linux-kernel - Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <ZFEVQmFGL3GxZMaf@nvidia.com>
Date:   Tue, 2 May 2023 10:50:58 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
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        Dave Chinner <david@...morbit.com>,
        Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

On Tue, May 02, 2023 at 03:47:43PM +0200, David Hildenbrand wrote:
> > Eventually we want to implement a mechanism where we can dynamically pin in response to RPCIT.
> 
> Okay, so IIRC we'll fail starting the domain early, that's good. And if we
> pin all guest memory (instead of small pieces dynamically), there is little
> existing use for file-backed RAM in such zPCI configurations (because memory
> cannot be reclaimed either way if it's all pinned), so likely there are no
> real existing users.

Right, this is VFIO, the physical HW can't tolerate not having pinned
memory, so something somewhere is always pinning it.

Which, again, makes it weird/wrong that this KVM code is pinning it
again :\

Jason