Re: [PATCH v2 03/15] block: Support data lifetime in the I/O priority bitfield

[Bart Van Assche <bvanassche@acm.org>]

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.