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

[John Hubbard <jhubbard@nvidia.com>]

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.

Tested-by: Sourab Gupta <sougupta@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>
---
 mm/gup.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 8e7dc2c6ee73..3ea661e67eea 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2784,12 +2784,17 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
 	mapping = READ_ONCE(folio->mapping);
 
 	/*
-	 * The mapping may have been truncated, in any case we cannot determine
-	 * if this mapping is safe - fall back to slow path to determine how to
-	 * proceed.
+	 * If the mapping is NULL (truncated, or never set), we cannot
+	 * determine whether the folio is file-backed, so a long-term writable
+	 * pin must fall back to the slow path.
+	 *
+	 * Otherwise, a NULL mapping proves this is not a secretmem folio
+	 * (secretmem folios always have a valid mapping to the secretmem
+	 * inode's address_space), so in that case, we can continue with the
+	 * fast path.
 	 */
 	if (!mapping)
-		return false;
+		return !reject_file_backed;
 
 	/* Anonymous folios pose no problem. */
 	mapping_flags = (unsigned long)mapping & FOLIO_MAPPING_FLAGS;

base-commit: 7f87a5ea75f011d2c9bc8ac0167e5e2d1adb1594
-- 
2.53.0