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

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Date:	Tue, 16 Feb 2010 18:53:30 +1100
From:	Nick Piggin <npiggin@...e.de>
To:	David Rientjes <rientjes@...gle.com>
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 Mon, Feb 15, 2010 at 11:41:49PM -0800, David Rientjes wrote:
> 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?

Ye, it's never worked with GFP_ATOMIC.


> 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 don't know. IMO we should never just randomly weaken or break such
flag as the page allocator API.

> 
> 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.

Thanks. Yes endlessly looping is far preferable to randomly oopsing
or corrupting memory.

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