As far as the rmap code is concerned, rmap_pages only alters the
page->mapping and page->index. It does it while holding the page
lock. However there are a few places that in presence of anon pages
are allowed to do rmap walks without the page lock (split_huge_page
and page_referenced_anon). Those places that are doing rmap walks
without taking the page lock first, must be updated to re-check that
the page->mapping didn't change after they obtained the anon_vma
lock. remap_pages takes the anon_vma lock for writing before altering
the page->mapping, so if the page->mapping is still the same after
obtaining the anon_vma lock (without the page lock), the rmap walks
can go ahead safely (and remap_pages will wait them to complete before
proceeding).

remap_pages serializes against itself with the page lock.

All other places taking the anon_vma lock while holding the mmap_sem
for writing, don't need to check if the page->mapping has changed
after taking the anon_vma lock, regardless of the page lock, because
remap_pages holds the mmap_sem for reading.

There's one constraint enforced to allow this simplification: the
source pages passed to remap_pages must be mapped only in one vma, but
this is not a limitation when used to handle userland page faults. The
source addresses passed to remap_pages should be set as VM_DONTCOPY
with MADV_DONTFORK to avoid any risk of the mapcount of the pages
increasing, if fork runs in parallel in another thread, before or
while remap_pages runs.