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Date:   Fri, 14 Aug 2020 09:14:11 +0100
From:   "hch@infradead.org" <hch@infradead.org>
To:     Damien Le Moal <Damien.LeMoal@wdc.com>
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Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append
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On Wed, Aug 05, 2020 at 07:35:28AM +0000, Damien Le Moal wrote:
> > the write pointer.  The only interesting addition is that we also want
> > to report where we wrote.  So I'd rather have RWF_REPORT_OFFSET or so.
> 
> That works for me. But that rules out having the same interface for raw block
> devices since O_APPEND has no meaning in that case. So for raw block devices, it
> will have to be through zonefs. That works for me, and I think it was your idea
> all along. Can you confirm please ?

Yes.  I don't think think raw syscall level access to the zone append
primitive makes sense.  Either use zonefs for a file-like API, or
use the NVMe pass through interface for 100% raw access.

> >  - take the exclusive per-inode (zone) lock and just issue either normal
> >    writes or zone append at your choice, relying on the lock to
> >    serialize other writers.  For the async case this means we need a
> >    lock than can be release in a different context than it was acquired,
> >    which is a little ugly but can be done.
> 
> Yes, that would be possible. But likely, this will also need calls to
> inode_dio_wait() to avoid ending up with a mix of regular write and zone append
> writes in flight (which likely would result in the regular write failing as the
> zone append writes would go straight to the device without waiting for the zone
> write lock like regular writes do).

inode_dio_wait is a really bad implementation of almost a lock.  I've
started some work that I need to finish to just replace it with proper
non-owner rwsems (or even the range locks Dave has been looking into).

> 
> This all sound sensible to me. One last point though, specific to zonefs: if the
> user opens a zone file with O_APPEND, I do want to have that necessarily mean
> "use zone append". And same for the "RWF_REPORT_OFFSET". The point here is that
> both O_APPEND and RWF_REPORT_OFFSET can be used with both regular writes and
> zone append writes, but none of them actually clearly specify if the
> application/user tolerates writing data to disk in a different order than the
> issuing order... So another flag to indicate "atomic out-of-order writes" (==
> zone append) ?

O_APPEND pretty much implies out of order, as there is no way for an
application to know which thread wins the race to write the next chunk.