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

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Message-ID: <2124945.3NMGunUBXV@nvdebian>
Date:   Thu, 1 Apr 2021 00:27:52 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     Jason Gunthorpe <jgg@...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 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. This check was in the 
original migration based implementation as I don't think it makes much sense 
to allow exclusive access to pinned pages given it indicates another device is 
possibly using it. 

> So the special logic is needed and the pinned page has to be copied
> and written as a normal pte, not dropped as a migration entry

Yep, if we end up allowing pinned pages to exist under these then that makes 
sense. Thanks for the clarification.

 - Alistair

> Jason
>