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From: Christoph Hellwig <hch@infradead.org>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Matthew Bobrowski <mbobrowski@mbobrowski.org>,
	Ritesh Harjani <riteshh@linux.ibm.com>,
	tytso@mit.edu, jack@suse.cz, adilger.kernel@dilger.ca,
	linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	hch@infradead.org, aneesh.kumar@linux.ibm.com
Subject: Re: [PATCH 0/5] ext4: direct IO via iomap infrastructure
Date: Sat, 24 Aug 2019 16:04:27 -0700
Message-ID: <20190824230427.GA32012@infradead.org> (raw)
In-Reply-To: <20190824035554.GA1037502@magnolia>

On Fri, Aug 23, 2019 at 08:55:54PM -0700, Darrick J. Wong wrote:
> I'm probably misunderstanding the ext4 extent cache horribly, but I keep
> wondering why any of this is necessary -- why can't ext4 track the
> unwritten status in the extent records directly?  And why is there all
> this strange "can merge" logic?  If you need to convert blocks X to Y
> to written state because a write to those blocks completed, isn't that
> just manipulation of a bunch of incore records?  And can't you just seek
> back and forth in the extent cache to look for adjacent records to merge
> with? <confuseD>

Same here.  I'm not an ext4 expert, but here is what we do in XFS, which
hopefully works in some form for ext4 a well:

 - when starting a direct I/O we allocate any needed blocks and do so
   as unwritten extent.  The extent tree code will merge them in
   whatever way that seems suitable
 - if the IOMAP_DIO_UNWRITTEN is set on the iomap at ->end_io time we
   call a function that walks the whole range covered by the ioend,
   and convert any unwritten extent to a normal written extent.  Any
   splitting and merging will be done as needed by the low-level
   extent tree code
 - this also means we don't need the xfs_ioen structure (which ext4)
   copied from for direct I/O at all (we used to have it initially,
   though including the time when ext4 copied this code).
 - we don't need the equivalent to the ext4_unwritten_wait call in
   ext4_file_write_iter because we serialize any non-aligned I/O
   instead of trying to optimize for weird corner cases


> (I'd really prefer not to go adding private fields all over the
> place...)

Agreed.

  reply index

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-12 12:52 Matthew Bobrowski
2019-08-12 12:52 ` [PATCH 1/5] ext4: introduce direct IO read code path using " Matthew Bobrowski
2019-08-12 17:18   ` Christoph Hellwig
2019-08-12 20:17     ` Matthew Wilcox
2019-08-13 10:45       ` Matthew Bobrowski
2019-08-12 12:52 ` [PATCH 2/5] ext4: move inode extension/truncate code out from ext4_iomap_end() Matthew Bobrowski
2019-08-12 17:18   ` Christoph Hellwig
2019-08-13 10:46     ` Matthew Bobrowski
2019-08-28 19:59   ` Jan Kara
2019-08-28 21:54     ` Matthew Bobrowski
2019-08-29  8:18       ` Jan Kara
2019-08-12 12:53 ` [PATCH 3/5] iomap: modify ->end_io() calling convention Matthew Bobrowski
2019-08-12 17:18   ` Christoph Hellwig
2019-08-13 10:43     ` Matthew Bobrowski
2019-08-12 12:53 ` [PATCH 4/5] ext4: introduce direct IO write code path using iomap infrastructure Matthew Bobrowski
2019-08-12 17:04   ` RITESH HARJANI
2019-08-13 12:58     ` Matthew Bobrowski
2019-08-13 14:35       ` Darrick J. Wong
2019-08-14  9:51         ` Matthew Bobrowski
2019-08-12 17:34   ` Christoph Hellwig
2019-08-13 10:45     ` Matthew Bobrowski
2019-08-28 20:26   ` Jan Kara
2019-08-28 22:32     ` Dave Chinner
2019-08-29  8:03       ` Jan Kara
2019-08-29 11:47       ` Matthew Bobrowski
2019-08-29 11:45     ` Matthew Bobrowski
2019-08-29 12:38       ` Jan Kara
2019-08-12 12:53 ` [PATCH 5/5] ext4: clean up redundant buffer_head direct IO code Matthew Bobrowski
2019-08-12 17:31 ` [PATCH 0/5] ext4: direct IO via iomap infrastructure RITESH HARJANI
2019-08-13 11:10   ` Matthew Bobrowski
2019-08-13 12:27     ` RITESH HARJANI
2019-08-14  9:48       ` Matthew Bobrowski
2019-08-14 11:58         ` RITESH HARJANI
2019-08-21 13:14       ` Matthew Bobrowski
2019-08-22 12:00         ` Matthew Bobrowski
2019-08-22 14:11           ` Ritesh Harjani
2019-08-24  3:18             ` Matthew Bobrowski
2019-08-24  3:55               ` Darrick J. Wong
2019-08-24 23:04                 ` Christoph Hellwig [this message]
2019-08-27  9:52                   ` Matthew Bobrowski
2019-08-28 12:05                     ` Matthew Bobrowski
2019-08-28 14:27                       ` Theodore Y. Ts'o
2019-08-28 18:02                         ` Jan Kara
2019-08-29  6:36                           ` Christoph Hellwig
2019-08-29 11:20                             ` Matthew Bobrowski
2019-08-29 14:41                               ` Christoph Hellwig
2019-08-23 13:43           ` [RFC 1/1] ext4: PoC implementation of option-1 Ritesh Harjani
2019-08-23 13:49             ` Ritesh Harjani

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