From 4ef4ac360101f8bb11b6486ce60cd60ca015be8c Mon Sep 17 00:00:00 2001 From: Mateusz Guzik Date: Thu, 17 Apr 2025 00:16:26 +0200 Subject: [PATCH] device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode_permission() The routine gets called for every path component during lookup. ->i_mode is going to be cached on account of permission checks, while ->i_rdev is an area which is most likely cache-cold. gcc 14.2 is kind enough to emit one branch: movzwl (%rbx),%eax mov %eax,%edx and $0xb000,%dx cmp $0x2000,%dx je 11bc This patch is lazy in that I don't know if the ->i_rdev branch makes any sense with the newly added mode check upfront. I am not changing any semantics here though. Signed-off-by: Mateusz Guzik Link: https://lore.kernel.org/20250416221626.2710239-3-mjguzik@gmail.com Signed-off-by: Christian Brauner --- include/linux/device_cgroup.h | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/include/linux/device_cgroup.h b/include/linux/device_cgroup.h index d02f32b7514e..0864773a57e8 100644 --- a/include/linux/device_cgroup.h +++ b/include/linux/device_cgroup.h @@ -18,15 +18,16 @@ static inline int devcgroup_inode_permission(struct inode *inode, int mask) { short type, access = 0; + if (likely(!S_ISBLK(inode->i_mode) && !S_ISCHR(inode->i_mode))) + return 0; + if (likely(!inode->i_rdev)) return 0; if (S_ISBLK(inode->i_mode)) type = DEVCG_DEV_BLOCK; - else if (S_ISCHR(inode->i_mode)) + else /* S_ISCHR by the test above */ type = DEVCG_DEV_CHAR; - else - return 0; if (mask & MAY_WRITE) access |= DEVCG_ACC_WRITE; -- 2.50.1