[LU-13826] fix compatibility for LL_IOC_MDC_GETINFO

Related — Wed, 9 Dec 2020 18:59:25 +0000

In master the LL_IOC_MDC_GETINFO definition changed when patch https://review.whamcloud.com/36674 "LU-10934 llite: integrate statx() API with Lustre" landed:

b2_10$ git grep '#define LL_IOC_MDC_GETINFO'
master:lustre/include/lustre/lustre_user.h:#define LL_IOC_MDC_GETINFO      _IOWR(IOC_MDC_TYPE, 23, struct lov_user_mds_data *)
master$ git grep '#define LL_IOC_MDC_GETINFO' 
lustre/include/uapi/linux/lustre/lustre_user.h:#define LL_IOC_MDC_GETINFO_OLD	_IOWR(IOC_MDC_TYPE, 23, struct lov_user_mds_data_v1 *)
lustre/include/uapi/linux/lustre/lustre_user.h:#define LL_IOC_MDC_GETINFO	_IOWR(IOC_MDC_TYPE, 23, struct lov_user_mds_data)

Which is obviously bad. It's also subtly bad in that the second ioctl depends on how lov_user_mds_data is defined, and it contains a different struct statx with different field names, so while the IOC numbers/structs are ABI compatible, it is not API compatible and applications using this header will break. For added confusion, this changed in 2.12.4 vs. 2.12.3. If we add a newer structure and update the definition as we have then should definition of LL_IOC_MDC_GETINFO change as well?

The suggestion is that we do:

#define LL_IOC_MDC_GETINFO_V1 _IOWR(IOC_MDC_TYPE, 23, struct lov_user_mds_data_v1 *)
#define LL_IOC_MDC_GETINFO_V2 _IOWR(IOC_MDC_TYPE, 23, struct lov_user_mds_data_v2)

in master and b2_12. Then we can use the explicitly versioned constants everywhere for the in-tree code, and declare LL_IOC_MDC_GETINFO in a compatible way, but external applications can select the version that they want explicitly.

Whamcloud Community JIRA

[LU-13826] fix compatibility for LL_IOC_MDC_GETINFO