linux-kernel - Re: [patch 08/11 -mmotm] oom: invoke oom killer for __GFP

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Date:	Mon, 11 May 2009 16:14:46 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	gregkh@...e.de, npiggin@...e.de, mel@....ul.ie,
	a.p.zijlstra@...llo.nl, cl@...ux-foundation.org,
	dave@...ux.vnet.ibm.com, san@...roid.com, arve@...roid.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 08/11 -mmotm] oom: invoke oom killer for __GFP_NOFAIL

On Mon, 11 May 2009 16:00:58 -0700 (PDT)
David Rientjes <rientjes@...gle.com> wrote:

> On Mon, 11 May 2009, Andrew Morton wrote:
> 
> > oh, well that was pretty useless then.  I was trying to find a handy
> > spot where we can avoid adding fastpath cycles.
> > 
> > How about we sneak it into the order>0 leg inside buffered_rmqueue()?
> > 
> 
> Wouldn't it be easier after my patch is merged to just check the oom 
> killer stack traces for such allocations and people complain about 
> unnecessary oom killing when memory is available but too fragmented?  The 
> gfp_flags and order are shown in the oom killer header.

That assumes that the oom-killer is triggered - in the typical
kernel developer testing, that won't happen.

I think what we should do here is to prevent people even attempting to
use __GFP_NOFAIL with higher-order allocations.

Are you aware of any callsite which is presently using __GFP_NOFAIL on
order>0 allocations?

I expect slub might cause this to happen due to its habit of using
larger-than-needed orders for small objects.  For example, cxgb3 is
passing __GFP_NOFAIL into alloc_skb().

> > 
> > --- a/mm/page_alloc.c~page-allocator-warn-if-__gfp_nofail-is-used-for-a-large-allocation
> > +++ a/mm/page_alloc.c
> > @@ -1130,6 +1130,20 @@ again:
> >  		list_del(&page->lru);
> >  		pcp->count--;
> >  	} else {
> > +		if (unlikely(gfp_mask & __GFP_NOFAIL)) {
> > +			/*
> > +			 * __GFP_NOFAIL is not to be used in new code.
> > +			 *
> > +			 * All __GFP_NOFAIL callers should be fixed so that they
> > +			 * properly detect and handle allocation failures.
> > +			 *
> > +			 * We most definitely don't want callers attempting to
> > +			 * allocate greater than single-page units with
> > +			 * __GFP_NOFAIL.
> > +			 */
> > +			WARN_ON_ONCE(order > 0);
> > +			return 0;
> > +		}
> >  		spin_lock_irqsave(&zone->lock, flags);
> >  		page = __rmqueue(zone, order, migratetype);
> >  		__mod_zone_page_state(zone, NR_FREE_PAGES, -(1 << order));
> 
> That "return 0" definitely needs to be removed, though :)

The inventor of copy-n-paste has a lot to answer for.
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