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

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Message-ID: <20210119183013.GB4605@ziepe.ca>
Date:   Tue, 19 Jan 2021 14:30:13 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, vbabka@...e.cz, mhocko@...e.com,
        david@...hat.com, osalvador@...e.de, dan.j.williams@...el.com,
        sashal@...nel.org, tyhicks@...ux.microsoft.com,
        iamjoonsoo.kim@....com, mike.kravetz@...cle.com,
        rostedt@...dmis.org, mingo@...hat.com, peterz@...radead.org,
        mgorman@...e.de, willy@...radead.org, rientjes@...gle.com,
        jhubbard@...dia.com, linux-doc@...r.kernel.org,
        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 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

Why does the zero page have to be movable?

Jason