From e631df89cd5d638a9d7c152dd9b0a92643efab3e Mon Sep 17 00:00:00 2001 From: Mateusz Guzik Date: Fri, 7 Nov 2025 15:21:47 +0100 Subject: [PATCH 1/3] fs: speed up path lookup with cheaper handling of MAY_EXEC The generic inode_permission() routine does work which is known to be of no significance for lookup. There are checks for MAY_WRITE, while the requested permission is MAY_EXEC. Additionally devcgroup_inode_permission() is called to check for devices, but it is an invariant the inode is a directory. Absent a ->permission func, execution lands in generic_permission() which checks upfront if the requested permission is granted for everyone. We can elide the branches which are guaranteed to be false and cut straight to the check if everyone happens to be allowed MAY_EXEC on the inode (which holds true most of the time). Moreover, filesystems which provide their own ->permission routine can take advantage of the optimization by setting the IOP_FASTPERM_MAY_EXEC flag on their inodes, which they can legitimately do if their MAY_EXEC handling matches generic_permission(). As a simple benchmark, as part of compilation gcc issues access(2) on numerous long paths, for example /usr/lib/gcc/x86_64-linux-gnu/12/crtendS.o Issuing access(2) on it in a loop on ext4 on Sapphire Rapids (ops/s): before: 3797556 after: 3987789 (+5%) Note: this depends on the not-yet-landed ext4 patch to mark inodes with cache_no_acl() Signed-off-by: Mateusz Guzik Link: https://patch.msgid.link/20251107142149.989998-2-mjguzik@gmail.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner --- fs/namei.c | 43 +++++++++++++++++++++++++++++++++++++++++-- include/linux/fs.h | 13 +++++++------ 2 files changed, 48 insertions(+), 8 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index 1d4d17f24fb2..94cb52b01022 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -540,6 +540,9 @@ static inline int do_inode_permission(struct mnt_idmap *idmap, * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC) * * Separate out file-system wide checks from inode-specific permission checks. + * + * Note: lookup_inode_permission_may_exec() does not call here. If you add + * MAY_EXEC checks, adjust it. */ static int sb_permission(struct super_block *sb, struct inode *inode, int mask) { @@ -602,6 +605,42 @@ int inode_permission(struct mnt_idmap *idmap, } EXPORT_SYMBOL(inode_permission); +/* + * lookup_inode_permission_may_exec - Check traversal right for given inode + * + * This is a special case routine for may_lookup() making assumptions specific + * to path traversal. Use inode_permission() if you are doing something else. + * + * Work is shaved off compared to inode_permission() as follows: + * - we know for a fact there is no MAY_WRITE to worry about + * - it is an invariant the inode is a directory + * + * Since majority of real-world traversal happens on inodes which grant it for + * everyone, we check it upfront and only resort to more expensive work if it + * fails. + * + * Filesystems which have their own ->permission hook and consequently miss out + * on IOP_FASTPERM can still get the optimization if they set IOP_FASTPERM_MAY_EXEC + * on their directory inodes. + */ +static __always_inline int lookup_inode_permission_may_exec(struct mnt_idmap *idmap, + struct inode *inode, int mask) +{ + /* Lookup already checked this to return -ENOTDIR */ + VFS_BUG_ON_INODE(!S_ISDIR(inode->i_mode), inode); + VFS_BUG_ON((mask & ~MAY_NOT_BLOCK) != 0); + + mask |= MAY_EXEC; + + if (unlikely(!