[PATCH v12 04/16] mm/gup: drop secretmem optimization from gup_fast_folio_allowed

Kalyazin, Nikita
2026-04-10T15:18:32Z

This drops an optimization in gup_fast_folio_allowed() where
secretmem_mapping() was only called if CONFIG_SECRETMEM=y. secretmem is
enabled by default since commit b758fe6df50d ("mm/secretmem: make it on
by default"), so the secretmem check did not actually end up elided in
most cases anymore anyway.

To make sure the fast path for ZONE_DEVICE pages (like Device DAX and
PCI P2PDMA) is still allowed, check for folio_is_zone_device() if
mapping is NULL.

This is in preparation of the generalization of handling mappings where
direct map entries of folios are set to not present. Currently,
mappings that match this description are secretmem mappings
(memfd_secret()). Later, some guest_memfd configurations will also fall
into this category.

Signed-off-by: Patrick Roy <patrick.roy@linux.dev>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Nikita Kalyazin <nikita.kalyazin@linux.dev>
---
mm/gup.c | 17 ++++++-----------
1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 8e7dc2c6ee73..e8367564d636 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2739,7 +2739,6 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
{
bool reject_file_backed = false;
struct address_space *mapping;
- bool check_secretmem = false;
unsigned long mapping_flags;

/*
@@ -2751,14 +2750,6 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
reject_file_backed = true;

/* We hold a folio reference, so we can safely access folio fields. */
-
- /* secretmem folios are always order-0 folios. */
- if (IS_ENABLED(CONFIG_SECRETMEM) && !folio_test_large(folio))
- check_secretmem = true;
-
- if (!reject_file_backed && !check_secretmem)
- return true;
-
if (WARN_ON_ONCE(folio_test_slab(folio)))
return false;

@@ -2787,9 +2778,13 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
* 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.
+ *
+ * ZONE_DEVICE folios (e.g. Device DAX, PCI P2PDMA) may legitimately
+ * have a NULL mapping. They are never secretmem/no-direct-map folios,
+ * so let them through.
*/
if (!mapping)
- return false;
+ return folio_is_zone_device(folio);

/* Anonymous folios pose no problem. */
mapping_flags = (unsigned long)mapping & FOLIO_MAPPING_FLAGS;
@@ -2800,7 +2795,7 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
* At this point, we know the mapping is non-null and points to an
* address_space object.
*/
- if (check_secretmem && secretmem_mapping(mapping))
+ if (secretmem_mapping(mapping))
return false;
/* The only remaining allowed file system is shmem. */
return !reject_file_backed || shmem_mapping(mapping);
--
2.50.1