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Date: Mon, 18 Sep 2017 08:31:41 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH] mm/memcg: avoid page count check for zone device
Message-ID: <20170918063141.6coovns7cb45bfly@dhcp22.suse.cz>
References: <20170914190011.5217-1-jglisse@redhat.com>
 <20170915070100.2vuxxxk2zf2yceca@dhcp22.suse.cz>
 <20170917174534.GC11906@redhat.com>
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To: Jerome Glisse <jglisse@redhat.com>
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>

On Sun 17-09-17 10:45:34, Jerome Glisse wrote:
> 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: Jerome 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: Jerome 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.

Andrew, could you fold this into the patch then?
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