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

[Kanchan Joshi <joshiiitr@gmail.com>]

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.
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 block-device these checks are not done, but there is another place
when it receives writes spanning beyond EOF and iov_iter_truncate()
adjusts it before sending it down.
And we return failure for that case in V4-  "Ref: [PATCH v4 3/6] uio:
return status with iov truncation"


-- 
Joshi