Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB

From: Matthew Wilcox <willy@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Ammar Faizi <ammarfaizi2@gnuweeb.org>,
	Drew DeVault <sir@cmpwn.com>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	io_uring Mailing List <io-uring@vger.kernel.org>,
	Jens Axboe <axboe@kernel.dk>,
	Pavel Begunkov <asml.silence@gmail.com>,
	linux-mm@kvack.org
Subject: Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB
Date: Tue, 16 Nov 2021 18:36:21 +0000	[thread overview]
Message-ID: <YZP6JSd4h45cyvsy@casper.infradead.org> (raw)
In-Reply-To: <20211115203530.62ff33fdae14927b48ef6e5f@linux-foundation.org>

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?