(inode->i_opflags & (IOP_FASTPERM | IOP_FASTPERM_MAY_EXEC)))) + return inode_permission(idmap, inode, mask); + + if (unlikely(((inode->i_mode & 0111) != 0111) || !no_acl_inode(inode))) + return inode_permission(idmap, inode, mask); + + return security_inode_permission(inode, mask); +} + /** * path_get - get a reference to a path * @path: path to get the reference to @@ -1855,7 +1894,7 @@ static inline int may_lookup(struct mnt_idmap *idmap, int err, mask; mask = nd->flags & LOOKUP_RCU ? MAY_NOT_BLOCK : 0; - err = inode_permission(idmap, nd->inode, mask | MAY_EXEC); + err = lookup_inode_permission_may_exec(idmap, nd->inode, mask); if (likely(!err)) return 0; @@ -1870,7 +1909,7 @@ static inline int may_lookup(struct mnt_idmap *idmap, if (err != -ECHILD) // hard error return err; - return inode_permission(idmap, nd->inode, MAY_EXEC); + return lookup_inode_permission_may_exec(idmap, nd->inode, 0); } static int reserve_stack(struct nameidata *nd, struct path *link) diff --git a/include/linux/fs.h b/include/linux/fs.h index c895146c1444..ff69734b9fde 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -659,13 +659,14 @@ is_uncached_acl(struct posix_acl *acl) return (long)acl & 1; } -#define IOP_FASTPERM 0x0001 -#define IOP_LOOKUP 0x0002 -#define IOP_NOFOLLOW 0x0004 -#define IOP_XATTR 0x0008 +#define IOP_FASTPERM 0x0001 +#define IOP_LOOKUP 0x0002 +#define IOP_NOFOLLOW 0x0004 +#define IOP_XATTR 0x0008 #define IOP_DEFAULT_READLINK 0x0010 -#define IOP_MGTIME 0x0020 -#define IOP_CACHED_LINK 0x0040 +#define IOP_MGTIME 0x0020 +#define IOP_CACHED_LINK 0x0040 +#define IOP_FASTPERM_MAY_EXEC 0x0080 /* * Inode state bits. Protected by inode->i_lock From 3e18f6256ecc1a1fce1fd8a7707c5daa162bd6b6 Mon Sep 17 00:00:00 2001 From: Mateusz Guzik Date: Fri, 7 Nov 2025 15:21:48 +0100 Subject: [PATCH 2/3] btrfs: utilize IOP_FASTPERM_MAY_EXEC Root filesystem was ext4, btrfs was mounted on /testfs. Then issuing access(2) in a loop on /testfs/repos/linux/include/linux/fs.h on Sapphire Rapids (ops/s): before: 3447976 after: 3620879 (+5%) Signed-off-by: Mateusz Guzik Link: https://patch.msgid.link/20251107142149.989998-3-mjguzik@gmail.com Acked-by: David Sterba Signed-off-by: Christian Brauner --- fs/btrfs/inode.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 3b1b3a0553ee..36c451a9a0bf 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -5837,6 +5837,8 @@ struct btrfs_inode *btrfs_iget(u64 ino, struct btrfs_root *root) if (ret) return ERR_PTR(ret); + if (S_ISDIR(inode->vfs_inode.i_mode)) + inode->vfs_inode.i_opflags |= IOP_FASTPERM_MAY_EXEC; unlock_new_inode(&inode->vfs_inode); return inode; } @@ -6788,8 +6790,11 @@ static int btrfs_create_common(struct inode *dir, struct dentry *dentry, } ret = btrfs_create_new_inode(trans, &new_inode_args); - if (!ret) + if (!ret) { + if (S_ISDIR(inode->i_mode)) + inode->i_opflags |= IOP_FASTPERM_MAY_EXEC; d_instantiate_new(dentry, inode); + } btrfs_end_transaction(trans); btrfs_btree_balance_dirty(fs_info); @@ -9169,6 +9174,11 @@ int btrfs_prealloc_file_range_trans(struct inode *inode, min_size, actual_len, alloc_hint, trans); } +/* + * NOTE: in case you are adding MAY_EXEC check for directories: + * we are marking them with IOP_FASTPERM_MAY_EXEC, allowing path lookup to + * elide calls here. + */ static int btrfs_permission(struct mnt_idmap *idmap, struct inode *inode, int mask) { From a0a28c4e41251a85b4b6db987a72ffbc8613e497 Mon Sep 17 00:00:00 2001 From: Mateusz Guzik Date: Fri, 7 Nov 2025 15:21:49 +0100 Subject: [PATCH 3/3] fs: retire now stale MAY_WRITE predicts in inode_permission() The primary non-MAY_WRITE consumer now uses lookup_inode_permission_may_exec(). Signed-off-by: Mateusz Guzik Link: https://patch.msgid.link/20251107142149.989998-4-mjguzik@gmail.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner --- fs/namei.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index 94cb52b01022..25cc9680e6dc 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -546,7 +546,7 @@ static inline int do_inode_permission(struct mnt_idmap *idmap, */ static int sb_permission(struct super_block *sb, struct inode *inode, int mask) { - if (unlikely(mask & MAY_WRITE)) { + if (mask & MAY_WRITE) { umode_t mode = inode->i_mode; /* Nobody gets write access to a read-only fs. */ @@ -577,7 +577,7 @@ int inode_permission(struct mnt_idmap *idmap, if (unlikely(retval)) return retval; - if (unlikely(mask & MAY_WRITE)) { + if (mask & MAY_WRITE) { /* * Nobody gets write access to an immutable file. */