Re: [PATCH v4 6/6] io_uring: add support for zone-append

[Kanchan Joshi <joshiiitr@gmail.com>]

On Fri, Jul 31, 2020 at 1:44 PM Damien Le Moal <Damien.LeMoal@wdc.com> wrote:
>
> On 2020/07/31 16:59, Kanchan Joshi wrote:
> > On Fri, Jul 31, 2020 at 12:29 PM Damien Le Moal <Damien.LeMoal@wdc.com> wrote:
> >>
> >> On 2020/07/31 15:45, hch@infradead.org wrote:
> >>> On Fri, Jul 31, 2020 at 06:42:10AM +0000, Damien Le Moal wrote:
> >>>>> - We may not be able to use RWF_APPEND, and need exposing a new
> >>>>> type/flag (RWF_INDIRECT_OFFSET etc.) user-space. Not sure if this
> >>>>> sounds outrageous, but is it OK to have uring-only flag which can be
> >>>>> combined with RWF_APPEND?
> >>>>
> >>>> Why ? Where is the problem ? O_APPEND/RWF_APPEND is currently meaningless for
> >>>> raw block device accesses. We could certainly define a meaning for these in the
> >>>> context of zoned block devices.
> >>>
> >>> We can't just add a meaning for O_APPEND on block devices now,
> >>> as it was previously silently ignored.  I also really don't think any
> >>> of these semantics even fit the block device to start with.  If you
> >>> want to work on raw zones use zonefs, that's what is exists for.
> >>
> >> Which is fine with me. Just trying to say that I think this is exactly the
> >> discussion we need to start with. What interface do we implement...
> >>
> >> Allowing zone append only through zonefs as the raw block device equivalent, all
> >> the O_APPEND/RWF_APPEND semantic is defined and the "return written offset"
> >> implementation in VFS would be common for all file systems, including regular
> >> ones. Beside that, there is I think the question of short writes... Not sure if
> >> short writes can currently happen with async RWF_APPEND writes to regular files.
> >> I think not but that may depend on the FS.
> >
> > generic_write_check_limits (called by generic_write_checks, used by
> > most FS) may make it short, and AFAIK it does not depend on
> > async/sync.
>
> Johannes has a patch (not posted yet) fixing all this for zonefs,
> differentiating sync and async cases, allow short writes or not, etc. This was
> done by not using generic_write_check_limits() and instead writing a
> zonefs_check_write() function that is zone append friendly.
>
> We can post that as a base for the discussion on semantic if you want...

There is no problem in about how-to-do-it. That part is simple - we
have the iocb, and sync/async can be known whether ki_complete
callback is set.
This point to be discussed was whether-to-allow-short-write-or-not if
we are talking about a generic file-append-returning-location.

That said, since we are talking about moving to indirect-offset in
io-uring, short-write is not an issue anymore I suppose (it goes back
to how it was).
But the unsettled thing is -  whether we can use O/RWF_APPEND with
indirect-offset (pointer) scheme.

> > This was one of the reason why we chose to isolate the operation by a
> > different IOCB flag and not by IOCB_APPEND alone.
>
> For zonefs, the plan is:
> * For the sync write case, zone append is always used.
> * For the async write case, if we see IOCB_APPEND, then zone append BIOs are
> used. If not, regular write BIOs are used.
>
> Simple enough I think. No need for a new flag.

Maybe simple if we only think of ZoneFS (how user-space sends
async-append and gets result is a common problem).
Add Block I/O in scope -  it gets slightly more complicated because it
has to cater to non-zoned devices. And there already is a
well-established understanding that append does nothing...so  code
like "if (flags & IOCB_APPEND) { do something; }" in block I/O path
may surprise someone resuming after a hiatus.
Add File I/O in scope - It gets further complicated. I think it would
make sense to make it opt-in rather than compulsory, but most of them
already implement a behavior for IOCB_APPEND. How to make it opt-in
without new flags.

New flags (FMODE_SOME_NAME, IOCB_SOME_NAME) serve that purpose.
Please assess the need (for isolation) considering all three cases.