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Subject: Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB
To:     Drew DeVault <sir@cmpwn.com>, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, io-uring@vger.kernel.org
Cc:     Pavel Begunkov <asml.silence@gmail.com>
References: <20211028080813.15966-1-sir@cmpwn.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <cc3d7fac-62e9-fe11-0cf1-3d9528d191a0@kernel.dk>
Date:   Thu, 28 Oct 2021 12:22:50 -0600
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On 10/28/21 2:08 AM, Drew DeVault wrote:
> This limit has not been updated since 2008, when it was increased to 64
> KiB at the request of GnuPG. Until recently, the main use-cases for this
> feature were (1) preventing sensitive memory from being swapped, as in
> GnuPG's use-case; and (2) real-time use-cases. In the first case, little
> memory is called for, and in the second case, the user is generally in a
> position to increase it if they need more.
> 
> The introduction of IOURING_REGISTER_BUFFERS adds a third use-case:
> preparing fixed buffers for high-performance I/O. This use-case will
> take as much of this memory as it can get, but is still limited to 64
> KiB by default, which is very little. This increases the limit to 8 MB,
> which was chosen fairly arbitrarily as a more generous, but still
> conservative, default value.
> ---
> It is also possible to raise this limit in userspace. This is easily
> done, for example, in the use-case of a network daemon: systemd, for
> instance, provides for this via LimitMEMLOCK in the service file; OpenRC
> via the rc_ulimit variables. However, there is no established userspace
> facility for configuring this outside of daemons: end-user applications
> do not presently have access to a convenient means of raising their
> limits.
> 
> The buck, as it were, stops with the kernel. It's much easier to address
> it here than it is to bring it to hundreds of distributions, and it can
> only realistically be relied upon to be high-enough by end-user software
> if it is more-or-less ubiquitous. Most distros don't change this
> particular rlimit from the kernel-supplied default value, so a change
> here will easily provide that ubiquity.

Agree with raising this limit, it is ridiculously low and we often get
reports from people that can't even do basic rings with it. Particularly
when bpf is involved as well, as it also dips into this pool.

On the production side at facebook, we do raise this limit as well.

Acked-by: Jens Axboe <axboe@kernel.dk>

-- 
Jens Axboe