Re: [net-next v2 08/11] devlink: track snapshot id usage count using an xarray

[Jiri Pirko <jiri@resnulli.us>]

Thu, Mar 26, 2020 at 04:51:54AM CET, jacob.e.keller@intel.com wrote:
>Each snapshot created for a devlink region must have an id. These ids
>are supposed to be unique per "event" that caused the snapshot to be
>created. Drivers call devlink_region_snapshot_id_get to obtain a new id
>to use for a new event trigger. The id values are tracked per devlink,
>so that the same id number can be used if a triggering event creates
>multiple snapshots on different regions.
>
>There is no mechanism for snapshot ids to ever be reused. Introduce an
>xarray to store the count of how many snapshots are using a given id,
>replacing the snapshot_id field previously used for picking the next id.
>
>The devlink_region_snapshot_id_get() function will use xa_alloc to
>insert an initial value of 1 value at an available slot between 0 and
>U32_MAX.
>
>The new __devlink_snapshot_id_increment() and
>__devlink_snapshot_id_decrement() functions will be used to track how
>many snapshots currently use an id.
>
>Drivers must now call devlink_snapshot_id_put() in order to release
>their reference of the snapshot id after adding region snapshots.
>
>By tracking the total number of snapshots using a given id, it is
>possible for the decrement() function to erase the id from the xarray
>when it is not in use.
>
>With this method, a snapshot id can become reused again once all
>snapshots that referred to it have been deleted via
>DEVLINK_CMD_REGION_DEL, and the driver has finished adding snapshots.
>
>This work also paves the way to introduce a mechanism for userspace to
>request a snapshot.
>
>Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

This looks good to me. 2 small nits below.
With or without changing them:
Reviewed-by: Jiri Pirko <jiri@mellanox.com>


>---
> drivers/net/ethernet/mellanox/mlx4/crdump.c |   3 +
> drivers/net/netdevsim/dev.c                 |   5 +-
> include/net/devlink.h                       |   4 +-
> net/core/devlink.c                          | 130 +++++++++++++++++++-
> 4 files changed, 134 insertions(+), 8 deletions(-)
>
>diff --git a/drivers/net/ethernet/mellanox/mlx4/crdump.c b/drivers/net/ethernet/mellanox/mlx4/crdump.c
>index 792951f6df0d..2700628f1689 100644
>--- a/drivers/net/ethernet/mellanox/mlx4/crdump.c
>+++ b/drivers/net/ethernet/mellanox/mlx4/crdump.c
>@@ -203,6 +203,9 @@ int mlx4_crdump_collect(struct mlx4_dev *dev)
> 	mlx4_crdump_collect_crspace(dev, cr_space, id);
> 	mlx4_crdump_collect_fw_health(dev, cr_space, id);
> 
>+	/* Release reference on the snapshot id */
>+	devlink_region_snapshot_id_put(devlink, id);
>+
> 	crdump_disable_crspace_access(dev, cr_space);
> 
> 	iounmap(cr_space);
>diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
>index b851fe63a75d..53ec891659eb 100644
>--- a/drivers/net/netdevsim/dev.c
>+++ b/drivers/net/netdevsim/dev.c
>@@ -44,6 +44,7 @@ static ssize_t nsim_dev_take_snapshot_write(struct file *file,
> 					    size_t count, loff_t *ppos)
> {
> 	struct nsim_dev *nsim_dev = file->private_data;
>+	struct devlink *devlink = priv_to_devlink(nsim_dev);

DaveM seems to frown upon breaking the reverse christmas tree like this.

Don't assign devlink here and do it later on instead to avoid the tree
breakage.


> 	void *dummy_data;
> 	int err;
> 	u32 id;
>@@ -54,13 +55,15 @@ static ssize_t nsim_dev_take_snapshot_write(struct file *file,
> 
> 	get_random_bytes(dummy_data, NSIM_DEV_DUMMY_REGION_SIZE);
> 
>-	err = devlink_region_snapshot_id_get(priv_to_devlink(nsim_dev), &id);
>+	err = devlink_region_snapshot_id_get(devlink, &id);
> 	if (err)
> 		pr_err("Failed to get snapshot id\n");
> 		return err;
> 	}
> 	err = devlink_region_snapshot_create(nsim_dev->dummy_region,
> 					     dummy_data, id);
>+	/* release the snapshot id regardless of errors */

I don't think this comment is necessary. Up to you.


>+	devlink_region_snapshot_id_put(devlink, id);
> 	if (err) {
> 		pr_err("Failed to create region snapshot\n");
> 		kfree(dummy_data);

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