Re: [PATCH 3/5] userfualtfd: Replace lru_cache functions with folio_add functions

[Andrew Morton]

On Wed, 2 Nov 2022 15:02:35 -0400 Peter Xu <peterx@xxxxxxxxxx> wrote:

> mfill_atomic_install_pte() checks page->mapping to detect whether one page
> is used in the page cache.  However as pointed out by Matthew, the page can
> logically be a tail page rather than always the head in the case of uffd
> minor mode with UFFDIO_CONTINUE.  It means we could wrongly install one pte
> with shmem thp tail page assuming it's an anonymous page.
> 
> It's not that clear even for anonymous page, since normally anonymous pages
> also have page->mapping being setup with the anon vma. It's safe here only
> because the only such caller to mfill_atomic_install_pte() is always
> passing in a newly allocated page (mcopy_atomic_pte()), whose page->mapping
> is not yet setup.  However that's not extremely obvious either.
> 
> For either of above, use page_mapping() instead.
> 
> And this should be stable material.

I added

Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem")