bcachefs: Restore journal write point at startup

This patch tweaks the journal recovery path so that we start writing
right after where we left off, instead of the next empty bucket. This is
partly prep work for supporting zoned devices, but it's also good to do
in general to avoid the journal completely filling up and getting stuck.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/journal_io.c

@@ -907,6 +907,7 @@ static void bch2_journal_read_device(struct closure *cl)
 	struct bch_fs *c = ca->fs;
 	struct journal_list *jlist =
 		container_of(cl->parent, struct journal_list, cl);
+	struct journal_replay *r;
 	struct journal_read_buf buf = { NULL, 0 };
 	u64 min_seq = U64_MAX;
 	unsigned i;
@@ -942,11 +943,29 @@ static void bch2_journal_read_device(struct closure *cl)
 	 * allocate
 	 */
 	while (ja->bucket_seq[ja->cur_idx] > min_seq &&
-	       ja->bucket_seq[ja->cur_idx] >
+	       ja->bucket_seq[ja->cur_idx] ==
 	       ja->bucket_seq[(ja->cur_idx + 1) % ja->nr])
 		ja->cur_idx = (ja->cur_idx + 1) % ja->nr;
 
-	ja->sectors_free = 0;
+	ja->sectors_free = ca->mi.bucket_size;
+
+	mutex_lock(&jlist->lock);
+	list_for_each_entry(r, jlist->head, list) {
+		for (i = 0; i < r->nr_ptrs; i++) {
+			if (r->ptrs[i].dev == ca->dev_idx &&
+			    sector_to_bucket(ca, r->ptrs[i].sector) == ja->buckets[ja->cur_idx]) {
+				unsigned wrote = (r->ptrs[i].sector % ca->mi.bucket_size) +
+					vstruct_sectors(&r->j, c->block_bits);
+
+				ja->sectors_free = min(ja->sectors_free,
+						       ca->mi.bucket_size - wrote);
+			}
+		}
+	}
+	mutex_unlock(&jlist->lock);
+
+	BUG_ON(ja->bucket_seq[ja->cur_idx] &&
+	       ja->sectors_free == ca->mi.bucket_size);
 
 	/*
 	 * Set dirty_idx to indicate the entire journal is full and needs to be