linux-kernel - Re: [PATCH] Increase default MLOCK

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Date:   Tue, 23 Nov 2021 19:59:53 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     David Hildenbrand <david@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Andrew Dona-Couch <andrew@...acou.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Drew DeVault <sir@...wn.com>,
        Ammar Faizi <ammarfaizi2@...weeb.org>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        io_uring Mailing List <io-uring@...r.kernel.org>,
        Pavel Begunkov <asml.silence@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB

On Tue, Nov 23, 2021 at 11:04:04PM +0100, Vlastimil Babka wrote:
> On 11/23/21 18:00, Jason Gunthorpe wrote:
> > 
> >> believe what you say and I trust your experience :) So could as well be
> >> that on such a "special" (or not so special) systems there should be a
> >> way to restrict it to privileged users only.
> > 
> > At this point RDMA is about as "special" as people running large
> > ZONE_MOVABLE systems, and the two are going to start colliding
> > heavily. The RDMA VFIO migration driver should be merged soon which
> > makes VMs using this stuff finally practical.
> 
> How does that work, I see the word migration, so does it cause pages to

Sorry I mean what is often called "VM live migration". Typically that
cannot be done if a PCI device is assigned to the VM as suspending and
the migrating a PCI device to another server is complicated. With
forthcoming hardware mlx5 can do this and thus the entire RDMA stack
becomes practically usable and performant within a VM.

> be migrated out of ZONE_MOVABLE before they are pinned?

GUP already does this automatically for FOLL_LONGTERM.

> Similarly for io-uring we could be migrating pages to be pinned so that
> the end up consolidated close together, and prevent pathologic
> situations like in David's reproducer. 

It is an interesting idea to have GUP do some kind of THP preserving
migration.

Jason