ceph: make seeky readdir more efficient

Current cephfs client uses string to indicate start position of
readdir. The string is last entry of previous readdir reply.
This approach does not work for seeky readdir because we can
not easily convert the new postion to a string. For seeky readdir,
mds needs to return dentries from the beginning. Client keeps
retrying if the reply does not contain the dentry it wants.

In current version of ceph, mds sorts CDentry in its cache in
hash order. Client also uses dentry hash to compose dir postion.
For seeky readdir, if client passes the hash part of dir postion
to mds. mds can avoid replying useless dentries.
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/dir.c

@@ -378,7 +378,11 @@ static int ceph_readdir(struct file *file, struct dir_context *ctx)
 				ceph_mdsc_put_request(req);
 				return -ENOMEM;
 			}
+		} else if (is_hash_order(ctx->pos)) {
+			req->r_args.readdir.offset_hash =
+				cpu_to_le32(fpos_hash(ctx->pos));
 		}
+
 		req->r_dir_release_cnt = fi->dir_release_count;
 		req->r_dir_ordered_cnt = fi->dir_ordered_count;
 		req->r_readdir_cache_idx = fi->readdir_cache_idx;

fs/ceph/inode.c

@@ -1482,10 +1482,17 @@ int ceph_readdir_prepopulate(struct ceph_mds_request *req,
 	if (test_bit(CEPH_MDS_R_ABORTED, &req->r_req_flags))
 		return readdir_prepopulate_inodes_only(req, session);
 
-	if (rinfo->hash_order && req->r_path2) {
-		last_hash = ceph_str_hash(ci->i_dir_layout.dl_dir_hash,
-					  req->r_path2, strlen(req->r_path2));
-		last_hash = ceph_frag_value(last_hash);
+	if (rinfo->hash_order) {
+		if (req->r_path2) {
+			last_hash = ceph_str_hash(ci->i_dir_layout.dl_dir_hash,
+						  req->r_path2,
+						  strlen(req->r_path2));
+			last_hash = ceph_frag_value(last_hash);
+		} else if (rinfo->offset_hash) {
+			/* mds understands offset_hash */
+			WARN_ON_ONCE(req->r_readdir_offset != 2);
+			last_hash = le32_to_cpu(rhead->args.readdir.offset_hash);
+		}
 	}
 
 	if (rinfo->dir_dir &&
@@ -1510,7 +1517,7 @@ int ceph_readdir_prepopulate(struct ceph_mds_request *req,
 	}
 
 	if (ceph_frag_is_leftmost(frag) && req->r_readdir_offset == 2 &&
-	    !(rinfo->hash_order && req->r_path2)) {
+	    !(rinfo->hash_order && last_hash)) {
 		/* note dir version at start of readdir so we can tell
 		 * if any dentries get dropped */
 		req->r_dir_release_cnt = atomic64_read(&ci->i_release_count);

fs/ceph/mds_client.c

@@ -189,6 +189,7 @@ static int parse_reply_info_dir(void **p, void *end,
 		info->dir_end = !!(flags & CEPH_READDIR_FRAG_END);
 		info->dir_complete = !!(flags & CEPH_READDIR_FRAG_COMPLETE);
 		info->hash_order = !!(flags & CEPH_READDIR_HASH_ORDER);
+		info->offset_hash = !!(flags & CEPH_READDIR_OFFSET_HASH);
 	}
 	if (num == 0)
 		goto done;

fs/ceph/mds_client.h

@@ -83,9 +83,10 @@ struct ceph_mds_reply_info_parsed {
 			struct ceph_mds_reply_dirfrag *dir_dir;
 			size_t			      dir_buf_size;
 			int                           dir_nr;
-			bool			      dir_complete;
 			bool			      dir_end;
+			bool			      dir_complete;
 			bool			      hash_order;
+			bool			      offset_hash;
 			struct ceph_mds_reply_dir_entry  *dir_entries;
 		};
 

include/linux/ceph/ceph_fs.h

@@ -365,6 +365,7 @@ extern const char *ceph_mds_op_name(int op);
 #define CEPH_READDIR_FRAG_END		(1<<0)
 #define CEPH_READDIR_FRAG_COMPLETE	(1<<8)
 #define CEPH_READDIR_HASH_ORDER		(1<<9)
+#define CEPH_READDIR_OFFSET_HASH	(1<<10)
 
 union ceph_mds_request_args {
 	struct {
@@ -384,6 +385,7 @@ union ceph_mds_request_args {
 		__le32 max_entries;          /* how many dentries to grab */
 		__le32 max_bytes;
 		__le16 flags;
+		__le32 offset_hash;
 	} __attribute__ ((packed)) readdir;
 	struct {
 		__le32 mode;