[Devel] [PATCH VZ10 v2 03/10] drivers/md/dm-qcow2: update metadata on whole cluster discard

[Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>]

Discard of a mapped cluster used to only punch a hole in the image
file: the L2 entry and refcounts were left untouched, so the cluster
remained allocated in qcow2 metadata forever.

Handle discards covering a whole cluster via the L1/L2 entry replace
machinery: prepare_cluster_discard() locks the L2 entry, then
issue_discard() punches the data cluster out of the image file and
queues the qio to write the zeroed L2 entry (and extended L2 bitmap)
in process_indexes_write(). Refcounts of the discarded cluster are
decremented in process_indexes_end() after the L2 writeback, like COW
does with its source clusters. Clusters shared with internal
snapshots or holding compressed data are not punched: only their
usage count is decremented.

Also teach revert_clusters_alloc() that a changed index may contain
a discard-cleared entry, which has no allocation to revert.

Partial cluster discards keep the previous behavior: punch a hole
without touching metadata.

Feature: dm-qcow2: block device over QCOW2 files driver
https://virtuozzo.atlassian.net/browse/VSTOR-139406
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
---
 drivers/md/dm-qcow2-map.c | 110 +++++++++++++++++++++++++++++++++++---
 drivers/md/dm-qcow2.h     |   2 +-
 2 files changed, 104 insertions(+), 8 deletions(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index 08d46eb177dd1..8d2c4e78a4c55 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -101,6 +101,12 @@ static loff_t bio_sector_to_file_pos(struct qcow2 *qcow2, struct qio *qio,
 	return map->data_clu_pos + bytes_off_in_cluster(qcow2, qio);
 }
 
+static bool qio_covers_full_clu(struct qcow2 *qcow2, struct qio *qio)
+{
+	return bytes_off_in_cluster(qcow2, qio) == 0 &&
+	       qio->bi_iter.bi_size == qcow2->clu_size;
+}
+
 static loff_t compressed_clu_end_pos(loff_t start, sector_t compressed_sectors)
 {
 	if (start % SECTOR_SIZE == 0)
@@ -1324,6 +1330,8 @@ static void revert_l_entries_update(struct qcow2 *qcow2, struct wb_desc *wbd)
 		set_u64_to_be_page(wbd->md->page, i, 0);
 		if (skip_odd && (i & 1))
 			continue; /* pos contains ext_l2 part of L2 entry */
+		if (!pos)
+			continue; /* no cluster was allocated */
 
 		spin_unlock(&qcow2->md_pages_lock);
 		pos &= ~LX_REFCOUNT_EXACTLY_ONE;
@@ -2015,7 +2023,12 @@ static int parse_metadata(struct qcow2 *qcow2, struct qio **qio,
 		return ret;
 
 	map->data_clu_pos = pos;
-	if (!write || !map->clu_is_cow)
+	if (!write)
+		return 0;
+
+	/* discards also need to update r1r2 */
+	if (!map->clu_is_cow &&
+	    !(op_is_discard((*qio)->bi_op) && qio_covers_full_clu(qcow2, *qio)))
 		return 0;
 
 	/* Now refcounters table/block */
@@ -3317,12 +3330,86 @@ static void issue_discard(struct qcow2_map *map, struct qio *qio)
 	int ret;
 
 	WARN_ON_ONCE(!(map->level & L2_LEVEL));
-	pos = bio_sector_to_file_pos(qcow2, qio, map);
-	ret = qcow2_punch_hole(qcow2->file, pos, qio->bi_iter.bi_size);
 
