On 10/12/23 18:08, Damien Le Moal wrote:
On 10/13/23 03:00, Bart Van Assche wrote:
We are having this discussion because bi_ioprio is sixteen bits wide and
because we don't want to make struct bio larger. How about expanding the
bi_ioprio field from 16 to 32 bits and to use separate bits for CDL
information and data lifetimes?
I guess we could do that as well. User side aio_reqprio field of struct aiocb,
which is used by io_uring and libaio, is an int, so 32-bits also. Changing
bi_ioprio to match that should not cause regressions or break user space I
think. Kernel uapi ioprio.h will need some massaging though.
Hmm ... are we perhaps looking at different kernel versions? This is
what I found:
$ git grep -nHE 'ioprio;|reqprio;' include/uapi/linux/{io_uring,aio_abi}.h
include/uapi/linux/aio_abi.h:89: __s16 aio_reqprio;
include/uapi/linux/io_uring.h:33: __u16 ioprio; /* ioprio for the
request */
The struct iocb used for asynchronous I/O has a size of 64 bytes and
does not have any holes. struct io_uring_sqe also has a size of 64 bytes
and does not have any holes either. The ioprio_set() and ioprio_get()
system calls use the data type int so these wouldn't need any changes to
increase the number of ioprio bits.
Reading Niklas's reply to Kanchan, I was reminded that using ioprio hint for
the lifetime may have one drawback: that information will be propagated to the
device only for direct IOs, no ? For buffered IOs, the information will be
lost. The other potential disadvantage of the ioprio interface is that we
cannot define ioprio+hint per file (or per inode really), unlike the old
write_hint that you initially reintroduced. Are these points blockers for the
user API you were thinking of ? How do you envision the user specifying
lifetime ? Per file ? Or are you thinking of not relying on the user to specify
that but rather the FS (e.g. f2fs) deciding on its own ? If it is the latter, I
think ioprio+hint is fine (it is simple). But if it is the former, the ioprio
API may not be the best suited for the job at hand.
The way I see it is that the primary purpose of the bits in the
bi_ioprio member that are used for the data lifetime is to allow
filesystems to provide data lifetime information to block drivers.
Specifying data lifetime information for direct I/O is convenient when
writing test scripts that verify whether data lifetime supports works
correctly. There may be other use cases but this is not my primary
focus.
I think that applications that want to specify data lifetime information
should use fcntl(fd, F_SET_RW_HINT, ...). It is up to the filesystem to
make sure that this information ends up in the bi_ioprio field. The
block layer is responsible for passing the information in the bi_ioprio
member to block drivers. Filesystems can support multiple policies for
combining the i_write_hint and other information into a data lifetime.
See also the whint_mode restored by patch 05/15 in this series.
Thanks,
Bart.
