Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

[Suren Baghdasaryan]

On Wed, Jan 15, 2025 at 7:38 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Jan 15, 2025 at 04:35:07PM +0100, Peter Zijlstra wrote:
>
> > Consider:
> >
> >     CPU0                              CPU1
> >
> >     rcu_read_lock();
> >     vma = vma_lookup(mm, vaddr);
> >
> >     ... cpu goes sleep for a *long time* ...
> >
> >                                       __vma_exit_locked();
> >                                       vma_area_free()
> >                                       ..
> >                                       vma = vma_area_alloc();
> >                                       vma_mark_attached();
> >
> >     ... comes back once vma is re-used ...
> >
> >     vma_start_read()
> >       vm_refcount_inc(); // success!!
> >
> > At which point we need to validate vma is for mm and covers vaddr, which
> > is what patch 15 does, no?

Correct. Sorry, I thought by "secondary validation" you only meant
vm_lock_seq check in vma_start_read(). Now I understand your point.
Yes, if the vma we found gets reused before we read-lock it then the
checks for mm and address range should catch a possibly incorrect vma.
If these checks fail, we retry. If they succeed we have the correct
vma even if it was recycled since we found it.

>
> Also, critically, we want these reads to happen *after* the refcount
> increment.

Yes, and I think the acquire fence in the
refcount_add_not_zero_limited() replacement should guarantee that
ordering.

>