[Devel] [PATCH VZ10 v2 03/10] drivers/md/dm-qcow2: update metadata on whole cluster discard
Pavel Tikhomirov
ptikhomirov at virtuozzo.com
Wed Aug 19 13:43:31 MSK 2026
On 8/12/26 22:19, Andrey Zhadchenko wrote:
> 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
revert_clusters_alloc is missing at this point, previous patches rename it.
> 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;
--
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.
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