Re: [PATCH v4 03/10] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem

[Peter Xu]

Axel,

On Thu, Apr 22, 2021 at 01:22:02PM -0700, Axel Rasmussen wrote:
> > +       if (page_in_cache)
> > +               page_add_file_rmap(page, false);
> > +       else
> > +               page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
> > +
> > +       /*
> > +        * Must happen after rmap, as mm_counter() checks mapping (via
> > +        * PageAnon()), which is set by __page_set_anon_rmap().
> > +        */
> > +       inc_mm_counter(dst_mm, mm_counter(page));
> 
> Actually, I've noticed that this is still slightly incorrect.
> 
> As Hugh pointed out, this works for the anon case, because
> page_add_new_anon_rmap() sets page->mapping.
> 
> But for the page_in_cache case, it doesn't work: unlike its anon
> counterpart, page_add_file_rmap() *does not* set page->mapping.

If it's already in the page cache, shouldn't it be set already in e.g. one
previous call to shmem_add_to_page_cache()?  Thanks,

-- 
Peter Xu