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

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Message-ID: <b97f1b15-fbcc-92a4-96ca-e918c2f6c7a3@kernel.dk>
Date:   Tue, 16 Nov 2021 11:55:41 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        Drew DeVault <sir@...wn.com>, 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 11/16/21 11:36 AM, Matthew Wilcox wrote:
> On Mon, Nov 15, 2021 at 08:35:30PM -0800, Andrew Morton wrote:
>> I'd also be interested in seeing feedback from the MM developers.
> [...]
>> Subject: Increase default MLOCK_LIMIT to 8 MiB
> 
> On the one hand, processes can already allocate at least this much
> memory that is non-swappable, just by doing things like opening a lot of
> files (allocating struct file & fdtable), using a lot of address space
> (allocating page tables), so I don't have a problem with it per se.
> 
> On the other hand, 64kB is available on anything larger than an IBM XT.
> Linux will still boot on machines with 4MB of RAM (eg routers).  For
> someone with a machine with only, say, 32MB of memory, this allows a
> process to make a quarter of the memory unswappable, and maybe that's
> not a good idea.  So perhaps this should scale over a certain range?
> 
> Is 8MB a generally useful amount of memory for an iouring user anyway?
> If you're just playing with it, sure, but if you have, oh i don't know,
> a database, don't you want to pin the entire cache and allow IO to the
> whole thing?

8MB is plenty for most casual use cases, which is exactly the ones that
we want to "just work" without requiring weird system level
modifications to increase the memlock limit.

For db etc server setups, you're going to be mucking with the setup
anyway, and then I don't see it as a big problem that you'll need to
increase it further. Because yes, that won't fit within 8MB if you start
doing registered buffers.

-- 
Jens Axboe