Re: [PATCH v2] ceph: don't use d_add in ceph_handle_snapdir

[Xiubo Li <xiubli@redhat.com>]

On 2021/3/17 4:39, Jeff Layton wrote:
> It's possible ceph_get_snapdir could end up finding a (disconnected)
> inode that already exists in the cache. Change the prototype for
> ceph_handle_snapdir to return a dentry pointer and have it use
> d_splice_alias so we don't end up with an aliased dentry in the cache.
>
> URL: https://tracker.ceph.com/issues/49843
> Reported-by: Al Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>   fs/ceph/dir.c   | 32 ++++++++++++++++++++------------
>   fs/ceph/file.c  |  7 +++++--
>   fs/ceph/super.h |  2 +-
>   3 files changed, 26 insertions(+), 15 deletions(-)
>
> v2:
>      zero out err var when ceph_handle_snapdir returns success
>
> diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
> index 113f669d71dd..570662dec3fe 100644
> --- a/fs/ceph/dir.c
> +++ b/fs/ceph/dir.c
> @@ -667,8 +667,8 @@ static loff_t ceph_dir_llseek(struct file *file, loff_t offset, int whence)
>   /*
>    * Handle lookups for the hidden .snap directory.
>    */
> -int ceph_handle_snapdir(struct ceph_mds_request *req,
> -			struct dentry *dentry, int err)
> +struct dentry *ceph_handle_snapdir(struct ceph_mds_request *req,
> +				   struct dentry *dentry, int err)
>   {
>   	struct ceph_fs_client *fsc = ceph_sb_to_client(dentry->d_sb);
>   	struct inode *parent = d_inode(dentry->d_parent); /* we hold i_mutex */
> @@ -676,18 +676,19 @@ int ceph_handle_snapdir(struct ceph_mds_request *req,
>   	/* .snap dir? */
>   	if (err == -ENOENT &&
>   	    ceph_snap(parent) == CEPH_NOSNAP &&
> -	    strcmp(dentry->d_name.name,
> -		   fsc->mount_options->snapdir_name) == 0) {
> +	    strcmp(dentry->d_name.name, fsc->mount_options->snapdir_name) == 0) {
> +		struct dentry *res;
>   		struct inode *inode = ceph_get_snapdir(parent);
> +
>   		if (IS_ERR(inode))
> -			return PTR_ERR(inode);
> -		dout("ENOENT on snapdir %p '%pd', linking to snapdir %p\n",
> -		     dentry, dentry, inode);
> -		BUG_ON(!d_unhashed(dentry));
> -		d_add(dentry, inode);
> -		err = 0;
> +			return ERR_CAST(inode);
> +		res = d_splice_alias(inode, dentry);
> +		dout("ENOENT on snapdir %p '%pd', linking to snapdir %p. Spliced dentry %p\n",
> +		     dentry, dentry, inode, res);
> +		if (res)
> +			dentry = res;
>   	}
> -	return err;
> +	return dentry;
>   }
>   
>   /*
> @@ -743,6 +744,7 @@ static struct dentry *ceph_lookup(struct inode *dir, struct dentry *dentry,
>   	struct ceph_fs_client *fsc = ceph_sb_to_client(dir->i_sb);
>   	struct ceph_mds_client *mdsc = ceph_sb_to_mdsc(dir->i_sb);
>   	struct ceph_mds_request *req;
> +	struct dentry *res;
>   	int op;
>   	int mask;
>   	int err;
> @@ -793,7 +795,13 @@ static struct dentry *ceph_lookup(struct inode *dir, struct dentry *dentry,
>   	req->r_parent = dir;
>   	set_bit(CEPH_MDS_R_PARENT_LOCKED, &req->r_req_flags);
>   	err = ceph_mdsc_do_request(mdsc, NULL, req);
> -	err = ceph_handle_snapdir(req, dentry, err);
> +	res = ceph_handle_snapdir(req, dentry, err);
> +	if (IS_ERR(res)) {
> +		err = PTR_ERR(res);
> +	} else {
> +		dentry = res;
> +		err = 0;
> +	}
>   	dentry = ceph_finish_lookup(req, dentry, err);
>   	ceph_mdsc_put_request(req);  /* will dput(dentry) */
>   	dout("lookup result=%p\n", dentry);
> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> index 209535d5b8d3..a6ef1d143308 100644
> --- a/fs/ceph/file.c
> +++ b/fs/ceph/file.c
> @@ -739,9 +739,12 @@ int ceph_atomic_open(struct inode *dir, struct dentry *dentry,
>   	err = ceph_mdsc_do_request(mdsc,
>   				   (flags & (O_CREAT|O_TRUNC)) ? dir : NULL,
>   				   req);
> -	err = ceph_handle_snapdir(req, dentry, err);
> -	if (err)
> +	dentry = ceph_handle_snapdir(req, dentry, err);
> +	if (IS_ERR(dentry)) {
> +		err = PTR_ERR(dentry);
>   		goto out_req;
> +	}
> +	err = 0;
>   
>   	if ((flags & O_CREAT) && !req->r_reply_info.head->is_dentry)
>   		err = ceph_handle_notrace_create(dir, dentry);
> diff --git a/fs/ceph/super.h b/fs/ceph/super.h
> index 188565d806b2..07a3fb52ae30 100644
> --- a/fs/ceph/super.h
> +++ b/fs/ceph/super.h
> @@ -1193,7 +1193,7 @@ extern const struct dentry_operations ceph_dentry_ops;
>   
>   extern loff_t ceph_make_fpos(unsigned high, unsigned off, bool hash_order);
>   extern int ceph_handle_notrace_create(struct inode *dir, struct dentry *dentry);
> -extern int ceph_handle_snapdir(struct ceph_mds_request *req,
> +extern struct dentry *ceph_handle_snapdir(struct ceph_mds_request *req,
>   			       struct dentry *dentry, int err);
>   extern struct dentry *ceph_finish_lookup(struct ceph_mds_request *req,
>   					 struct dentry *dentry, int err);

LGTM.

Reviewed-by: Xiubo Li <xiubli@redhat.com>