md/raid5-cache: adjust the write position of the empty block if no data blocks

When recovery is complete, we write an empty block and record his
position first, then make the data-only stripes rewritten done,
the location of the empty block as the last checkpoint position
to write into the super block. And we should update last_checkpoint
to this empty block position.

------------------------------------------------------------------
|  old log       | empty block | data only stripes | invalid log |
------------------------------------------------------------------
^                ^                                 ^
|                |- log->last_checkpoint           |- log->log_start
|                |- log->last_cp_seq               |- log->next_checkpoint
|- log->seq=n    |- log->seq=10+n

At the same time, if there is no data-only stripes, this scene may appear,
| meta1 | meta2 | meta3 |
meta 1 is valid, meta 2 is invalid. meta 3 could be valid. so we should
The solution is we create a new meta in meta2 with its seq == meta1's
seq + 10 and let superblock points to meta2.

Signed-off-by: JackieLiu <liuyun01@kylinos.cn>
Reviewed-by: Zhengyuan Liu <liuzhengyuan@kylinos.cn>
Reviewed-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid5-cache.c

@@ -2077,7 +2077,6 @@ r5c_recovery_rewrite_data_only_stripes(struct r5l_log *log,
 		return -ENOMEM;
 	}
 
-	ctx->seq += 10;
 	list_for_each_entry_safe(sh, next, &ctx->cached_list, lru) {
 		struct r5l_meta_block *mb;
 		int i;
@@ -2137,6 +2136,8 @@ static int r5l_recovery_log(struct r5l_log *log)
 	struct mddev *mddev = log->rdev->mddev;
 	struct r5l_recovery_ctx ctx;
 	int ret;
+	sector_t pos;
+	struct stripe_head *sh;
 
 	ctx.pos = log->last_checkpoint;
 	ctx.seq = log->last_cp_seq;
@@ -2154,6 +2155,18 @@ static int r5l_recovery_log(struct r5l_log *log)
 	if (ret)
 		return ret;
 
+       pos = ctx.pos;
+       ctx.seq += 10;
+
+	if (ctx.data_only_stripes == 0) {
+		log->next_checkpoint = ctx.pos;
+		r5l_log_write_empty_meta_block(log, ctx.pos, ctx.seq++);
+		ctx.pos = r5l_ring_add(log, ctx.pos, BLOCK_SECTORS);
+	} else {
+		sh = list_last_entry(&ctx.cached_list, struct stripe_head, lru);
+		log->next_checkpoint = sh->log_start;
+	}
+
 	if ((ctx.data_only_stripes == 0) && (ctx.data_parity_stripes == 0))
 		pr_debug("md/raid:%s: starting from clean shutdown\n",
 			 mdname(mddev));
@@ -2171,10 +2184,9 @@ static int r5l_recovery_log(struct r5l_log *log)
 	}
 
 	log->log_start = ctx.pos;
-	log->next_checkpoint = ctx.pos;
 	log->seq = ctx.seq;
-	r5l_log_write_empty_meta_block(log, ctx.pos, ctx.seq);
-	r5l_write_super(log, ctx.pos);
+	log->last_checkpoint = pos;
+	r5l_write_super(log, pos);
 	return 0;
 }