Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB

From: David Hildenbrand
Date: Mon Nov 22 2021 - 13:26:24 EST

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.


David / dhildenb