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

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Message-ID: <fdf1c610-6c9a-befd-a284-b8a552b4c225@kernel.dk>
Date:   Thu, 4 Nov 2021 08:44:24 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Cyril Hrubis <chrubis@...e.cz>
Cc:     Drew DeVault <sir@...wn.com>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, io-uring@...r.kernel.org,
        Pavel Begunkov <asml.silence@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB

On 11/4/21 8:27 AM, Cyril Hrubis wrote:
> Hi!
>>> 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.
> 
> We are raising this limit to 2MB for LTP testcases as well, otherwise we
> get failures when we run a few bpf tests in quick succession.> 
> Acked-by: Cyril Hrubis <chrubis@...e.cz>

Andrew, care to pick this one up for 5.16?

-- 
Jens Axboe