[Devel] [PATCH v4 VZ9 4/5] dm-qcow2: add merge_backward set_eventfd command

[Andrey Zhadchenko]

On 3/3/25 10:37, Pavel Tikhomirov wrote:
> This eventfd can be used to get an event when merge_backward start work
> have finished and is waiting for completion.
> 
> Note: The eventfd can be changed even while work is running.
> 
> Locking:
> 
> The backward_merge.eventfd_ctx is protected from being released by
> tgt->ctl_mutex.
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-100466
> Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
> 
> --
> v2: Always report that work finished, e.g. also on error or then it was
> canceled, this should be more consistent from the userspace perspective.
> v4: Address Andrey's reveiw: signal that we are at completion waiting on
> change of eventfd.
> ---
>   drivers/md/dm-qcow2-cmd.c | 42 ++++++++++++++++++++++++++++++++++++++-
>   drivers/md/dm-qcow2.h     |  2 ++
>   2 files changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-qcow2-cmd.c b/drivers/md/dm-qcow2-cmd.c
> index 04a992f3ebba6..f16b4f731ca5a 100644
> --- a/drivers/md/dm-qcow2-cmd.c
> +++ b/drivers/md/dm-qcow2-cmd.c
> @@ -5,6 +5,8 @@
>   #include <linux/device-mapper.h>
>   #include <linux/sched/signal.h>
>   #include <linux/file.h>
> +#include <linux/eventfd.h>
> +#include <linux/minmax.h>
>   #include <linux/error-injection.h>
>   #include "dm-qcow2.h"
>   
> @@ -197,6 +199,8 @@ void qcow2_merge_backward_work(struct work_struct *work)
>   
>   	mutex_lock(&tgt->ctl_mutex);
>   	if (tgt->backward_merge.state != BACKWARD_MERGE_START) {
> +		if (tgt->backward_merge.eventfd_ctx)
> +			eventfd_signal(tgt->backward_merge.eventfd_ctx, 1);
>   		mutex_unlock(&tgt->ctl_mutex);
>   		return;
>   	}
> @@ -249,6 +253,8 @@ void qcow2_merge_backward_work(struct work_struct *work)
>   		/* Finish merge */
>   		tgt->backward_merge.state = BACKWARD_MERGE_WAIT_COMPLETION;
>   	}
> +	if (tgt->backward_merge.eventfd_ctx)
> +		eventfd_signal(tgt->backward_merge.eventfd_ctx, 1);

It would be a bit better if we also set a different values for error or 
success, but it is not necessary, as either complete will fail or we do 
get_progress and see error

>   	mutex_unlock(&tgt->ctl_mutex);
>   }
>   
> @@ -312,6 +318,27 @@ static bool qcow2_backward_merge_should_stop(struct qcow2_target *tgt)
>   	return READ_ONCE(tgt->backward_merge.state) == BACKWARD_MERGE_STOP;
>   }
>   
> +#define QCOW2_FILE_UNBIND -1
> +
> +static int qcow2_merge_backward_set_eventfd(struct qcow2_target *tgt, int efd)
> +{
> +	struct eventfd_ctx *ctx = NULL;
> +
> +	ctx = efd == QCOW2_FILE_UNBIND ? NULL : eventfd_ctx_fdget(efd);
> +	if (IS_ERR(ctx))
> +		return PTR_ERR(ctx);
> +
> +	mutex_lock(&tgt->ctl_mutex);
> +	swap(ctx, tgt->backward_merge.eventfd_ctx);
> +	if (ctx)
> +		eventfd_ctx_put(ctx);
> +	if (tgt->backward_merge.eventfd_ctx &&
> +	    tgt->backward_merge.state == BACKWARD_MERGE_WAIT_COMPLETION)
> +		eventfd_signal(tgt->backward_merge.eventfd_ctx, 1);
> +	mutex_unlock(&tgt->ctl_mutex);
> +	return 0;
> +}
> +
>   static struct qcow2 *qcow2_get_img(struct qcow2_target *tgt, u32 img_id, u8 *ref_index)
>   {
>   	struct qcow2 *qcow2;
> @@ -470,14 +497,27 @@ int qcow2_message(struct dm_target *ti, unsigned int argc, char **argv,
>   		ret = qcow2_get_event(tgt, result, maxlen);
>   		goto out;
>   	} else if (!strcmp(argv[0], "merge_backward")) {
> -		if (argc != 2) {
> +		if (argc < 2) {
>   			ret = -EINVAL;
>   			goto out;
>   		}
>   		if (!strcmp(argv[1], "cancel")) {
> +			if (argc != 2) {
> +				ret = -EINVAL;
> +				goto out;
> +			}
>   			qcow2_merge_backward_cancel(tgt);
>   			ret = 0;
>   			goto out;
> +		} else if (!strcmp(argv[1], "set_eventfd")) {
> +			int efd;
> +
> +			if (argc != 3 || kstrtoint(argv[2], 10, &efd)) {
> +				ret = -EINVAL;
> +				goto out;
> +			}
> +			ret = qcow2_merge_backward_set_eventfd(tgt, efd);
> +			goto out;
>   		}
>   	}
>   
> diff --git a/drivers/md/dm-qcow2.h b/drivers/md/dm-qcow2.h
> index bebfdc50ed6d4..c4956e3fd0eb7 100644
> --- a/drivers/md/dm-qcow2.h
> +++ b/drivers/md/dm-qcow2.h
> @@ -5,6 +5,7 @@
>   #include <linux/percpu-refcount.h>
>   #include <linux/device-mapper.h>
>   #include <linux/fs.h>
> +#include <linux/eventfd.h>
>   #include "dm-core.h"
>   
>   #define DM_MSG_PREFIX "qcow2"
> @@ -161,6 +162,7 @@ struct qcow2_backward_merge {
>   	struct work_struct work;
>   	enum qcow2_backward_merge_state state;
>   	int error;
> +	struct eventfd_ctx *eventfd_ctx;
>   };
>   
>   struct qcow2_target {