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Date:   Wed, 2 Mar 2022 12:55:59 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
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        Pedro Gomes <pedrodemargomes@gmail.com>,
        Oded Gabbay <oded.gabbay@gmail.com>, linux-mm@kvack.org
Subject: Re: [PATCH RFC 12/13] mm/gup: trigger FAULT_FLAG_UNSHARE when
 R/O-pinning a possibly shared anonymous page
Message-ID: <20220302165559.GU219866@nvidia.com>
References: <20220224122614.94921-1-david@redhat.com>
 <20220224122614.94921-13-david@redhat.com>
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On Thu, Feb 24, 2022 at 01:26:13PM +0100, David Hildenbrand wrote:
> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
> on the page table entry will end up replacing the mapped anonymous page
> due to COW, resulting in the GUP pin no longer being consistent with the
> page actually mapped into the page table.
> 
> The possible ways to deal with this situation are:
>  (1) Ignore and pin -- what we do right now.
>  (2) Fail to pin -- which would be rather surprising to callers and
>      could break user space.
>  (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
>      pins.

How does this mesh with the common FOLL_FORCE|FOLL_WRITE|FOLL_PIN
pattern used for requesting read access? Can they be converted to
just FOLL_WRITE|FOLL_PIN after this?

Jason