[PATCH v3] mm: shmem: always support large folios for internal shmem mount

[Baolin Wang]

Currently, when shmem mounts are initialized, they only use 'sbinfo->huge' to
determine whether the shmem mount supports large folios. However, for anonymous
shmem, whether it supports large folios can be dynamically configured via sysfs
interfaces, so setting or not setting mapping_set_large_folios() during initialization
cannot accurately reflect whether anonymous shmem actually supports large folios,
which has already caused some confusion[1].

As discussed with David[2], for anonymous shmem we can treat it as always potentially
having large folios. Therefore, always support large folios for the internal shmem
mount (e.g., anonymous shmem), and which large order allocations are allowed can be
configured dynamically via the 'shmem_enabled' interfaces.

[1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@xxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/all/875dc63b-0cd2-49e5-8b0d-3fb062789813@xxxxxxxxxx/
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
Changes from v2:
 - Always support large folios for internal shmem mount, per David.
Changes from v1:
 - Update the comments and commit message, per Lance.
---
 mm/shmem.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 4ecefe02881d..769ef37b1ea9 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)
+	/*
+	 * Always support large folios for the internal shmem mount (e.g.,
+	 * anonymous shmem), and which large order allocations are allowed
+	 * can be configured dynamically via the 'shmem_enabled' interfaces.
+	 *
+	 * For tmpfs, honour the 'huge=' mount option to determine whether
+	 * large folios are supported.
+	 *
+	 * 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