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

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Date:   Tue, 19 Jan 2021 14:47:51 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pavel Tatashin <pasha.tatashin@...een.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>,
        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 01:34:26PM -0500, Pavel Tatashin wrote:
> 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

Things assuming GUP doesn't break COW, making all GUP WRITE was
already tried and revered for some other reason

> > 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?

The zero page is always zero, it is never written to. What does cow
matter?

Jason