linux-kernel - Re: [PATCH 04/20] arm: get rid of superfluous __GFP

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Date:	Fri, 29 Apr 2016 11:41:52 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org
Subject: Re: [PATCH 04/20] arm: get rid of superfluous __GFP_REPEAT

On Thu 28-04-16 17:08:31, Michal Hocko wrote:
> On Thu 28-04-16 15:55:45, Russell King - ARM Linux wrote:
> > On Thu, Apr 28, 2016 at 03:23:50PM +0200, Michal Hocko wrote:
> > > From: Michal Hocko <mhocko@...e.com>
> > > 
> > > __GFP_REPEAT has a rather weak semantic but since it has been introduced
> > > around 2.6.12 it has been ignored for low order allocations.
> > > 
> > > PGALLOC_GFP uses __GFP_REPEAT but none of the allocation which uses
> > > this flag is for more than order-2. This means that this flag has never
> > > been actually useful here because it has always been used only for
> > > PAGE_ALLOC_COSTLY requests.
> > 
> > I'm unconvinced.  Back in 2013, I was seeing a lot of failures, so:
> > 
> > commit 8c65da6dc89ccb605d73773b1dd617e72982d971
> > Author: Russell King <rmk+kernel@....linux.org.uk>
> > Date:   Sat Nov 30 12:52:31 2013 +0000
> > 
> >     ARM: pgd allocation: retry on failure
> > 
> >     Make pgd allocation retry on failure; we really need this to succeed
> >     otherwise fork() can trigger OOMs.
> > 
> >     Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
> > 
> > Maybe something has changed again in the MM layer which makes this flag
> > unnecessary again, and it was a temporary blip around that time, I don't
> > know.
> 
> PAGE_ALLOC_COSTLY_ORDER is defined to order 3 since 2007 and even before
> the code was doing
> -               if ((order <= 3) || (gfp_mask & __GFP_REPEAT))
> +               if ((order <= PAGE_ALLOC_COSTLY_ORDER) ||
> +                                               (gfp_mask & __GFP_REPEAT))
>                         do_retry = 1;
> 
> So an order-2 allocation which is the case for this particular code now
> will trigger the OOM killer and fail only when the current task is
> killed by the OOM killer. Other than that order-2 is basically
> GFP_NOFAIL. Have a look at __alloc_pages_slowpath() for more details.

Does this explanation help?
-- 
Michal Hocko
SUSE Labs