mm/memory: move page_count() check into validate_page_before_insert()

mm/memory: move page_count() check into validate_page_before_insert()

Patch series "mm/memory: cleanly support zeropage in vm_insert_page*(),
vm_map_pages*() and vmf_insert_mixed()", v2.

There is interest in mapping zeropages via vm_insert_pages() [1] into
MAP_SHARED mappings.

For now, we only get zeropages in MAP_SHARED mappings via
vmf_insert_mixed() from FSDAX code, and I think it's a bit shaky in some
cases because we refcount the zeropage when mapping it but not necessarily
always when unmapping it ...  and we should actually never refcount it.

It's all a bit tricky, especially how zeropages in MAP_SHARED mappings
interact with GUP (FOLL_LONGTERM), mprotect(), write-faults and s390x
forbidding the shared zeropage (rewrite [2] s now upstream).

This series tries to take the careful approach of only allowing the
zeropage where it is likely safe to use (which should cover the existing
FSDAX use case and [1]), preventing that it could accidentally get mapped
writable during a write fault, mprotect() etc, and preventing issues with
FOLL_LONGTERM in the future with other users.

Tested with a patch from Vincent that uses the zeropage in context of
[1].

[1] https://lkml.kernel.org/r/20240430111354.637356-1-vdonnefort@google.com
[2] https://lkml.kernel.org/r/20240411161441.910170-1-david@redhat.com

This patch (of 3):

We'll now also cover the case where insert_page() is called from
__vm_insert_mixed(), which sounds like the right thing to do.

Link: https://lkml.kernel.org/r/20240522125713.775114-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>