Re: [PATCH v5 04/11] blksnap: header file of the module interface

[Sergei Shtepa <sergei.shtepa@veeam.com>]

Hi!
Thanks for the review.

On 7/17/23 20:57, Thomas Weißschuh wrote:
> Subject:
> Re: [PATCH v5 04/11] blksnap: header file of the module interface
> From:
> Thomas Weißschuh <thomas@t-8ch.de>
> Date:
> 7/17/23, 20:57
> 
> To:
> Sergei Shtepa <sergei.shtepa@veeam.com>
> CC:
> axboe@kernel.dk, hch@infradead.org, corbet@lwn.net, snitzer@kernel.org, viro@zeniv.linux.org.uk, brauner@kernel.org, dchinner@redhat.com, willy@infradead.org, dlemoal@kernel.org, jack@suse.cz, ming.lei@redhat.com, linux-block@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Donald Buczek <buczek@molgen.mpg.de>
> 
> 
> On 2023-06-12 15:52:21+0200, Sergei Shtepa wrote:
> 
>> [..]
>> diff --git a/include/uapi/linux/blksnap.h b/include/uapi/linux/blksnap.h
>> new file mode 100644
>> index 000000000000..2fe3f2a43bc5
>> --- /dev/null
>> +++ b/include/uapi/linux/blksnap.h
>> @@ -0,0 +1,421 @@
>> [..]
>> +/**
>> + * struct blksnap_snapshotinfo - Result for the command
>> + *	&blkfilter_ctl_blksnap.blkfilter_ctl_blksnap_snapshotinfo.
>> + *
>> + * @error_code:
>> + *	Zero if there were no errors while holding the snapshot.
>> + *	The error code -ENOSPC means that while holding the snapshot, a snapshot
>> + *	overflow situation has occurred. Other error codes mean other reasons
>> + *	for failure.
>> + *	The error code is reset when the device is added to a new snapshot.
>> + * @image:
>> + *	If the snapshot was taken, it stores the block device name of the
>> + *	image, or empty string otherwise.
>> + */
>> +struct blksnap_snapshotinfo {
>> +	__s32 error_code;
>> +	__u8 image[IMAGE_DISK_NAME_LEN];
> Nitpick:
> 
> Seems a bit weird to have a signed error code that is always negative.
> Couldn't this be an unsigned number or directly return the error from
> the ioctl() itself?

Yes, it's a good idea to pass the error code as an unsigned value.
And this positive value can be passed in case of successful execution
of ioctl(), but I would not like to put different error signs in one value.

> 
>> +};
>> +
>> +/**
>> + * DOC: Interface for managing snapshots
>> + *
>> + * Control commands that are transmitted through the blksnap module interface.
>> + */
>> +enum blksnap_ioctl {
>> +	blksnap_ioctl_version,
>> +	blksnap_ioctl_snapshot_create,
>> +	blksnap_ioctl_snapshot_destroy,
>> +	blksnap_ioctl_snapshot_append_storage,
>> +	blksnap_ioctl_snapshot_take,
>> +	blksnap_ioctl_snapshot_collect,
>> +	blksnap_ioctl_snapshot_wait_event,
>> +};
>> +
>> +/**
>> + * struct blksnap_version - Module version.
>> + *
>> + * @major:
>> + *	Version major part.
>> + * @minor:
>> + *	Version minor part.
>> + * @revision:
>> + *	Revision number.
>> + * @build:
>> + *	Build number. Should be zero.
>> + */
>> +struct blksnap_version {
>> +	__u16 major;
>> +	__u16 minor;
>> +	__u16 revision;
>> +	__u16 build;
>> +};
>> +
>> +/**
>> + * define IOCTL_BLKSNAP_VERSION - Get module version.
>> + *
>> + * The version may increase when the API changes. But linking the user space
>> + * behavior to the version code does not seem to be a good idea.
>> + * To ensure backward compatibility, API changes should be made by adding new
>> + * ioctl without changing the behavior of existing ones. The version should be
>> + * used for logs.
>> + *
>> + * Return: 0 if succeeded, negative errno otherwise.
>> + */
>> +#define IOCTL_BLKSNAP_VERSION							\
>> +	_IOW(BLKSNAP, blksnap_ioctl_version, struct blksnap_version)
> Shouldn't this be _IOR()?
> 
>   "_IOW means userland is writing and kernel is reading. _IOR
>   means userland is reading and kernel is writing."
> 
> The other ioctl definitions seem to need a review, too.
> 

Yeah. I need to replace _IOR and _IOW in all ioctl.
Thanks!