Date:   Mon, 2 Mar 2020 11:38:12 -0800
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     netdev@...r.kernel.org, valex@...lanox.com, linyunsheng@...wei.com,
        lihong.yang@...el.com, kuba@...nel.org
Subject: Re: [RFC PATCH v2 14/22] devlink: implement DEVLINK_CMD_REGION_NEW

On 3/2/2020 9:41 AM, Jiri Pirko wrote:
> Sat, Feb 15, 2020 at 12:22:13AM CET, jacob.e.keller@...el.com wrote:
>> Implement support for the DEVLINK_CMD_REGION_NEW command for creating
>> snapshots. This new command parallels the existing
>> DEVLINK_CMD_REGION_DEL.
>>
>> In order for DEVLINK_CMD_REGION_NEW to work for a region, the new
>> ".snapshot" operation must be implemented in the region's ops structure.
>>
>> The desired snapshot id may be provided. If the requested id is already
>> in use, an error will be reported. If no id is provided one will be
>> selected in the same way as a triggered snapshot.
>>
>> In either case, the reference count for that id will be incremented
>> in the snapshot IDR.
>>
>> Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
>> ---
>> .../networking/devlink/devlink-region.rst     | 12 +++-
>> include/net/devlink.h                         |  6 ++
>> net/core/devlink.c                            | 72 +++++++++++++++++++
>> 3 files changed, 88 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/networking/devlink/devlink-region.rst b/Documentation/networking/devlink/devlink-region.rst
>> index 1a7683e7acb2..a24faf2b6b7a 100644
>> --- a/Documentation/networking/devlink/devlink-region.rst
>> +++ b/Documentation/networking/devlink/devlink-region.rst
>> @@ -20,6 +20,11 @@ address regions that are otherwise inaccessible to the user.
>> Regions may also be used to provide an additional way to debug complex error
>> states, but see also :doc:`devlink-health`
>>
>> +Regions may optionally support capturing a snapshot on demand via the
>> +``DEVLINK_CMD_REGION_NEW`` netlink message. A driver wishing to allow
>> +requested snapshots must implement the ``.snapshot`` callback for the region
>> +in its ``devlink_region_ops`` structure.
>> +
>> example usage
>> -------------
>>
>> @@ -40,8 +45,11 @@ example usage
>>     # Delete a snapshot using:
>>     $ devlink region del pci/0000:00:05.0/cr-space snapshot 1
>>
>> -    # Trigger (request) a snapshot be taken:
>> -    $ devlink region trigger pci/0000:00:05.0/cr-space
> 
> Odd. It is actually "devlink region dump". There is no trigger.
> 
> 
>> +    # Request an immediate snapshot, if supported by the region
>> +    $ devlink region new pci/0000:00:05.0/cr-space
> 
> Without ID? I would personally require snapshot id always. Without it,
> it looks like you are creating region.
> 

Not specifying an ID causes the ID to be auto-selected. I suppose
support for that doesn't need to be kept.

> 
>> +
>> +    # Request an immediate snapshot with a specific id
>> +    $ devlink region new pci/0000:00:05.0/cr-space snapshot 5
>>
>>     # Dump a snapshot:
>>     $ devlink region dump pci/0000:00:05.0/fw-health snapshot 1
>> diff --git a/include/net/devlink.h b/include/net/devlink.h
>> index 3a5ff6bea143..3cd0ff2040b2 100644
>> --- a/include/net/devlink.h
>> +++ b/include/net/devlink.h
>> @@ -498,10 +498,16 @@ struct devlink_info_req;
>>  * struct devlink_region_ops - Region operations
>>  * @name: region name
>>  * @destructor: callback used to free snapshot memory when deleting
>> + * @snapshot: callback to request an immediate snapshot. On success,
>> + *            the data variable must be updated to point to the snapshot data.
>> + *            The function will be called while the devlink instance lock is
>> + *            held.
>>  */
>> struct devlink_region_ops {
>> 	const char *name;
>> 	void (*destructor)(const void *data);
>> +	int (*snapshot)(struct devlink *devlink, struct netlink_ext_ack *extack,
>> +			u8 **data);
> 
> Please have the same type here and for destructor. "u8 *" I guess.
> 
Sure. My only concern would be if that causes a compiler warning when
passing kfree/vfree to the destructor pointer. Alternatively we could
use void **data, but it's definitely interpreted as a byte stream by the
devlink core code.

