linux-kernel - Re: [PATCH v5 08/14] mm/gup: do not allow zero page for pinned pages

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Message-ID: <CA+CK2bBKbht34Hkg9YvhwYAiAjd3NMd_+Eir9wfx+07V-Y2TTA@mail.gmail.com>
Date:   Tue, 19 Jan 2021 13:34:26 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        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>,
        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 v5 08/14] mm/gup: do not allow zero page for pinned pages

On Tue, Jan 19, 2021 at 1:30 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Mon, Jan 18, 2021 at 11:39:14PM -0500, Pavel Tatashin wrote:
> > Zero page should not be used for long term pinned pages. Once pages
> > are pinned their physical addresses cannot changed until they are unpinned.
> >
> > Guarantee to always return real pages when they are pinned by adding
> > FOLL_WRITE.
> >
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
> > ---
> >  mm/gup.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
>
> No, this will definitely break things

What will break

>
> Why does the zero page have to be movable?

It is not even about being movable, we can't cow pinned pages returned
by GUP call, how can we use zero page for that?

>
> Jason