Re: [GIT PULL] locking/urgent for v6.17-rc1

[Linus Torvalds]

On Sat, 9 Aug 2025 at 14:02, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> please pull a locking/urgent fix for v6.17-rc1.

Ok, so this clearly wasn't a fix.

> Thomas Gleixner (1):
>       futex: Move futex cleanup to __mmdrop()

So this causes problems, because __mmdrop is not done in thread
context, and the kvfree() calls then cause issues:

  https://lore.kernel.org/all/20250821102721.6deae493@xxxxxxxxxx/
  https://lore.kernel.org/all/20250818131902.5039-1-hdanton@xxxxxxxx/

Hilf Danton sent out a patch, but honestly, that patch looks like pure
bandaid, and will make the exit path horribly much slower by moving
things into workqueues. It might not be visible in profiles exactly
*because* it's then hidden in workqueues, but it's not great.

I think it's a mistake to allow vmalloc'ing those hashes in the first
place, and I suggest the local hash be size-limited to the point where
it's just a kmalloc() and thus works in all contexts.

Or maybe the mistake was the mm-private hashing in the first place.
Maybe that hash shouldn't be allocated at mm_alloc() ->
futex_mm_init() at all. Only initialized by the futex code when
needed, and then dropped in exit_mmap().

So the problems seem deeper than just "free'd in the wrong context".

           Linus