Re: [PATCH RFC v3 13/13] uprobes: add speculative lockless VMA to inode resolution

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Aug 12, 2024 at 11:18 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> On Mon, Aug 12, 2024 at 09:29:17PM -0700, Andrii Nakryiko wrote:
> > Now that files_cachep is SLAB_TYPESAFE_BY_RCU, we can safely access
> > vma->vm_file->f_inode lockless only under rcu_read_lock() protection,
> > attempting uprobe look up speculatively.
> >
> > We rely on newly added mmap_lock_speculation_{start,end}() helpers to
> > validate that mm_struct stays intact for entire duration of this
> > speculation. If not, we fall back to mmap_lock-protected lookup.
> >
> > This allows to avoid contention on mmap_lock in absolutely majority of
> > cases, nicely improving uprobe/uretprobe scalability.
> >
>
> Here I have to admit to being mostly ignorant about the mm, so bear with
> me. :>
>
> I note the result of find_active_uprobe_speculative is immediately stale
> in face of modifications.
>
> The thing I'm after is that the mmap_lock_speculation business adds
> overhead on archs where a release fence is not a de facto nop and I
> don't believe the commit message justifies it. Definitely a bummer to
> add merely it for uprobes. If there are bigger plans concerning it
> that's a different story of course.
>
> With this in mind I have to ask if instead you could perhaps get away
> with the already present per-vma sequence counter?

per-vma sequence counter does not implement acquire/release logic, it
relies on vma->vm_lock for synchronization. So if we want to use it,
we would have to add additional memory barriers here. This is likely
possible but as I mentioned before we would need to ensure the
pagefault path does not regress. OTOH mm->mm_lock_seq already halfway
there (it implements acquire/release logic), we just had to ensure
mmap_write_lock() increments mm->mm_lock_seq.

So, from the release fence overhead POV I think whether we use
mm->mm_lock_seq or vma->vm_lock, we would still need a proper fence
here.