linux-kernel - Re: [PATCH 1/5] mm: Add __GFP_NO_OOM

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Message-Id: <200905090155.44635.rjw@sisk.pl>
Date:	Sat, 9 May 2009 01:55:43 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, fengguang.wu@...el.com,
	linux-pm@...ts.linux-foundation.org, pavel@....cz,
	torvalds@...ux-foundation.org, jens.axboe@...cle.com,
	alan-jenkins@...fmail.co.uk, linux-kernel@...r.kernel.org,
	kernel-testers@...r.kernel.org
Subject: Re: [PATCH 1/5] mm: Add __GFP_NO_OOM_KILL flag

On Thursday 07 May 2009, David Rientjes wrote:
> On Thu, 7 May 2009, Rafael J. Wysocki wrote:
> 
> > OK, let's try with __GFP_NO_OOM_KILL first.  If there's too much disagreement,
> > I'll use the freezer-based approach instead.
> > 
> 
> Third time I'm going to suggest this, and I'd like a response on why it's 
> not possible instead of being ignored.
> 
> All of your tasks are in D state other than kthreads, right?  That means 
> they won't be in the oom killer (thus no zones are oom locked), so you can 
> easily do this
> 
> 	struct zone *z;
> 	for_each_populated_zone(z)
> 		zone_set_flag(z, ZONE_OOM_LOCKED);
> 
> and then
> 
> 	for_each_populated_zone(z)
> 		zone_clear_flag(z, ZONE_OOM_LOCKED);
> 
> The serialization is done with trylocks so this will never invoke the oom 
> killer because all zones in the allocator's zonelist will be oom locked.

Well, that might have been a good idea if it actually had worked. :-(

> Why does this not work for you?

If I set image_size to something below "hard core working set" +
totalreserve_pages, preallocate_image_memory() hangs the
box (please refer to the last patch I sent,
http://patchwork.kernel.org/patch/22423/).

However, with the freezer-based disabling of the OOM killer it doesn't hang
under the same test conditions.

The difference appears to be that using your approach makes
__alloc_pages_internal() loop forever between the !try_set_zone_oom() test and
restart:, while it should go to nopage: in that situation.

So, I think I'll stick to the Andrew's approach with using __GFP_NO_OOM_KILL.

Thanks,
Rafael
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