-	if (ret)
-		qio->bi_status = errno_to_blk_status(ret);
-	qio_endio(qio);
+	if (!map->clu_is_cow) {
+		pos = bio_sector_to_file_pos(qcow2, qio, map);
+		ret = qcow2_punch_hole(qcow2->file, pos, qio->bi_iter.bi_size);
+
+		if (ret) {
+			qio->bi_status = errno_to_blk_status(ret);
+			qio_endio(qio);
+			return;
+		}
+	}
+
+	/* Clear metadata if needed */
+	if (qio->flags & QIO_IS_DISCARD_FL) {
+		qio->queue_list_id = QLIST_INDEXES_WRITE;
+		qcow2_dispatch_qios(qcow2, qio, NULL);
+	} else {
+		qio_endio(qio);
+	}
+}
+
+static bool qio_discard_updates_metadata(struct qcow2 *qcow2, struct qio *qio,
+					 struct qcow2_map *map)
+{
+	if (!(map->level & L2_LEVEL) || !map->data_clu_alloced)
+		return false;
+	if (qio_covers_full_clu(qcow2, qio))
+		return true;
+	return false;
+}
+
+/*
+ * Discard changes metadata: prepare L2 entry update. It gets locked
+ * here, new value is written in process_indexes_write(), and
+ * refcounts are handled after the L2 writeback in process_indexes_end().
+ *
+ * Discard covering the whole cluster replaces the entry and unuses
+ * the discarded (or COW source) cluster.
+ */
+static int prepare_cluster_discard(struct qcow2 *qcow2, struct qio **qio,
+				   struct qcow2_map *map)
+{
+	u32 index_in_page = map->l2.index_in_page;
+	struct md_page *md = map->l2.md;
+	loff_t unuse_pos, unuse_end;
+	struct qio_ext *ext;
+	int ret;
+
+	WARN_ON_ONCE(!(map->level & L2_LEVEL) || !map->data_clu_alloced);
+
+	spin_lock_irq(&qcow2->md_pages_lock);
+	if (delay_if_dirty(qcow2, md, index_in_page, qio) ||
+	    __delay_if_writeback(qcow2, md, index_in_page, qio, true) ||
+	    (qcow2->ext_l2 &&
+	     delay_if_dirty(qcow2, md, index_in_page + 1, qio))) {
+		spin_unlock_irq(&qcow2->md_pages_lock);
+		return 0;
+	}
+	spin_unlock_irq(&qcow2->md_pages_lock);
+
+	if (map->clu_is_cow) {
+		/* Cluster is shared or compressed. Decrement refcount. */
+		unuse_pos = map->cow_clu_pos;
+		unuse_end = map->cow_clu_end;
+	} else {
+		/* Nobody else refers the cluster: unuse it after discard */
+		unuse_pos = map->data_clu_pos;
+		unuse_end = map->data_clu_pos + qcow2->clu_size;
+	}
+
+	ret = prepare_l_entry_replace(qcow2, map, *qio, md, index_in_page,
+				      unuse_pos, unuse_end, L2_LEVEL);
+	if (ret <= 0)
+		return ret;
+
+	ext = (*qio)->ext;
+	ext->lx_md = md;
+
+	(*qio)->flags |= QIO_IS_DISCARD_FL;
+	return 1;
 }
 
 static int handle_metadata(struct qcow2 *qcow2, struct qio **qio,
@@ -3344,6 +3431,9 @@ static int handle_metadata(struct qcow2 *qcow2, struct qio **qio,
 		/* Nothing to COW or L1 is mapped exactly once */
 		qio_endio(*qio);
 		ret = 0;
+	} else if (unlikely(op_is_discard((*qio)->bi_op)) &&
+		   qio_discard_updates_metadata(qcow2, *qio, map)) {
+		ret = prepare_cluster_discard(qcow2, qio, map);
 	} else if (write &&
 		   (!qio_is_fully_alloced(qcow2, *qio, map) || map->clu_is_cow)) {
 		if (map->clu_is_cow) {
@@ -3365,7 +3455,7 @@ static int handle_metadata(struct qcow2 *qcow2, struct qio **qio,
 				qio_endio(*qio);
 				ret = 0;
 			}
-			/* Otherwise issue_discard(). TODO: update L2 */
+			/* Otherwise issue_discard(). */
 		} else {
 			/* Wants L1 or L2 entry allocation */
 			ret = prepare_l1l2_allocation(qcow2, *qio, map);
@@ -3479,6 +3569,12 @@ static void process_one_qio(struct qcow2 *qcow2, struct qio *qio)
 
 	write = op_is_write(qio->bi_op);
 
+	/* Discard with prepared metadata update, see prepare_cluster_discard() */
+	if (unlikely(qio->flags & QIO_IS_DISCARD_FL)) {
+		issue_discard(&map, qio);
+		return;
+	}
+
 	if (unlikely(map.compressed)) {
 		/* Compressed qio never uses sub-clus */
 		submit_read_compressed(&map, qio, write);
diff --git a/drivers/md/dm-qcow2.h b/drivers/md/dm-qcow2.h
index 230e7a4a34e76..8a24e04130e4d 100644
--- a/drivers/md/dm-qcow2.h
+++ b/drivers/md/dm-qcow2.h
@@ -340,7 +340,7 @@ struct qio {
 	blk_status_t bi_status;
 #define QIO_FREE_ON_ENDIO_FL	(1 << 0) /* Free this qio memory from qio_endio() */
 #define QIO_IS_MERGE_FL		(1 << 3) /* This is service merge qio */
-#define QIO_IS_DISCARD_FL	(1 << 4) /* This zeroes index on backward merge */
+#define QIO_IS_DISCARD_FL	(1 << 4) /* This zeroes index (discard or backward merge) */
 #define QIO_IS_L1COW_FL		(1 << 5) /* This qio only wants COW at L1 */
 #define QIO_SPLIT_INHERITED_FLAGS (QIO_IS_DISCARD_FL)
 	u8 flags;
-- 
2.43.5