[Devel] [PATCH VZ10 03/10] drivers/md/dm-qcow2: update metadata on whole cluster discard
Andrey Zhadchenko
andrey.zhadchenko at virtuozzo.com
Wed Aug 12 21:56:31 MSK 2026
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 at 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
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