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device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode_permission()

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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>
This commit is contained in:
Mateusz Guzik 2025-04-17 00:16:26 +02:00 • committed by Christian Brauner
parent 875ccc0ddc
commit 4ef4ac3601
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1 changed files with 4 additions and 3 deletions
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7
include/linux/device_cgroup.h
View file

@ -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;