[PATCH v2] mm: shmem: don't set large-order range for internal shmem mount

[Baolin Wang]

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 shmem mount (e.g. anonymous shmem), to avoid confusion,
as discussed in the previous thread[1].

[1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@xxxxxxxxxxxxxxxxx/
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
Changes from v1:
 - Update the comments and commit message, per Lance.
---
 mm/shmem.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 4ecefe02881d..568e1baee90d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3088,8 +3088,16 @@ 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 shmem mount is configured dynamically
+	 * via the 'shmem_enabled' interfaces, so there is no need to set a
+	 * large order range for the internal shmem mount's 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