linux-kernel - Re: [PATCH] vmscan: do not throttle kthreads due to too_many

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Message-ID: <20151125162756.GJ29014@esperanza>
Date:	Wed, 25 Nov 2015 19:27:57 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Vlastimil Babka <vbabka@...e.cz>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vmscan: do not throttle kthreads due to too_many_isolated

On Wed, Nov 25, 2015 at 04:45:13PM +0100, Vlastimil Babka wrote:
> On 11/25/2015 04:36 PM, Vladimir Davydov wrote:
> > Block device drivers often hand off io request processing to kernel
> > threads (example: device mapper). If such a thread calls kmalloc, it can
> > dive into direct reclaim path and end up waiting for too_many_isolated
> > to return false, blocking writeback. This can lead to a dead lock if the
> 
> Shouldn't such allocation lack __GFP_IO to prevent this and other kinds of
> deadlocks? And/or have mempools?

Not necessarily. loopback is an example: it can call
grab_cache_write_begin -> add_to_page_cache_lru with GFP_KERNEL.

> PF_KTHREAD looks like a big hammer to me that will solve only one
> potential problem...

This problem can result in processes hanging forever. Any ideas how this
could be fixed in a better way?

Thanks,
Vladimir
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