mm/gup: fix GUP-fast fallback for NULL-mapping order-0 folios

Since commit f002882ca369 ("mm: merge folio_is_secretmem() and
folio_fast_pin_allowed() into gup_fast_folio_allowed()"),
gup_fast_folio_allowed() falls back to the slow path for any order-0 folio
with a NULL mapping when CONFIG_SECRETMEM=y.  This causes a performance
regression for drivers that allocate pages with alloc_page() and insert
them into VMAs via vm_insert_page().  These pages legitimately have a NULL
folio->mapping, but they cannot be secretmem pages.

Secretmem pages are always added to the secretmem inode's page cache via
filemap_add_folio(), which sets folio->mapping to the inode's i_mapping. 
A folio with a NULL mapping can never be a secretmem folio.  The
NULL-mapping check was intended to handle truncated file-backed pages (a
reject_file_backed concern), not secretmem detection.

When only check_secretmem is true (and reject_file_backed is false), a
NULL mapping is sufficient to prove the folio is not secretmem, so the
fast path can proceed.

Link: https://lore.kernel.org/20260708005745.164928-1-jhubbard@nvidia.com
Fixes: f002882ca369 ("mm: merge folio_is_secretmem() and folio_fast_pin_allowed() into gup_fast_folio_allowed()")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Tested-by: Sourab Gupta <sougupta@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions