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

[Peter Zijlstra]

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?

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