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

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Date:   Tue, 18 May 2021 19:45:05 -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 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.

> 
> We can optimize this into the various variants above, but logically
> device exclusive stop existing during fork.

Makes sense, I think that's indeed what this patch did at least for the COW
case, so I think Alistair did address that comment.  It's just that I think we
need to drop the other !COW case (imho that should correspond to the changes in
copy_nonpresent_pte()) in this patch to guarantee it.

I also hope we don't make copy_pte_range() even more complicated just to do the
lock_page() right, so we could fail the fork() if the lock is hard to take.

-- 
Peter Xu