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

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Since commit f002882ca3 ("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: f002882ca3 ("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>
This commit is contained in:
John Hubbard 2026-07-07 17:57:45 -07:00 committed by Andrew Morton
parent 4e1fbffb33
commit c494788faf
1 changed files with 9 additions and 4 deletions
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13
mm/gup.c
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@ -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;