We currently always chase a pointer inode->i_sb->s_user_ns whenever we need to map a uid/gid. Linus reported in [1] that this is noticable during path lookup. In the majority of cases we don't need to bother with that pointer chase because the inode won't be located on a filesystem that's mounted in a user namespace. The user namespace of the superblock cannot ever change once it's mounted. So introduce and raise IOP_USERNS on all inodes and check for that flag in i_user_ns() when we retrieve the user namespace. Additionally, we now inline all low-level idmapping helpers. [1]: https://lore.kernel.org/CAHk-=whJgRDtxTudTQ9HV8BFw5-bBsu+c8Ouwd_PrPqPB6_KEQ@xxxxxxxxxxxxxx Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> --- Christian Brauner (3): inode: add fastpath for filesystem user namespace retrieval mnt_idmapping: add struct mnt_idmap to header mnt_idmapping: inline all low-level helpers fs/inode.c | 6 ++ fs/mnt_idmapping.c | 165 ---------------------------------------- include/linux/fs.h | 5 +- include/linux/mnt_idmapping.h | 168 +++++++++++++++++++++++++++++++++++++++-- include/linux/uidgid.h | 23 +++++- include/linux/user_namespace.h | 23 +----- kernel/user_namespace.c | 2 + 7 files changed, 195 insertions(+), 197 deletions(-) --- base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8 change-id: 20250416-work-mnt_idmap-s_user_ns-eb57ee83e1d6
