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

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Date:   Mon, 22 Nov 2021 19:26:14 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Dona-Couch <andrew@...acou.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Drew DeVault <sir@...wn.com>
Cc:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        io_uring Mailing List <io-uring@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB

On 22.11.21 18:55, Andrew Dona-Couch wrote:
> Forgive me for jumping in to an already overburdened thread.  But can
> someone pushing back on this clearly explain the issue with applying
> this patch?

It will allow unprivileged users to easily and even "accidentally"
allocate more unmovable memory than it should in some environments. Such
limits exist for a reason. And there are ways for admins/distros to
tweak these limits if they know what they are doing.

> 
> The only concerns I've heard are that it doesn't go far enough.  That
> another strategy (that everyone seems to agree would be a fair bit more
> effort) could potentially achieve the same goal and then some.  Isn't
> that exactly what's meant by "don't let perfection be the enemy of the
> good"? The saying is not talking about literal perfection -- the idea is
> that you make progress where you can, and that incremental progress and
> broader changes are not necessarily in conflict.
> 
> This tiny patch could be a step in the right direction.  Why does this
> thread need dozens of replies?

Because it does something controversial. Send controversial patches,
receive many opinions, it's that simple.

This is not a step into the right direction. This is all just trying to
hide the fact that we're exposing FOLL_LONGTERM usage to random
unprivileged users.

Maybe we could instead try getting rid of FOLL_LONGTERM usage and the
memlock limit in io_uring altogether, for example, by using mmu
notifiers. But I'm no expert on the io_uring code.

-- 
Thanks,

David / dhildenb