[Devel] [PATCH VZ9 v3] dm-qcow2: allow specifying depth to merge intermediate images
Pavel Tikhomirov
ptikhomirov at virtuozzo.com
Thu Sep 18 13:56:02 MSK 2025
On 9/18/25 18:45, Andrey Zhadchenko wrote:
> dm-qcow2 is almost ready to merge intermediate images, thanks
> to separated metadata tables and qio being per-qcow2.
> So this patch mostly propagates which qcow2 we want to use
> instead of tgt->top.
>
> qcow2_merge_backward_complete is a bit harder due to in-middle
> replacement and needing to recalculate img_id for a part of
> image chain.
>
> https://virtuozzo.atlassian.net/browse/VSTOR-101375
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
> ---
>
> v2:
> - changed depth to u32
> - reverted changes in set_backward_merge_in_process(): do queue stop
> for not top qcow2 images at it's own level
> - changed saving qcow2 in backward_merge to saving pointer to the
> qcow2 which we later intend to change to lower delta
>
> v3: simplified depth lookup in qcow2_merge_backward_start()
>
> drivers/md/dm-qcow2-cmd.c | 59 ++++++++++++++++++++++++++++-----------
> drivers/md/dm-qcow2.h | 1 +
> 2 files changed, 44 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/md/dm-qcow2-cmd.c b/drivers/md/dm-qcow2-cmd.c
> index e77052ad61e6b..2e4384360ca3d 100644
> --- a/drivers/md/dm-qcow2-cmd.c
> +++ b/drivers/md/dm-qcow2-cmd.c
> @@ -116,9 +116,9 @@ static int qcow2_service_iter(struct qcow2_target *tgt, struct qcow2 *qcow2,
> }
> ALLOW_ERROR_INJECTION(qcow2_service_iter, ERRNO);
>
> -static int qcow2_merge_common(struct qcow2_target *tgt)
> +static int qcow2_merge_common(struct qcow2_target *tgt, struct qcow2 *qcow2)
> {
> - struct qcow2 *qcow2 = tgt->top, *lower = qcow2->lower;
> + struct qcow2 *lower = qcow2->lower;
> u32 clu_size = qcow2->clu_size;
> loff_t end = lower->hdr.size;
>
> @@ -139,9 +139,8 @@ static int qcow2_merge_forward(struct qcow2_target *tgt)
> }
> ALLOW_ERROR_INJECTION(qcow2_merge_forward, ERRNO);
>
> -static int qcow2_break_l1cow(struct qcow2_target *tgt)
> +static int qcow2_break_l1cow(struct qcow2_target *tgt, struct qcow2 *qcow2)
> {
> - struct qcow2 *qcow2 = tgt->top;
> loff_t end = qcow2->hdr.size;
> loff_t step = (u64)qcow2->l2_entries * qcow2->clu_size;
>
> @@ -154,6 +153,15 @@ static void set_backward_merge_in_process(struct qcow2_target *tgt,
> {
> LIST_HEAD(list);
>
> + /*
> + * There are no writes if it is not the top qcow2 image.
> + * so we do not need to stop and flush requests when setting
> + */
> + if (qcow2 != tgt->top && set) {
> + qcow2->backward_merge_in_process = set;
> + return;
> + }
> +
> /*
> * To avoid race between allocations and COWS
> * we completely stop queueing qios and wait
> @@ -240,13 +248,20 @@ static int qcow2_merge_backward_progress(struct qcow2_target *tgt,
>
> static int qcow2_merge_backward_set_eventfd(struct qcow2_target *tgt, int efd);
>
> -static int qcow2_merge_backward_start(struct qcow2_target *tgt, int efd)
> +static int qcow2_merge_backward_start(struct qcow2_target *tgt, int efd, u32 depth)
> {
> - struct qcow2 *qcow2 = tgt->top, *lower = qcow2->lower;
> + struct qcow2 *qcow2, *lower, **pqcow2 = &tgt->top;
> int ret;
>
> lockdep_assert_held(&tgt->ctl_mutex);
>
> + while (depth-- && *pqcow2)
> + pqcow2 = &(*pqcow2)->lower;
> +
> + qcow2 = *pqcow2;
> + if (!qcow2)
> + return -ENOENT;
> + lower = qcow2->lower;
> if (!lower)
> return -ENOENT;
> if (!(lower->file->f_mode & FMODE_WRITE))
> @@ -263,6 +278,7 @@ static int qcow2_merge_backward_start(struct qcow2_target *tgt, int efd)
> if (ret)
> return ret;
>
> + tgt->backward_merge.pqcow2 = pqcow2;
> tgt->backward_merge.state = BACKWARD_MERGE_START;
> __backward_merge_update_stage(tgt, BACKWARD_MERGE_STAGE_START);
> tgt->backward_merge.error = 0;
> @@ -291,7 +307,7 @@ void qcow2_merge_backward_work(struct work_struct *work)
> __backward_merge_update_stage(tgt, BACKWARD_MERGE_STAGE_BREAK_L1COW);
> mutex_unlock(&tgt->ctl_mutex);
>
> - qcow2 = tgt->top;
> + qcow2 = *tgt->backward_merge.pqcow2;
> lower = qcow2->lower;
>
> /*
> @@ -300,7 +316,7 @@ void qcow2_merge_backward_work(struct work_struct *work)
> * we'd have to freeze IO going to all data clusters
> * under every L1 entry related to several snapshots.
