Re: [PATCH 04/20] arm: get rid of superfluous __GFP_REPEAT

From	Michal Hocko <mhocko@kernel.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH 04/20] arm: get rid of superfluous __GFP_REPEAT
Date	2016-04-29 11:50 +0200
Message-ID	<rtddE-3Cj-11@gated-at.bofh.it> (permalink)
References	<rsUaZ-3LP-3@gated-at.bofh.it> <rsUkI-3Su-39@gated-at.bofh.it> <rsVA5-50h-1@gated-at.bofh.it> <rsVJM-5u4-17@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

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@suse.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@arm.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@arm.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

Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

[PATCH 04/20] arm: get rid of superfluous __GFP_REPEAT Michal Hocko <mhocko@kernel.org> - 2016-04-28 15:40 +0200
  Re: [PATCH 04/20] arm: get rid of superfluous __GFP_REPEAT Russell King - ARM Linux <linux@arm.linux.org.uk> - 2016-04-28 17:00 +0200
    Re: [PATCH 04/20] arm: get rid of superfluous __GFP_REPEAT Michal Hocko <mhocko@kernel.org> - 2016-04-28 17:10 +0200
      Re: [PATCH 04/20] arm: get rid of superfluous __GFP_REPEAT Michal Hocko <mhocko@kernel.org> - 2016-04-29 11:50 +0200

csiph-web