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From: Kanchan Joshi <joshi.k@samsung.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: axboe@kernel.dk, viro@zeniv.linux.org.uk, bcrl@kvack.org,
	asml.silence@gmail.com, Damien.LeMoal@wdc.com,
	linux-fsdevel@vger.kernel.org, mb@lightnvm.io,
	linux-kernel@vger.kernel.org, linux-aio@kvack.org,
	io-uring@vger.kernel.org, linux-block@vger.kernel.org,
	selvakuma.s1@samsung.com, nj.shetty@samsung.com,
	javier.gonz@samsung.com, Arnav Dawn <a.dawn@samsung.com>
Subject: Re: [PATCH v2 1/2] fs,block: Introduce RWF_ZONE_APPEND and handling in direct IO path
Date: Sat, 27 Jun 2020 02:45:14 +0530
Message-ID: <20200626211514.GA24762@test-zns> (raw)
In-Reply-To: <20200626085846.GA24962@infradead.org>


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On Fri, Jun 26, 2020 at 09:58:46AM +0100, Christoph Hellwig wrote:
>To restate my previous NAK:
>
>A low-level protocol detail like RWF_ZONE_APPEND has absolutely no
>business being exposed in the Linux file system interface.
>
>And as mentioned before I think the idea of returning the actual
>position written for O_APPEND writes totally makes sense, and actually
>is generalizable to all files.  Together with zonefs that gives you a
>perfect interface for zone append.
>
>On Thu, Jun 25, 2020 at 10:45:48PM +0530, Kanchan Joshi wrote:
>> Introduce RWF_ZONE_APPEND flag to represent zone-append.
>
>And no one but us select few even know what zone append is, nevermind
>what the detailed semantics are.  If you add a userspace API you need
>to very clearly document the semantics inluding errors and corner cases.

For block IO path (which is the scope of this patchset) there is no
probelm in using RWF_APPEND for zone-append, because it does not do
anything for block device. We can use that, avoiding introduction of
RWF_ZONE_APPEND in user-space.

In kernel, will it be fine to keep IOCB_ZONE_APPEND apart from
IOCB_APPEND? Reason being, this can help to isolate the code meant only
for zone-append from the one that is already present for conventional
append.

Snippet from quick reference -

static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
        ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
        if (flags & RWF_APPEND)
                ki->ki_flags |= IOCB_APPEND;
+       if (flags & RWF_ZONE_APPEND) {
+               /* currently support block device only */
+               umode_t mode = file_inode(ki->ki_filp)->i_mode;
+
+               if (!(S_ISBLK(mode)))
+                       return -EOPNOTSUPP;
+               ki->ki_flags |= IOCB_ZONE_APPEND;
+       }


As for file I/O in future, I see a potential problem with RWF_APPEND.
In io_uring, zone-append requires bit of pre/post processing, which
ideally should be done only for zone-append case. A ZoneFS file using
RWF_APPEND as a mean to invoke zone-append vs a regular file (hosted on
some other FS) requiring conventional RWF_APPEND - both will execute
that processing.
Is there a good way to differentiate ZoneFS file from another file which
only wants use conventional file-append?

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  reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20200625171829epcas5p268486a0780571edb4999fc7b3caab602@epcas5p2.samsung.com>
2020-06-25 17:15 ` [PATCH v2 0/2] zone-append support in io-uring and aio Kanchan Joshi
     [not found]   ` <CGME20200625171834epcas5p226a24dfcb84cfa83fe29a2bd17795d85@epcas5p2.samsung.com>
2020-06-25 17:15     ` [PATCH v2 1/2] fs,block: Introduce RWF_ZONE_APPEND and handling in direct IO path Kanchan Joshi
2020-06-26  2:50       ` Damien Le Moal
2020-06-29 18:32         ` Kanchan Joshi
2020-06-30  0:37           ` Damien Le Moal
2020-06-30  7:40             ` Kanchan Joshi
2020-06-30  7:52               ` Damien Le Moal
2020-06-30  7:56                 ` Damien Le Moal
2020-06-30  8:16                   ` Kanchan Joshi
2020-06-26  8:58       ` Christoph Hellwig
2020-06-26 21:15         ` Kanchan Joshi [this message]
2020-06-27  6:51           ` Christoph Hellwig
     [not found]   ` <CGME20200625171838epcas5p449183e12770187142d8d55a9bf422a8d@epcas5p4.samsung.com>
2020-06-25 17:15     ` [PATCH v2 2/2] io_uring: add support for zone-append Kanchan Joshi
2020-06-25 19:40       ` Pavel Begunkov
2020-06-26  3:11   ` [PATCH v2 0/2] zone-append support in io-uring and aio Damien Le Moal
2020-06-26  6:37     ` javier.gonz
2020-06-26  6:56       ` Damien Le Moal
2020-06-26  7:03         ` javier.gonz@samsung.com
2020-06-26 22:15     ` Kanchan Joshi
2020-06-30 12:46   ` Matthew Wilcox

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