> 
>> };
>>
>> struct devlink_fmsg;
>> diff --git a/net/core/devlink.c b/net/core/devlink.c
>> index 9571063846cc..b5d1b21e5178 100644
>> --- a/net/core/devlink.c
>> +++ b/net/core/devlink.c
>> @@ -4045,6 +4045,71 @@ static int devlink_nl_cmd_region_del(struct sk_buff *skb,
>> 	return 0;
>> }
>>
>> +static int
>> +devlink_nl_cmd_region_new(struct sk_buff *skb, struct genl_info *info)
>> +{
>> +	struct devlink *devlink = info->user_ptr[0];
>> +	struct devlink_region *region;
>> +	const char *region_name;
>> +	u32 snapshot_id;
>> +	u8 *data;
>> +	int err;
>> +
>> +	if (!info->attrs[DEVLINK_ATTR_REGION_NAME]) {
>> +		NL_SET_ERR_MSG_MOD(info->extack, "No region name provided");
>> +		return -EINVAL;
>> +	}
>> +
>> +	region_name = nla_data(info->attrs[DEVLINK_ATTR_REGION_NAME]);
>> +	region = devlink_region_get_by_name(devlink, region_name);
>> +	if (!region) {
>> +		NL_SET_ERR_MSG_MOD(info->extack,
> 
> In devlink.c, please don't wrap here.
> 

For any of these?

> 
>> +				   "The requested region does not exist");
>> +		return -EINVAL;
>> +	}
>> +
>> +	if (!region->ops->snapshot) {
>> +		NL_SET_ERR_MSG_MOD(info->extack,
>> +				   "The requested region does not support taking an immediate snapshot");
>> +		return -EOPNOTSUPP;
>> +	}
>> +
>> +	if (region->cur_snapshots == region->max_snapshots) {
>> +		NL_SET_ERR_MSG_MOD(info->extack,
>> +				   "The region has reached the maximum number of stored snapshots");
>> +		return -ENOMEM;
>> +	}
>> +
>> +	if (info->attrs[DEVLINK_ATTR_REGION_SNAPSHOT_ID]) {
>> +		/* __devlink_region_snapshot_create will take care of
>> +		 * inserting the snapshot id into the IDR if necessary.
>> +		 */
>> +		snapshot_id = nla_get_u32(info->attrs[DEVLINK_ATTR_REGION_SNAPSHOT_ID]);
>> +
>> +		if (devlink_region_snapshot_get_by_id(region, snapshot_id)) {
>> +			NL_SET_ERR_MSG_MOD(info->extack,
>> +					   "The requested snapshot id is already in use");
>> +			return -EEXIST;
>> +		}
>> +	} else {
>> +		snapshot_id = __devlink_region_snapshot_id_get(devlink);
>> +	}
>> +
>> +	err = region->ops->snapshot(devlink, info->extack, &data);
> 
> Don't you put the "id"? Looks like a leak.
> 

The id is put into the devlink_region_snapshot_create, the driver code
doesn't need to know about it as far as I can tell.

Currently the ids are managed by an IDR which stores a reference count
of how many snapshots use it.

Use of "NULL" is done so that devlink_region_snapshot_id_get can
"pre-allocate" the ID without assigning snapshots, assuming that a later
call to the devlink_region_snapshot_create will find that id and create
or increment it's refcount.

This complexity comes from the fact that the current code requires the
ability to re-use the same snapshot id for different regions in the same
devlink. This devlink_region_snapshot_id_get must return IDs which are
unique across all regions. If a user does DEVLINK_CMD_REGION_NEW with an
ID, it would only be used by a single snapshot. We need to make sure
that this doesn't confuse devlink_region_snapshot_id_get. Additionally,
I wanted to make sure that the snapshot IDs could be re-used once the
related snapshots have been deleted.

> 
>> +	if (err)
>> +		return err;
>> +
>> +	err = __devlink_region_snapshot_create(region, data, snapshot_id);
>> +	if (err)
>> +		goto err_free_snapshot_data;
>> +
>> +	return 0;
>> +
>> +err_free_snapshot_data:
>> +	region->ops->destructor(data);
>> +	return err;
>> +}
>> +
>> static int devlink_nl_cmd_region_read_chunk_fill(struct sk_buff *msg,
>> 						 struct devlink *devlink,
>> 						 u8 *chunk, u32 chunk_size,
>> @@ -6358,6 +6423,13 @@ static const struct genl_ops devlink_nl_ops[] = {
>> 		.flags = GENL_ADMIN_PERM,
>> 		.internal_flags = DEVLINK_NL_FLAG_NEED_DEVLINK,
>> 	},
>> +	{
>> +		.cmd = DEVLINK_CMD_REGION_NEW,
>> +		.validate = GENL_DONT_VALIDATE_STRICT | GENL_DONT_VALIDATE_DUMP,
>> +		.doit = devlink_nl_cmd_region_new,
>> +		.flags = GENL_ADMIN_PERM,
>> +		.internal_flags = DEVLINK_NL_FLAG_NEED_DEVLINK,
>> +	},
>> 	{
>> 		.cmd = DEVLINK_CMD_REGION_DEL,
>> 		.validate = GENL_DONT_VALIDATE_STRICT | GENL_DONT_VALIDATE_DUMP,
>> -- 
>> 2.25.0.368.g28a2d05eebfb
>>

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