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

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Date:	Tue, 5 May 2009 16:40:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	rientjes@...gle.com, 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 Wed, 6 May 2009 01:20:34 +0200
"Rafael J. Wysocki" <rjw@...k.pl> wrote:

> On Wednesday 06 May 2009, Andrew Morton wrote:
> > On Wed, 6 May 2009 00:19:35 +0200
> > "Rafael J. Wysocki" <rjw@...k.pl> wrote:
> > 
> > > > > +			&& !processes_are_frozen()) {
> > > > >  		if (!try_set_zone_oom(zonelist, gfp_mask)) {
> > > > >  			schedule_timeout_uninterruptible(1);
> > > > >  			goto restart;
> > > > 
> > > > Cool, that looks like the semantics of __GFP_NO_OOM_KILL without requiring 
> > > > a new gfp flag.  Thanks.
> > > 
> > > Well, you're welcome.
> > > 
> > > BTW, I think that Andrew was actually right when he asked if I checked whether
> > > the existing __GFP_NORETRY would work as-is for __GFP_FS set and
> > > __GFP_NORETRY unset.  Namely, in that case we never reach the code before
> > > nopage: that checks __GFP_NORETRY, do we?
> > > 
> > > So I think we shouldn't modify the 'else if' condition above and check for
> > > !processes_are_frozen() at the beginning of the block below.
> > 
> > Confused.
> > 
> > I'm suspecting that hibernation can allocate its pages with
> > __GFP_FS|__GFP_WAIT|__GFP_NORETRY|__GFP_NOWARN, and the page allocator
> > will dtrt: no oom-killings.
> > 
> > In which case, processes_are_frozen() is not needed at all?
> 
> __GFP_NORETRY alone causes it to fail relatively quickly, but I'll try with
> the combination.

OK.  __GFP_WAIT is the big hammer.

> Anyway, even if the hibernation code itself doesn't trigger the OOM killer,
> but anyone else allocates memory in parallel or after we've preallocated the
> image memory, that may still trigger it.  So it seems processes_are_frozen()
> may still be useful?

Could be.  But only kernel threads are active at this time (yes?), and they
won't have much work to do because userspace is asleep.

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