linux-kernel - [PATCH v2 0/4] permission check avoidance during lookup

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Message-ID: <20251106180103.923856-1-mjguzik@gmail.com>
Date: Thu,  6 Nov 2025 19:00:58 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org,
	tytso@....edu,
	torvalds@...ux-foundation.org,
	josef@...icpanda.com,
	linux-btrfs@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH v2 0/4] permission check avoidance during lookup

To quote from patch 1:
<quote>
Vast majority of real-world lookups happen on directories which are
traversable by anyone. Figuring out that this holds for a given inode
can be done when instantiating it or changing permissions, avoiding the
overhead during lookup. Stats below.

A simple microbench of stating /usr/include/linux/fs.h on ext4 in a loop
on Sapphire Rapids (ops/s):
before: 3640352
after:  3797258 (+4%)
</quote>

During a kernel build about 90% of all lookups managed to skip
permission checks in my setup, see the commit message for a breakdown.

WARNING: more testing is needed for correctness, but I'm largely happy
with the state as is.

WARNING: I'm assuming the following bit is applied:
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 78ea864fa8cd..eaf776cd4175 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5518,6 +5518,10 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
                goto bad_inode;
        brelse(iloc.bh);
 
+       /* Initialize the "no ACL's" state for the simple cases */
+       if (!ext4_test_inode_state(inode, EXT4_STATE_XATTR) && !ei->i_file_acl)
+               cache_no_acl(inode);
+
        unlock_new_inode(inode);
        return inode;

Lack of the patch does not affect correctness, but it does make the
patch ineffective for ext4. I did not include it in the posting as other
people promised to sort it out.

Discussion is here with an ack from Jan:
https://lore.kernel.org/linux-fsdevel/kn44smk4dgaj5rqmtcfr7ruecixzrik6omur2l2opitn7lbvfm@rm4y24fcfzbz/T/#m30d6cea6be48e95c0d824e98a328fb90c7a5766d
and full thread:
https://lore.kernel.org/linux-fsdevel/kn44smk4dgaj5rqmtcfr7ruecixzrik6omur2l2opitn7lbvfm@rm4y24fcfzbz/T/#t

v2:
- productize
- btrfs and tmpfs support

Mateusz Guzik (4):
  fs: speed up path lookup with cheaper MAY_EXEC checks
  ext4: opt-in for IOP_MAY_FAST_EXEC
  btrfs: opt-in for IOP_MAY_FAST_EXEC
  tmpfs: opt-in for IOP_MAY_FAST_EXEC

 fs/attr.c          |  1 +
 fs/btrfs/inode.c   | 12 +++++-
 fs/ext4/inode.c    |  2 +
 fs/ext4/namei.c    |  1 +
 fs/namei.c         | 95 +++++++++++++++++++++++++++++++++++++++++++++-
 fs/posix_acl.c     |  1 +
 fs/xattr.c         |  1 +
 include/linux/fs.h | 21 +++++++---
 mm/shmem.c         |  9 +++++
 9 files changed, 134 insertions(+), 9 deletions(-)

-- 
2.48.1