Re: [PATCH] mm/rmap: remove unnecessary page_table_lock

Yajun Deng
2024-04-23T07:53:28Z

April 22, 2024 at 7:24 PM, "David Hildenbrand" <david@redhat.com> wrote:

>
> On 22.04.24 12:52, Yajun Deng wrote:
>
> >
> > page_table_lock is a lock that for page table, we won't change page
> >
> > table in __anon_vma_prepare(). As we can see, it works well in
> >
> > anon_vma_clone(). They do the same operation.
> >
>
> We are reusing mm->page_table_lock to serialize, not the *actual* low-level page table locks that really protect PTEs.
>
> With that locking gone, there would be nothing protection vma->anon_vma.
>
> Note that anon_vma_clone() is likely called with the mmap_lock held in write mode, which is not the case for __anon_vma_prepare() ...

Yes, anon_vma_clone() is called with the mmap_lock held. I added mmap_assert_write_locked(dst->vm_mm) to prove it.
I added mmap_assert_write_locked(vma->vm_mm) in __anon_vma_prepare() at the same time, it shows __anon_vma_prepare()
is also called with the mmap_lock held too.

>
> I think this change is wrong.
>
> -- Cheers,
>
> David / dhildenb
>