On 11/4/2021 12:14 AM, Christoph Hellwig wrote:

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What is the actual use case here?

One of the immediate use-case is to use this interface with XFS 
scrubbing infrastructure [1] (by replacing any SCSI calls e.g. sg_io() 
with BLKVERIFY ioctl() calls corresponding to REQ_OP_VERIFY) and 
eventually allow and extend other file systems to use it for scrubbing.

[1] man xfs_scrub :-
-x     Read all file data extents to look for disk errors.
               xfs_scrub will issue O_DIRECT reads to the block device
               directly.  If the block device is a SCSI disk, it will
               instead issue READ VERIFY commands directly to the disk.
               If media errors are found, the error report will include
               the disk offset, in bytes.  If the media errors affect a
               file, the report will also include the inode number and
               file offset, in bytes.  These actions will confirm that
               all file data blocks can be read from storage.