linux-kernel - [PATCH 2/2] device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode

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Message-ID: <20250416221626.2710239-3-mjguzik@gmail.com>
Date: Thu, 17 Apr 2025 00:16:26 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: torvalds@...ux-foundation.org,
	viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH 2/2] 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@...il.com>
---
 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