linux-kernel - Re: [PATCH v4 05/10] mm/gup: migrate pinned pages out of movable zone

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Message-ID: <CA+CK2bBLZu3X5P3qkaTV7To4qx2W79Y1MQdpJM3134LtkzkhXA@mail.gmail.com>
Date:   Wed, 13 Jan 2021 14:14:11 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 05/10] mm/gup: migrate pinned pages out of movable zone

On Fri, Dec 18, 2020 at 8:08 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Fri 18-12-20 07:24:53, Pavel Tatashin wrote:
> > On Fri, Dec 18, 2020 at 4:43 AM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Thu 17-12-20 13:52:38, Pavel Tatashin wrote:
> > > > +      * 1. Pinned pages: (long-term) pinning of movable pages is avoided
> > > > +      *    when pages are pinned and faulted, but it is still possible that
> > > > +      *    address space already has pages in ZONE_MOVABLE at the time when
> > > > +      *    pages are pinned (i.e. user has touches that memory before
> > > > +      *    pinning). In such case we try to migrate them to a different zone,
> > > > +      *    but if migration fails the pages can still end-up pinned in
> > > > +      *    ZONE_MOVABLE. In such case, memory offlining might retry a long
> > > > +      *    time and will only succeed once user application unpins pages.
> > >
> > > I still dislike this. Pinning can fail so there shouldn't be any reasons
> > > to break MOVABLE constrain for something that can be handled. If
> > > anything there should be a very good reasoning behind this decision
> > > documented.
> >
> > This is basically current behaviour, after patch 8, we can never pin
> > pages in the movable zone, so I will update this comment in that
> > patch.
>
> Then it would be much easier for review to state that the existing
> behavior is unchanged and do not update this comment just to remove it
> in a later patch. Because this patch should be straightforward change of
> the condition which pages to migrate (+some renaming which should be
> reasonably easy to follow).

Makes sense, I will update this comment correctly right away when the
behaviour changes.