dm log writes: make sure super sector log updates are written in order

Currently, although we submit super bios in order (and super.nr_entries
is incremented by each logged entry), submit_bio() is async so each
super sector may not be written to log device in order and then the
final nr_entries may be smaller than it should be.

This problem can be reproduced by the xfstests generic/455 with ext4:

  QA output created by 455
 -Silence is golden
 +mark 'end' does not exist

Fix this by serializing submission of super sectors to make sure each
is written to the log disk in order.

Fixes: 0e9cebe ("dm: add log writes target")
Cc: stable@vger.kernel.org
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Suggested-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-log-writes.c

@@ -60,6 +60,7 @@
 
 #define WRITE_LOG_VERSION 1ULL
 #define WRITE_LOG_MAGIC 0x6a736677736872ULL
+#define WRITE_LOG_SUPER_SECTOR 0
 
 /*
  * The disk format for this is braindead simple.
@@ -115,6 +116,7 @@ struct log_writes_c {
 	struct list_head logging_blocks;
 	wait_queue_head_t wait;
 	struct task_struct *log_kthread;
+	struct completion super_done;
 };
 
 struct pending_block {
@@ -180,6 +182,14 @@ static void log_end_io(struct bio *bio)
 	bio_put(bio);
 }
 
+static void log_end_super(struct bio *bio)
+{
+	struct log_writes_c *lc = bio->bi_private;
+
+	complete(&lc->super_done);
+	log_end_io(bio);
+}
+
 /*
  * Meant to be called if there is an error, it will free all the pages
  * associated with the block.
@@ -215,7 +225,8 @@ static int write_metadata(struct log_writes_c *lc, void *entry,
 	bio->bi_iter.bi_size = 0;
 	bio->bi_iter.bi_sector = sector;
 	bio_set_dev(bio, lc->logdev->bdev);
-	bio->bi_end_io = log_end_io;
+	bio->bi_end_io = (sector == WRITE_LOG_SUPER_SECTOR) ?
+			  log_end_super : log_end_io;
 	bio->bi_private = lc;
 	bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
 
@@ -418,11 +429,18 @@ static int log_super(struct log_writes_c *lc)
 	super.nr_entries = cpu_to_le64(lc->logged_entries);
 	super.sectorsize = cpu_to_le32(lc->sectorsize);
 
-	if (write_metadata(lc, &super, sizeof(super), NULL, 0, 0)) {
+	if (write_metadata(lc, &super, sizeof(super), NULL, 0,
+			   WRITE_LOG_SUPER_SECTOR)) {
 		DMERR("Couldn't write super");
 		return -1;
 	}
 
+	/*
+	 * Super sector should be writen in-order, otherwise the
+	 * nr_entries could be rewritten incorrectly by an old bio.
+	 */
+	wait_for_completion_io(&lc->super_done);
+
 	return 0;
 }
 
@@ -531,6 +549,7 @@ static int log_writes_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	INIT_LIST_HEAD(&lc->unflushed_blocks);
 	INIT_LIST_HEAD(&lc->logging_blocks);
 	init_waitqueue_head(&lc->wait);
+	init_completion(&lc->super_done);
 	atomic_set(&lc->io_blocks, 0);
 	atomic_set(&lc->pending_blocks, 0);