RE: [PATCH v4 2/2] io_uring: add support for IORING_OP_GETDENTS

[Tavian Barnes <tavianator@tavianator.com>]

 > IORING_OP_GETDENTS behaves much like getdents64(2) and takes the same
 > arguments, but with a small twist: it takes an additional offset
 > argument, and reading from the specified directory starts at the given
 > offset.
 >
 > For the first IORING_OP_GETDENTS call on a directory, the offset
 > parameter can be set to zero, and for subsequent calls, it can be
 > set to the ->d_off field of the last struct linux_dirent64 returned
 > by the previous IORING_OP_GETDENTS call.
 >
 > Internally, if necessary, IORING_OP_GETDENTS will vfs_llseek() to
 > the right directory position before calling vfs_getdents().

For my envisioned use case it would be important to support reading from
the current file position when offset == -1 (IORING_FEAT_RW_CUR_POS).
Among other things, this would let me fully read a directory with fewer
round-trips.  Normally after the first getdents(), another one must be
issued to distinguish between EOF and a short first read.  It would be
nice to do both calls with linked SQEs so I could immediately know that
I've reached the end of the directory.

 > IORING_OP_GETDENTS may or may not update the specified directory's
 > file offset, and the file offset should not be relied upon having
 > any particular value during or after an IORING_OP_GETDENTS call.

Obviously for the above to work, we'd have to get rid of this 
limitation.  Is that possible?