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

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Dec 18, 2024 at 8:18 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Dec 18, 2024 at 07:50:34AM -0800, Suren Baghdasaryan wrote:
>
> > I think vma_start_write() should be done inside
> > vms_gather_munmap_vmas() for __mmap_prepare() to work correctly:
> >
> > __mmap_prepare
> >     vms_gather_munmap_vmas
> >     vms_clean_up_area // clears PTEs
> > ...
> > __mmap_complete
> >     vms_complete_munmap_vmas
>
> I'm unsure what exactly you mean; __split_vma() will start_write on the
> one that is broken up and the rest won't actually change until
> vms_complete_munmap_vmas().

Ah, sorry, I missed the write-locking in the __split_vma(). Looks like
indeed vma_start_write() is not needed in vms_gather_munmap_vmas().

>
> > If we do not write-lock the vmas inside vms_gather_munmap_vmas(), we
> > will be clearing PTEs from under a discoverable vma.
>
> You will not. vms_complete_munmap_vmas() will call remove_vma() to
> remove PTEs IIRC, and if you do start_write() and detach() before
> dropping mmap_lock_write, you should be good.

Ok, I think we will have to move mmap_write_downgrade() inside
vms_complete_munmap_vmas() to be called after remove_vma().
vms_clear_ptes() is using vmas, so we can't move remove_vma() before
mmap_write_downgrade().

>
> > There might be other places like this too but I think we can move
> > vma_mark_detach() like you suggested without moving vma_start_write()
> > and that should be enough.
>
> I really don't see why you can't move vma_start_write() -- note that by
> moving that after you've unhooked the vmas from the mm (which you have
> by that point) you get the sync point you wanted.
>
>