linux-kernel - Re: [PATCH 1/2] mm: free large amount of 0-order pages in workqueue

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Date:	Tue, 31 Mar 2015 15:31:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	linux-kernel@...r.kernel.org, mhocko@...e.cz,
	Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	linux-mm@...ck.org (open list:MEMORY MANAGEMENT)
Subject: Re: [PATCH 1/2] mm: free large amount of 0-order pages in workqueue

On Tue, 31 Mar 2015 18:11:32 -0400 Sasha Levin <sasha.levin@...cle.com> wrote:

> Freeing pages became a rather costly operation, specially when multiple debug
> options are enabled. This causes hangs when an attempt to free a large amount
> of 0-order is made. Two examples are vfree()ing large block of memory, and
> punching a hole in a shmem filesystem.
> 
> To avoid that, move any free operations that involve batching pages into a
> list to a workqueue handler where they could be freed later.

eek.

__free_pages() is going to be a hot path for someone - it has 500+
callsites.

And this patch might cause problems for rt_prio() tasks which run for a
long time, starving out the workqueue thread.  And probably other stuff
I didn't think of...

What whacky debug option is actually causing this?  Full-page poisoning?



Stick a cond_resched() in __vunmap() ;)
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