Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount
From: Baolin Wang
Date: Fri Apr 17 2026 - 08:51:37 EST
On 4/17/26 5:52 PM, David Hildenbrand (Arm) wrote:
On 4/17/26 11:27, Baolin Wang wrote:
On 4/17/26 5:21 PM, David Hildenbrand (Arm) wrote:
On 4/17/26 05:25, Baolin Wang wrote:
Currently, when shmem mounts are initialized, they only use 'sbinfo-
huge' todetermine 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);
Two questions from a non-fs person about the semantics here:
a) Can sbinfo->huge be triggered later, for example, through a remount
(staring at shmem_reconfigure())
For tmpfs, yes.
So, we could pass this check here, not setting
mapping_set_large_folios(), but later someone toggles it and we missed
to set mapping_set_large_folios()?
Indeed. Good point.
Or would we always go through another __shmem_get_inode() after a remount?
Not really. There could be files created before remount whose mappings don't support large folios (with 'huge=never' option), while files created after remount will have mappings that support large folios (if remounted with 'huge=always' option).
It looks like the previous commit 5a90c155defa was also problematic. The huge mount option has introduced a lot of tricky issues:(
Now I think Zi's previous suggestion should be able to clean up this mess? That is, calling mapping_set_large_folios() unconditionally for all shmem mounts, and revisiting Kefeng's first version to fix the performance issue.
[1] https://lore.kernel.org/all/20240914140613.2334139-1-wangkefeng.wang@xxxxxxxxxx/