linux-kernel - Re: [RFC PATCH 3/3] mm, fault_around: do not take a reference to a locked page

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Message-ID: <20181121071132.GD12932@dhcp22.suse.cz>
Date:   Wed, 21 Nov 2018 08:11:32 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <OSalvador@...e.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [RFC PATCH 3/3] mm, fault_around: do not take a reference to a
 locked page

On Tue 20-11-18 17:47:21, Hugh Dickins wrote:
> On Tue, 20 Nov 2018, Michal Hocko wrote:
> 
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > filemap_map_pages takes a speculative reference to each page in the
> > range before it tries to lock that page. While this is correct it
> > also can influence page migration which will bail out when seeing
> > an elevated reference count. The faultaround code would bail on
> > seeing a locked page so we can pro-actively check the PageLocked
> > bit before page_cache_get_speculative and prevent from pointless
> > reference count churn.
> > 
> > Cc: "Kirill A. Shutemov" <kirill@...temov.name>
> > Suggested-by: Jan Kara <jack@...e.cz>
> > Signed-off-by: Michal Hocko <mhocko@...e.com>
> 
> Acked-by: Hugh Dickins <hughd@...gle.com>

Thanks!

> though I think this patch is more useful to the avoid atomic ops,
> and unnecessary dirtying of the cacheline, than to avoid the very
> transient elevation of refcount, which will not affect page migration
> very much.

Are you sure it would really be transient? In other words is it possible
that the fault around can block migration repeatedly under refault heavy
workload? I just couldn't convince myself, to be honest.
-- 
Michal Hocko
SUSE Labs