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

[Peter Zijlstra <peterz@infradead.org>]

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().

> 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.

> 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.