Re: [PATCH v2] audit: Avoid excessive dput/dget in audit_context setup and reset paths

On 2/3/26 6:26 PM, Al Viro wrote:
> On Tue, Feb 03, 2026 at 09:50:02PM +0000, Al Viro wrote:
>> On Tue, Feb 03, 2026 at 03:32:04PM -0500, Waiman Long wrote:
>>
>>> That is actually a concern that I have at the back of my mind. I can modify
>>> the patch to cache only the dentry and do get/put the mount every time which
>>> is much cheaper as it is a percpu counter.  In that way, a chdir(2) followed
>>> by a umount(2) shouldn't cause a -EBUSY. Right?
>> Quite - it will just retain a reference to dentry, with filesystem shutdown
>> being very unhappy about somebody retaining references to objects on the
>> filesystem about to be taken out...
> Sarcasm aside, I wonder if we could do the following trick:
> 	* a new primitive for "grab or borrow pwd", similar to what fdget()
> does for struct file.  If current->fs is shared, do what we do now and return
> true; otherwise just copy the contents of current->fs->pwd return false.
> 	* paired primitive that would take a boolean + struct path * and
> do path_put() if boolean is true.
> 	* syscalls that might alter ->fs, ->fs->pwd or add extra references to
> ->fs would start with grabbing an extra ref on entry and drop it in the end;
> that would make that primitive safe to use there.
> 	* audit using that thing and storing the result along with the copy
> of pwd; on the way out it would use the "put unless borrowed" primitive.
>
> Might or might not be useful - hard to tell without knowing the job mix of those
> audit-afflicted production systems.
>
> I'll try to put something along those lines together...

Interesting. So are you thinking about a reference on the pwd inside the 
fs_struct? I am thinking about instead of getting a reference to 
pwd.dentry and pwd.mnt, we can just get a reference to the pwd itself. 
We have to grab the fs lock in get_fs_pwd() anyway. This spinlock 
doesn't show up as being contended in the customer bug report as each 
process can has its own fs_struct instead of many processing sharing the 
same working directory.

In the case of audit, fs->pwd is copied out at the beginning of the 
open* system call and then put back at the end of that system call. So 
it should last a pretty short time, i.e. the reference will be released 
shortly. However, that will make the set_fs_pwd() call a bit tricky to 
implement, but I think it is still doable.

Cheers,
Longman