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[46.135.69.142]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-acb3d128c9esm194762866b.108.2025.04.16.15.16.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 16 Apr 2025 15:16:43 -0700 (PDT) From: Mateusz Guzik To: brauner@kernel.org Cc: torvalds@linux-foundation.org, viro@zeniv.linux.org.uk, jack@suse.cz, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Mateusz Guzik Subject: [PATCH 2/2] device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode_permission() Date: Thu, 17 Apr 2025 00:16:26 +0200 Message-ID: <20250416221626.2710239-3-mjguzik@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250416221626.2710239-1-mjguzik@gmail.com> References: <20250416221626.2710239-1-mjguzik@gmail.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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 --- 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;