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

[Damien Le Moal <dlemoal@kernel.org>]

On 10/13/23 03:00, Bart Van Assche wrote:
> On 10/11/23 18:02, Damien Le Moal wrote:
>> Some have stated interest in CDL in NVMe-oF context, which could
>> imply that combining CDL and lifetime may be something useful to do
>> in that space...
> 
> 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.

> This patch does not make struct bio bigger because it changes a three
> byte hole with a one byte hole:

Yeah, but if the kernel is compiled with struct randomization, that does not
really apply, doesn't it ?

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.

-- 
Damien Le Moal
Western Digital Research