linux-kernel - Re: [PATCH v7 5/8] mm: Device exclusive memory access

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Message-ID: <20210331134604.GK1463678@nvidia.com>
Date:   Wed, 31 Mar 2021 10:46:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     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,
        kvm-ppc@...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,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v7 5/8] mm: Device exclusive memory access

On Thu, Apr 01, 2021 at 12:27:52AM +1100, Alistair Popple wrote:
> On Thursday, 1 April 2021 12:18:54 AM AEDT Jason Gunthorpe wrote:
> > On Wed, Mar 31, 2021 at 11:59:28PM +1100, Alistair Popple wrote:
> > 
> > > I guess that makes sense as the split could go either way at the
> > > moment but I should add a check to make sure this isn't used with
> > > pinned pages anyway.
> > 
> > Is it possible to have a pinned page under one of these things? If I
> > pin it before you migrate it then it remains pinned but hidden under
> > the swap entry?
> 
> At the moment yes. But I had planned (and this reminded me) to add a check to 
> prevent marking pinned pages for exclusive access. 

How do you even do that without races with GUP fast?

Jason