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

[Mateusz Guzik <mjguzik@gmail.com>]

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?