> */
> - ret = qcow2_break_l1cow(tgt);
> + ret = qcow2_break_l1cow(tgt, qcow2);
> if (ret) {
> QC_ERR(tgt->ti, "Can't break L1 COW");
> goto out_err;
> @@ -316,7 +332,7 @@ void qcow2_merge_backward_work(struct work_struct *work)
>
> /* Start merge */
> backward_merge_update_stage(tgt, BACKWARD_MERGE_STAGE_RUNNING);
> - ret = qcow2_merge_common(tgt);
> + ret = qcow2_merge_common(tgt, qcow2);
> if (ret) {
> set_backward_merge_in_process(tgt, qcow2, false);
> ret2 = qcow2_set_image_file_features(lower, false);
> @@ -350,16 +366,23 @@ void qcow2_merge_backward_work(struct work_struct *work)
>
> static int qcow2_merge_backward_complete(struct qcow2_target *tgt)
> {
> - struct qcow2 *qcow2 = tgt->top, *lower = qcow2->lower;
> + struct qcow2 *qcow2 = *tgt->backward_merge.pqcow2, *i;
> int ret;
>
> lockdep_assert_held(&tgt->ctl_mutex);
>
> if (tgt->backward_merge.state != BACKWARD_MERGE_WAIT_COMPLETION)
> return -EBUSY;
> +
> + *tgt->backward_merge.pqcow2 = qcow2->lower;
> +
> + i = tgt->top;
> + while (i != qcow2->lower) {
> + i->img_id--;
> + i = i->lower;
> + }
> +
> __backward_merge_update_stage(tgt, BACKWARD_MERGE_STAGE_COMPLETING);
> -
> - tgt->top = lower;
> smp_wmb(); /* Pairs with qcow2_ref_inc() */
> qcow2_inflight_ref_switch(tgt); /* Pending qios */
> qcow2_flush_deferred_activity(tgt, qcow2); /* Delayed md pages */
> @@ -396,7 +419,7 @@ void qcow2_merge_backward_cancel(struct qcow2_target *tgt)
> } else if (tgt->backward_merge.state == BACKWARD_MERGE_STOP) {
> flush = true;
> } else if (tgt->backward_merge.state == BACKWARD_MERGE_WAIT_COMPLETION) {
> - set_backward_merge_in_process(tgt, tgt->top, false);
> + set_backward_merge_in_process(tgt, *tgt->backward_merge.pqcow2, false);
> tgt->backward_merge.state = BACKWARD_MERGE_STOPPED;
> __backward_merge_update_stage(tgt, BACKWARD_MERGE_STAGE_NONE);
> }
> @@ -571,7 +594,7 @@ int qcow2_message(struct dm_target *ti, unsigned int argc, char **argv,
> {
> struct qcow2_target *tgt = to_qcow2_target(ti);
> int ret = -EPERM;
> - u32 val, val2;
> + u32 val, val2, depth = 0;
> int efd;
>
> if (!capable(CAP_SYS_ADMIN))
> @@ -652,11 +675,15 @@ int qcow2_message(struct dm_target *ti, unsigned int argc, char **argv,
> } else if (!strcmp(argv[0], "merge_backward")) {
> /* argc >= 2 */
> if (!strcmp(argv[1], "start")) {
> - if (argc != 3 || kstrtoint(argv[2], 10, &efd) || efd < 0) {
> + if (argc < 3 || argc > 4 || kstrtoint(argv[2], 10, &efd) || efd < 0) {
> ret = -EINVAL;
> goto out_unlock;
> }
> - ret = qcow2_merge_backward_start(tgt, efd);
> + if (argc == 4 && kstrtou32(argv[3], 10, &depth)) {
> + ret = -EINVAL;
> + goto out_unlock;
> + }
> + ret = qcow2_merge_backward_start(tgt, efd, depth);
> } else if (!strcmp(argv[1], "complete")) {
> if (argc != 2) {
> ret = -EINVAL;
> diff --git a/drivers/md/dm-qcow2.h b/drivers/md/dm-qcow2.h
> index 5aa00c6a5ebd5..c50b578fb8b50 100644
> --- a/drivers/md/dm-qcow2.h
> +++ b/drivers/md/dm-qcow2.h
> @@ -172,6 +172,7 @@ enum qcow2_backward_merge_stage {
>
> struct qcow2_backward_merge {
> struct work_struct work;
> + struct qcow2 **pqcow2;
> enum qcow2_backward_merge_state state;
> int error;
> struct eventfd_ctx *eventfd_ctx;
--
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.
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