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

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Message-Id: <200905072336.11383.rjw@sisk.pl>
Date:	Thu, 7 May 2009 23:36:10 +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, Andrew Morton wrote:
> 
> > > > 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.
> > > > 
> > > > Why does this not work for you?
> > > 
> > > Well, it might work too, but why are you insisting?  How's it better than
> > > __GFP_NO_OOM_KILL, actually?
> > > 
> > > Andrew, what do you think about this?
> > 
> > I don't think I understand the proposal.  Is it to provide a means by
> > which PM can go in and set a state bit against each and every zone?  If
> > so, that's still a global boolean, only messier.
> > 
> 
> Why can't it be global while preallocating memory for hibernation since 
> nothing but kthreads could allocate at this point and if the system is oom 
> then the oom killer wouldn't be able to do anything anyway since it can't 
> kill them?
> 
> The fact is that _all_ allocations here are implicitly __GFP_NO_OOM_KILL 
> whether it specifies it or not since the oom killer would simply kill a 
> task in D state which can't exit or free memory and subsequent allocations 
> would make the oom killer a no-op because there's an eligible task with 
> TIF_MEMDIE set.  The only thing you're saving with __GFP_NO_OOM_KILL is 
> calling the oom killer in a first place and killing an unresponsive task 

That's exactly what we're trying to do.  We don't want tasks to get killed just
because we're freeing memory for hibernation image.

> but that would have to happen anyway when thawed since the system is oom 
> (or otherwise lockup for GFP_KERNEL with order < PAGE_ALLOC_COSTLY_ORDER).

Are you sure?  The image memory is freed before thawing tasks.
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