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

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,