Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

[Peter Zijlstra]

On Thu, Dec 19, 2024 at 01:18:23PM -0500, Liam R. Howlett wrote:

> > For RCU lookups only the mas tree matters -- and its left present there.
> > 
> > If you really want to block RCU readers, I would suggest punching a hole
> > in the mm_mt. All the traditional code won't notice anyway, this is all
> > with mmap_lock held for writing.
> 
> We don't want to block all rcu readers, we want to block the rcu readers
> that would see the problem - that is, anyone trying to read a particular
> area.
> 
> Right now we can page fault in unpopulated vmas while writing other vmas
> to the tree.  We are also moving more users to rcu reading to use the
> vmas they need without waiting on writes to finish.
> 
> Maybe I don't understand your suggestion, but I would think punching a
> hole would lose this advantage?

My suggestion was to remove the range stuck in mas_detach from mm_mt.
That is exactly the affected range, no?