linux-kernel - Re: Is it safe for kthreadd to drain_all

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Date:   Thu, 6 Apr 2017 14:06:14 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: Is it safe for kthreadd to drain_all_pages?

On Wed, Apr 05, 2017 at 01:59:49PM -0700, Hugh Dickins wrote:
> Hi Mel,
> 
> I suspect that it's not safe for kthreadd to drain_all_pages();
> but I haven't studied flush_work() etc, so don't really know what
> I'm talking about: hoping that you will jump to a realization.
> 

You're right, it's not safe. If kthreadd is creating the workqueue
thread to do the drain and it'll recurse into itself.

> 4.11-rc has been giving me hangs after hours of swapping load.  At
> first they looked like memory leaks ("fork: Cannot allocate memory");
> but for no good reason I happened to do "cat /proc/sys/vm/stat_refresh"
> before looking at /proc/meminfo one time, and the stat_refresh stuck
> in D state, waiting for completion of flush_work like many kworkers.
> kthreadd waiting for completion of flush_work in drain_all_pages().
> 

It's asking itself to do work in all likelihood.

> Patch below has been running well for 36 hours now:
> a bit too early to be sure, but I think it's time to turn to you.
> 

I think the patch is valid but like Michal, would appreciate if you
could run the patch he linked to see if it also side-steps the same
problem. 

Good spot!

-- 
Mel Gorman
SUSE Labs