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

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Date:   Wed, 19 May 2021 10:09:33 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.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 Wed, May 19, 2021 at 10:28:42AM -0300, Jason Gunthorpe wrote:
> On Tue, May 18, 2021 at 07:45:05PM -0400, Peter Xu wrote:
> > On Tue, May 18, 2021 at 08:03:27PM -0300, Jason Gunthorpe wrote:
> > > Logically during fork all these device exclusive pages should be
> > > reverted back to their CPU pages, write protected and the CPU page PTE
> > > copied to the fork.
> > > 
> > > We should not copy the device exclusive page PTE to the fork. I think
> > > I pointed to this on an earlier rev..
> > 
> > Agreed.  Though please see the question I posted in the other thread: now I am
> > not very sure whether we'll be able to mark a page as device exclusive if that
> > page has mapcount>1.
> 
> IMHO it is similar to write protect done by filesystems on shared
> mappings - all VMAs with a copy of the CPU page have to get switched
> to the device exclusive PTE. This is why the rmap stuff is involved in
> the migration helpers

Right, I think Alistair corrected me there that I missed the early COW
happening in GUP.

Actually even without that GUP triggering early COW it won't be a problem,
because as long as one child mm restored the pte from exclusive to normal
(before any further COW happens) device exclusiveness is broken in the mmu
notifiers, and after that point all previous-exclusive ptes actually becomes
the same as a very normal PageAnon.  Then it's very sane to even not have the
original page in parent process, because we know each COWed page will contain
all the device atomic modifications (so we don't really have the requirement to
return the original page to parent).

Sorry for the noise.

-- 
Peter Xu