On 2/19/26 5:20 PM, Paul Moore wrote:
On Thu, Feb 12, 2026 at 1:09 PM Waiman Long <[email protected]> wrote:
When the audit subsystem is enabled, it can do a lot of get_fs_pwd()
calls to get references to fs->pwd and then releasing those references
back with path_put() later. That may cause a lot of spinlock contention
on a single pwd's dentry lock because of the constant changes to the
reference count when there are many processes on the same working
directory actively doing open/close system calls. This can cause
noticeable performance regresssion when compared with the case where
the audit subsystem is turned off especially on systems with a lot of
CPUs which is becoming more common these days.
A simple and elegant solution to avoid this kind of performance
regression is to add a common pool of extra fs->pwd references inside
the fs_struct. When a caller needs a pwd reference, it can borrow one
from pool, if available, to avoid an explicit path_get(). When it is
time to release the reference, it can put it back into the common pool
if fs->pwd isn't changed before without doing a path_put(). We still
need to acquire the fs's spinlock, but fs_struct is more distributed
and it is less common to have many tasks sharing a single fs_struct.
A new set of get_fs_pwd_pool/put_fs_pwd_pool() APIs are introduced
with this patch to enable other subsystems to acquire and release
a pwd reference from the common pool without doing unnecessary
path_get/path_put().
Besides fs/fs_struct.c, the copy_mnt_ns() function of fs/namespace.c is
also modified to properly handle the extra pwd references, if available.
Signed-off-by: Waiman Long <[email protected]>
---
fs/fs_struct.c | 26 +++++++++++++++++++++-----
fs/namespace.c | 8 ++++++++
include/linux/fs_struct.h | 30 +++++++++++++++++++++++++++++-
3 files changed, 58 insertions(+), 6 deletions(-)
...
diff --git a/fs/namespace.c b/fs/namespace.c
index c58674a20cad..a2323ba84d76 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -40,6 +41,33 @@ static inline void get_fs_pwd(struct fs_struct *fs, struct
path *pwd)
read_sequnlock_excl(&fs->seq);
}
+/* Acquire a pwd reference from the pwd_refs pool, if available */
+static inline void get_fs_pwd_pool(struct fs_struct *fs, struct path *pwd)
+{
+ read_seqlock_excl(&fs->seq);
+ *pwd = fs->pwd;
+ if (fs->pwd_refs)
+ fs->pwd_refs--;
+ else
+ path_get(pwd);
+ read_sequnlock_excl(&fs->seq);
+}
+
+/* Release a pwd reference back to the pwd_refs pool, if appropriate */
+static inline void put_fs_pwd_pool(struct fs_struct *fs, struct path *pwd)
+{
+ bool put = false;
+
+ read_seqlock_excl(&fs->seq);
+ if ((fs->pwd.dentry == pwd->dentry) && (fs->pwd.mnt == pwd->mnt))
+ fs->pwd_refs++;
+ else
+ put = true;
+ read_sequnlock_excl(&fs->seq);
+ if (put)
+ path_put(pwd);
+}
This is a nitpick, and perhaps I'm missing something, but I think you
could skip the local 'put' boolean by setting 'pwd' to NULL in the
pool case, e.g.
static inline void put_fs_pwd_pool(fs, pwd)
{
read_seqlock_excl(&fs)
if (fs == pwd) {
fs->pwd_refs++
pwd = NULL
}
read_sequnlock_excl(&fs)
if (pwd)
path_put(pwd)
}
Thanks for the suggestion. It does make the function simpler. I have
included that change in my v4 patch.
Al & Christian, are you OK with taking these patches for the next v7.1
release or do you have other suggested changes you would like to see?
Thanks,
Longman
