Re: [PATCH v6 13/16] mm: introduce vma_ensure_detached()

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Dec 17, 2024 at 2:26 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Dec 16, 2024 at 11:24:16AM -0800, Suren Baghdasaryan wrote:
> > vma_start_read() can temporarily raise vm_refcnt of a write-locked and
> > detached vma:
> >
> > // vm_refcnt==1 (attached)
> > vma_start_write()
> >     vma->vm_lock_seq = mm->mm_lock_seq
> >
> >                     vma_start_read()
> >                        vm_refcnt++; // vm_refcnt==2
> >
> > vma_mark_detached()
> >     vm_refcnt--; // vm_refcnt==1
> >
> > // vma is detached but vm_refcnt!=0 temporarily
> >
> >                        if (vma->vm_lock_seq == mm->mm_lock_seq)
> >                            vma_refcount_put()
> >                                vm_refcnt--; // vm_refcnt==0
> >
> > This is currently not a problem when freeing the vma because RCU grace
> > period should pass before kmem_cache_free(vma) gets called and by that
> > time vma_start_read() should be done and vm_refcnt is 0. However once
> > we introduce possibility of vma reuse before RCU grace period is over,
> > this will become a problem (reused vma might be in non-detached state).
> > Introduce vma_ensure_detached() for the writer to wait for readers until
> > they exit vma_start_read().
>
> So aside from the lockdep problem (which I think is fixable), the normal
> way to fix the above is to make dec_and_test() do the kmem_cache_free().
>
> Then the last user does the free and everything just works.

I see your point. Let me reply in the other patch where you have more
comments about this.