device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode_permiss...
authorMateusz Guzik <mjguzik@gmail.com>
Wed, 16 Apr 2025 22:16:26 +0000 (00:16 +0200)
committerChristian Brauner <brauner@kernel.org>
Mon, 21 Apr 2025 08:27:59 +0000 (10:27 +0200)
commit4ef4ac360101f8bb11b6486ce60cd60ca015be8c
treebb3e1417fd6fbfeb255e93faa29784efdff821fe
parent875ccc0ddceead3998d9ffd1e68f1290efa1f9a9
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 <inode_permission+0xec>

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 <mjguzik@gmail.com>
Link: https://lore.kernel.org/20250416221626.2710239-3-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
include/linux/device_cgroup.h