linux-kernel - Re: [PATCH v8 5/8] mm: Device exclusive memory access

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Date:   Tue, 18 May 2021 14:33:34 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
        nouveau@...ts.freedesktop.org, bskeggs@...hat.com,
        akpm@...ux-foundation.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        jhubbard@...dia.com, rcampbell@...dia.com, jglisse@...hat.com,
        hch@...radead.org, daniel@...ll.ch, willy@...radead.org,
        bsingharora@...il.com, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v8 5/8] mm: Device exclusive memory access

On Tue, May 18, 2021 at 01:27:42PM -0400, Peter Xu wrote:

> I also have a pure and high level question regarding a process fork() when
> there're device exclusive ptes: would the two processes then own the device
> together?  Is this a real usecase?

If the pages are MAP_SHARED then yes. All VMAs should point at the
same device_exclusive page and all VMA should migrate back to CPU
pages together.

> Indeed it'll be odd for a COW page since for COW page then it means after
> parent/child writting to the page it'll clone into two, then it's a mistery on
> which one will be the one that "exclusived owned" by the device..

For COW pages it is like every other fork case.. We can't reliably
write-protect the device_exclusive page during fork so we must copy it
at fork time.

Thus three reasonable choices:
 - Copy to a new CPU page
 - Migrate back to a CPU page and write protect it
 - Copy to a new device exclusive page

Jason