linux-kernel - Re: [PATCH] vfio/type1: Unpin zero pages

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Date:   Wed, 7 Sep 2022 20:07:02 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lpivarc@...hat.com" <lpivarc@...hat.com>,
        "Liu, Jingqi" <jingqi.liu@...el.com>,
        "Lu, Baolu" <baolu.lu@...el.com>
Subject: Re: [PATCH] vfio/type1: Unpin zero pages

On Wed, Sep 07, 2022 at 02:24:16PM -0600, Alex Williamson wrote:

> Also, I want to clarify, is this a recommendation relative to the
> stable patch proposed here, or only once we get rid of shared zero page
> pinning?  We can't simply do accounting on the shared zero page since a
> single user can overflow the refcount.

Yes, here I would account properly in a way that keeps working for
future GUP changes because if something goes wrong with this simple
patch it has a simple fix.

Trialing it will get some good data to inform what David's patch
should do.

Overall have the feeling that a small group of people might grumble
that their limits break, but with a limit adjustment they can probably
trivially move on. It would be very interesting to see if someone
feels like the issue is important enough to try and get something
changed.

You could also fix it by just using FOLL_FORCE (like RDMA/io_uring
does), which fixes the larger issue Kevin noted that the ROM doesn't
become visible to DMA.

Jason