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

[Waiman Long <longman@redhat.com>]

On 2/19/26 5:20 PM, Paul Moore wrote:
> On Thu, Feb 12, 2026 at 1:09 PM Waiman Long <longman@redhat.com> 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@redhat.com>
>> ---
>>   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