Re: [RESEND PATCH v3 1/2] fs: Add a pool of extra fs->pwd references to fs_struct

[Paul Moore]

On Thu, Feb 12, 2026 at 1:09 PM Waiman Long <longman@xxxxxxxxxx> 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 <longman@xxxxxxxxxx>
> ---
>  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)
}

-- 
paul-moore.com