From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751599AbdIQRpm (ORCPT <rfc822;w@1wt.eu>);
        Sun, 17 Sep 2017 13:45:42 -0400
Received: from mx1.redhat.com ([209.132.183.28]:56416 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751410AbdIQRpl (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Sun, 17 Sep 2017 13:45:41 -0400
DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com B11B1C07188D
Authentication-Results: ext-mx07.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com
Authentication-Results: ext-mx07.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=jglisse@redhat.com
Date: Sun, 17 Sep 2017 10:45:34 -0700
From: Jerome Glisse <jglisse@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>
Subject: Re: [PATCH] mm/memcg: avoid page count check for zone device
Message-ID: <20170917174534.GC11906@redhat.com>
References: <20170914190011.5217-1-jglisse@redhat.com>
 <20170915070100.2vuxxxk2zf2yceca@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20170915070100.2vuxxxk2zf2yceca@dhcp22.suse.cz>
User-Agent: Mutt/1.8.3 (2017-05-23)
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Sun, 17 Sep 2017 17:45:41 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Sep 15, 2017 at 09:01:00AM +0200, Michal Hocko wrote:
> On Thu 14-09-17 15:00:11, jglisse@redhat.com wrote:
> > From: Jérôme Glisse <jglisse@redhat.com>
> > 
> > Fix for 4.14, zone device page always have an elevated refcount
> > of one and thus page count sanity check in uncharge_page() is
> > inappropriate for them.
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> > Reported-by: Evgeny Baskakov <ebaskakov@nvidia.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Side note. Wouldn't it be better to re-organize the check a bit? It is
> true that this is VM_BUG so it is not usually compiled in but when it
> preferably checks for unlikely cases first while the ref count will be
> 0 in the prevailing cases. So can we have
> 	VM_BUG_ON_PAGE(page_count(page) && !is_zone_device_page(page) &&
> 			!PageHWPoison(page), page);
> 
> I would simply fold this nano optimization into the patch as you are
> touching it already. Not sure it is worth a separate commit.

I am traveling sorry for late answer. This nano optimization make sense
Andrew do you want me to respin or should we leave it be ? I don't mind
either way.

Cheers,
Jérôme