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

[Damien Le Moal]

On 2020/07/31 16:59, Kanchan Joshi wrote:
> On Fri, Jul 31, 2020 at 12:29 PM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote:
>>
>> On 2020/07/31 15:45, hch@xxxxxxxxxxxxx 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...

> 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.

-- 
Damien Le Moal
Western Digital Research