linux-kernel - Re: [PATCH] Improve Atheros ethernet driver not to do order 4 GFP

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Message-ID: <20151203081646.GB9264@dhcp22.suse.cz>
Date:	Thu, 3 Dec 2015 09:16:47 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Chris Snook <chris.snook@...il.com>
Cc:	David Miller <davem@...emloft.net>, pavel@....cz, akpm@...l.org,
	linux-kernel@...r.kernel.org, jcliburn@...il.com,
	netdev@...r.kernel.org, rjw@...ysocki.net, linux-mm@...ck.org,
	nic-devel@...lcomm.com, ronangeles@...il.com, ebiederm@...ssion.com
Subject: Re: [PATCH] Improve Atheros ethernet driver not to do order 4
 GFP_ATOMIC allocation

On Wed 02-12-15 22:43:31, Chris Snook wrote:
> On Tue, Dec 1, 2015 at 12:35 PM David Miller <davem@...emloft.net> wrote:
> 
> > From: Michal Hocko <mhocko@...nel.org>
> > Date: Mon, 30 Nov 2015 14:21:29 +0100
> >
> > > On Sat 28-11-15 15:51:13, Pavel Machek wrote:
> > >>
> > >> atl1c driver is doing order-4 allocation with GFP_ATOMIC
> > >> priority. That often breaks  networking after resume. Switch to
> > >> GFP_KERNEL. Still not ideal, but should be significantly better.
> > >
> > > It is not clear why GFP_KERNEL can replace GFP_ATOMIC safely neither
> > > from the changelog nor from the patch context.
> >
> > Earlier in the function we do a GFP_KERNEL kmalloc so:
> >
> > ¯\_(ツ)_/¯
> >
> > It should be fine.
> >
> 
> AFAICT, the people who benefit from GFP_ATOMIC are the people running all
> their storage over NFS/iSCSI who are suspending their machines while
> they're so busy they don't have any clean order 4 pagecache to drop, and
> want the machine to panic rather than hang.

Why would GFP_KERNEL order-4 allocation hang? It will fail if there are
not >=4 order pages available even after reclaim and/or compaction.
GFP_ATOMIC allocations should be used only when an access to memory
reserves is really required. If the allocation just doesn't want to
invoke direct reclaim then GFP_NOWAIT is a more suitable alternative.

-- 
Michal Hocko
SUSE Labs
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