Re: [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm

Kees Cook
2023-11-08T19:25:39Z

On Wed, Nov 08, 2023 at 01:03:33AM +0100, Mateusz Guzik wrote:
> [...]
> >>@[
> >> __pv_queued_spin_lock_slowpath+1
> >> _raw_spin_lock_irq+43
> >> wait_for_completion+141
> >> stop_one_cpu+127
> >> sched_exec+165
> >
> > There's the suspicious sched_exec() I was talking about! :)
> >
> > I think it needs to be moved, and perhaps _later_ instead of earlier?
> > Hmm...
> >
>
> I'm getting around 3.4k execs/s. However, if I "taskset -c 3
> ./static-doexec 1" the number goes up to about 9.5k and lock
> contention disappears from the profile. So off hand looks like the
> task is walking around the box when it perhaps could be avoided -- it
> is idle apart from running the test. Again this is going to require a
> serious look instead of ad hoc pokes.

Hm, that is pretty interesting. I'll see if I can go find the original
rationale for adding sched_exec() in there...

> Side note I actually read your patch this time around instead of
> skimming through it and assuming it did what I thought.
>
> do_filp_open is of course very expensive and kmalloc + kfree are slow.
> On top of it deallocating a file object even after a failed open was
> very expensive due to delegation to task_work (recently fixed).
>
> What I claim should be clear-cut faster is that lookup as in the
> original patch and only messing with file allocation et al if it
> succeeds.

I'm less familiar with the VFS guts here -- I'm open to alternatives! :)

--
Kees Cook