from ongoing truncation").  It should make it into
the stable kernels in the near future.  Can you reproduce this issue?

Thanks,
Johannes

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