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

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Date:   Wed, 24 Nov 2021 11:34:05 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     David Hildenbrand <david@...hat.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, 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 Wed, Nov 24, 2021 at 03:14:00PM +0100, David Hildenbrand wrote:

> I'm not aware of any where you can fragment 50% of all pageblocks in the
> system as an unprivileged user essentially consuming almost no memory
> and essentially staying inside well-defined memlock limits. But sure if
> there are "many" people will be able to come up with at least one
> comparable thing. I'll be happy to learn.

If the concern is that THP's can be DOS'd then any avenue that renders
the system out of THPs is a DOS attack vector. Including all the
normal workloads that people run and already complain that THPs get
exhausted.

A hostile userspace can only quicken this process.

> My position that FOLL_LONGTERM for unprivileged users is a strong no-go
> stands as it is.

As this basically excludes long standing pre-existing things like
RDMA, XDP, io_uring, and more I don't think this can be the general
answer for mm, sorry.

Sure, lets stop now since I don't think we can agree.

Jason