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@xxxxxxxxx>
---
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;
--
2.48.1
