linux-kernel - Re: Dirty pages underflow on 3.14.23

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Date:	Wed, 7 Jan 2015 13:28:58 -0800
From:	Simon Kirby <sim@...tway.ca>
To:	Holger Hoffst?tte <holger.hoffstaette@...glemail.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: Dirty pages underflow on 3.14.23

On Wed, Jan 07, 2015 at 10:57:46AM +0000, Holger Hoffst?tte wrote:

> On Tue, 06 Jan 2015 12:54:43 -0500, Mikulas Patocka wrote:
> 
> > I can't reprodce it. It happened just once.
> > 
> > That patch is supposed to fix an occasional underflow by a single page -
> > while my meminfo showed underflow by 22952KiB (5738 pages).
> 
> You are probably looking for:
> commit 835f252c6debd204fcd607c79975089b1ecd3472
> "aio: fix uncorrent dirty pages accouting when truncating AIO ring buffer"
> 
> It definitely went into 3.14.26, don't know about 3.16.x.

I can confirm that a MySQL shutdown/restart triggers it for me, even
immediately following a fresh boot:

# uname -a ; grep '^nr_dirty ' /proc/vmstat; /etc/init.d/mysql restart; \
             grep '^nr_dirty ' /proc/vmstat
Linux blue 3.16.6-blue #51 Mon Oct 20 14:00:47 PDT 2014 i686 GNU/Linux
nr_dirty 13
[ ok ] Stopping MySQL database server: mysqld.
[ ok ] Starting MySQL database server: mysqld . ..
[info] Checking for tables which need an upgrade, are corrupt or were not closed cleanly..
nr_dirty 4294967245

Hmm...A possibly-related issue...Before trying this, after a fresh boot,
/proc/vmstat showed:

nr_alloc_batch 4294541205

and after the restart, it shows:

nr_alloc_batch 161

...anyway, git cherry-pick ce4b66be6cd964e84363afd4a603633dd061b3b8 on
3.16.6 tree does seem to fix nr_dirty from underflowing...Yay!

Still, nr_alloc_batch reads as 4294254379 after MySQL restart, and now
seems to stay up there.

Simon-
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