linux-kernel - Re: [patch -mm 8/9 v2] oom: avoid oom killer for lowmem allocations

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Date:	Mon, 15 Feb 2010 23:41:49 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Nick Piggin <npiggin@...e.de>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Lubos Lunak <l.lunak@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch -mm 8/9 v2] oom: avoid oom killer for lowmem
 allocations

On Tue, 16 Feb 2010, Nick Piggin wrote:

> > As I already explained when you first brought this up, the possibility of 
> > not invoking the oom killer is not unique to GFP_DMA, it is also possible 
> > for GFP_NOFS.  Since __GFP_NOFAIL is deprecated and there are no current 
> > users of GFP_DMA | __GFP_NOFAIL, that warning is completely unnecessary.  
> > We're not adding any additional __GFP_NOFAIL allocations.
> 
> Completely agree with this request. Actually, I think even better you
> should just add && !(gfp_mask & __GFP_NOFAIL). Deprecated doesn't mean
> it is OK to break the API (callers *will* oops or corrupt memory if
> __GFP_NOFAIL returns NULL).
> 

... unless it's used with GFP_ATOMIC, which we've always returned NULL 
for when even ALLOC_HARDER can't find pages, right?

I'm wondering where this strong argument in favor of continuing to support 
__GFP_NOFAIL was when I insisted we call the oom killer for them even for 
allocations over PAGE_ALLOC_COSTLY_ORDER when __alloc_pages_nodemask() was 
refactored back in 2.6.31.  The argument was that nobody is allocating 
that high of orders of __GFP_NOFAIL pages so we don't need to free memory 
for them and that's where the deprecation of the modifier happened in the 
first place.  Ultimately, we did invoke the oom killer for those 
allocations because there's no chance of forward progress otherwise and, 
unlike __GFP_DMA, GFP_KERNEL | __GFP_NOFAIL actually is popular.  

I'll add this check to __alloc_pages_may_oom() for the !(gfp_mask & 
__GFP_NOFAIL) path since we're all content with endlessly looping.
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