mm: wire up tail page poisoning over ->mappings

Tail pages have a sanity check on ->mapping fields, not all of them but
only upon index>2, for now.  It's because we reused ->mapping fields of
the tail pages index=1,2 for other things.

Define a macro for "max index of tail pages that got ->mapping field
reused" on top of folio definition, because when we grow folio tail pages
we'd want to boost this too together.

Then wire everything up using that macro.

Don't try to poison the ->mapping field in prep_compound_tail() for tail
pages <=TAIL_MAPPING_REUSED_MAX because it's wrong.  For example, the 1st
tail page already reused ->mapping field as _nr_pages_mapped.  It didn't
already blow up only because we luckily always prepare tail pages before
preparing the head, then prep_compound_head() will update
folio->_nr_pages_mapped so as to void the poisoning.  This should make it
always safe again, even e.g.  if we prep the head first.

Clean up free_tail_page_prepare() along the way on checking ->mapping
poisoning to also leverage the new macro.

Link: https://lkml.kernel.org/r/20230815210659.430010-1-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>