Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks

[Matthew Wilcox <willy@infradead.org>]

On Sat, Apr 13, 2024 at 02:46:56PM -0700, Suren Baghdasaryan wrote:
> On Fri, Apr 12, 2024 at 8:31 AM Matthew Wilcox <willy@infradead.org> wrote:
> >  - Rename lock_vma() to uffd_lock_vma() because it really is uffd
> >    specific.
> 
> I'm planning to expand the scope of lock_vma() and reuse it for
> /proc/pid/maps reading under per-VMA locks. No objection to renaming
> it for now but I'll likely rename it back later once it's used in more
> places.

That would seem like a mistake.  The uffd lock_vma() will create an
anon_vma for VMAs that don't have one, and you wouldn't want that.
It seems to me that lock_vma_under_rcu() does everything you want except
the fallback to mmap_read_lock().  And I'm not sure there's a good way
to package that up ... indeed, I don't see why you'd want the "take
the mmap_lock, look up the VMA, drop the mmap read lock" part at all --
once you've got the mmap_lock, just hold it until you're done.