[PATCH] mm: shmem: don't set large-order range for internal anonymous shmem mapping

Baolin Wang
2026-04-07T06:07:53Z

Anonymous shmem large order allocations are dynamically controlled via the
global THP sysfs knob (/sys/kernel/mm/transparent_hugepage/shmem_enabled)
and the per-size mTHP knobs (/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled).

Therefore, anonymous shmem uses shmem_allowable_huge_orders() to check
which large orders are allowed, rather than relying on mapping_max_folio_order().
Moreover, mapping_max_folio_order() is intended to control large order
allocations only for tmpfs mounts. Clarify this by not setting a large-order
range for internal anonymous shmem mappings, to avoid confusion, as discussed
in the previous thread[1].

[1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@linux.alibaba.com/
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
mm/shmem.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 4ecefe02881d..a60fe067969c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3088,8 +3088,17 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
if (sbinfo->noswap)
mapping_set_unevictable(inode->i_mapping);

- /* Don't consider 'deny' for emergencies and 'force' for testing */
- if (sbinfo->huge)
+ /*
+ * Only set the large order range for tmpfs mounts. The large order
+ * selection for the internal anonymous shmem mount is configured
+ * dynamically via the 'shmem_enabled' interfaces, so there is no
+ * need to set a large order range for the internal anonymous shmem
+ * mapping.
+ *
+ * Note: Don't consider 'deny' for emergencies and 'force' for
+ * testing.
+ */
+ if (sbinfo->huge && !(sb->s_flags & SB_KERNMOUNT))
mapping_set_large_folios(inode->i_mapping);

switch (mode & S_IFMT) {
--
2.47.3