Re: [PATCH v2 1/2] ceph/mdsc: Move CEPH_CAP_PIN reference when r_parent is updated

[Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>]

On Thu, 2025-09-04 at 10:11 +0000, Alex Markuze wrote:
> When the parent directory lock is not held, req->r_parent can become stale and is updated to point to the correct inode.
> However, the associated CEPH_CAP_PIN reference was not being adjusted.
> The CEPH_CAP_PIN is a reference on an inode that is tracked for accounting purposes.
> Moving this pin is important to keep the accounting balanced. When the pin was not moved from the old parent to the new one, it created two problems:
> The reference on the old, stale parent was never released, causing a reference leak.
> A reference for the new parent was never acquired, creating the risk of a reference underflow later in ceph_mdsc_release_request().
> This patch corrects the logic by releasing the pin from the old parent and acquiring it for the new parent when r_parent is switched.
> This ensures reference accounting stays balanced.

This explanation looks much cleaner. :)

Thanks,
Slava.

> 
> Signed-off-by: Alex Markuze <amarkuze@redhat.com>
> Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
> ---
>  fs/ceph/mds_client.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index ce0c129f4651..4e5926f36e8d 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -3053,12 +3053,19 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
>  	 */
>  	if (!parent_locked && req->r_parent && path_info1.vino.ino &&
>  	    ceph_ino(req->r_parent) != path_info1.vino.ino) {
> +		struct inode *old_parent = req->r_parent;
>  		struct inode *correct_dir = ceph_get_inode(mdsc->fsc->sb, path_info1.vino, NULL);
>  		if (!IS_ERR(correct_dir)) {
>  			WARN_ONCE(1, "ceph: r_parent mismatch (had %llx wanted %llx) - updating\n",
> -				  ceph_ino(req->r_parent), path_info1.vino.ino);
> -			iput(req->r_parent);
> +			          ceph_ino(old_parent), path_info1.vino.ino);
> +			/*
> +			 * Transfer CEPH_CAP_PIN from the old parent to the new one.
> +			 * The pin was taken earlier in ceph_mdsc_submit_request().
> +			 */
> +			ceph_put_cap_refs(ceph_inode(old_parent), CEPH_CAP_PIN);
> +			iput(old_parent);
>  			req->r_parent = correct_dir;
> +			ceph_get_cap_refs(ceph_inode(req->r_parent), CEPH_CAP_PIN);
>  		}
>  	}
>