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From: Matthew Bobrowski <mbobrowski@mbobrowski.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: 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,
	david@fromorbit.com, darrick.wong@oracle.com
Subject: Re: [PATCH v3 5/6] ext4: introduce direct IO write path using iomap infrastructure
Date: Tue, 17 Sep 2019 22:39:01 +1000	[thread overview]
Message-ID: <20190917123901.GB17286@bobrowski> (raw)
In-Reply-To: <20190917090233.GB29487@infradead.org>

On Tue, Sep 17, 2019 at 02:02:33AM -0700, Christoph Hellwig wrote:
> On Tue, Sep 17, 2019 at 02:30:15PM +0530, Ritesh Harjani wrote:
> > So if we have a delayed buffered write to a file,
> > in that case we first only update inode->i_size and update
> > i_disksize at writeback time
> > (i.e. during block allocation).
> > In that case when we call for ext4_dio_write_iter
> > since offset + len > i_disksize, we call for ext4_update_i_disksize().
> > 
> > Now if writeback for some reason failed. And the system crashes, during the
> > DIO writes, after the blocks are allocated. Then during reboot we may have
> > an inconsistent inode, since we did not add the inode into the
> > orphan list before we updated the inode->i_disksize. And journal replay
> > may not succeed.
> > 
> > 1. Can above actually happen? I am still not able to figure out the
> >    race/inconsistency completely.
> > 2. Can you please help explain under what other cases
> >    it was necessary to call ext4_update_i_disksize() in DIO write paths?
> > 3. When will i_disksize be out-of-sync with i_size during DIO writes?
> 
> None of the above seems new in this patchset, does it?

That's correct.

*Ritesh - FWIW, I think you'll find the answers to your questions above by
 referring to the following commits:

 1) 73fdad00b208b
 2) 45d8ec4d9fd54

If you drop the check (offset + count > EXT4_I(inode)->i_disksize) and the
call to ext4_update_i_disksize(), under some workloads i.e. "generic/475"
you'll generally end up with metadata corruption.

> That being said I found the early size update odd. XFS updates the on-disk
> size only at I/O completion time to deal with various races including the
> potential exposure of stale data.

Indeed a little odd. But, I think delalloc/writeback implementation is
possibly to blame here (based on what's detailed in 45d8ec4d9fd54)? Ideally, I
had the same approach as XFS in mind, but I couldn't do it.

--<M>--

  parent reply	other threads:[~2019-09-17 12:39 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-12 11:03 [PATCH v3 0/6] ext4: port direct IO to iomap infrastructure Matthew Bobrowski
2019-09-12 11:03 ` [PATCH v3 1/6] ext4: introduce direct IO read path using " Matthew Bobrowski
2019-09-16 12:00   ` Christoph Hellwig
2019-09-16 13:07     ` Matthew Bobrowski
2019-09-12 11:04 ` [PATCH v3 2/6] ext4: move inode extension/truncate code out from ext4_iomap_end() Matthew Bobrowski
2019-09-23 16:21   ` Jan Kara
2019-09-24  9:50     ` Matthew Bobrowski
2019-09-24 13:13     ` Jan Kara
2019-09-12 11:04 ` [PATCH v3 3/6] iomap: split size and error for iomap_dio_rw ->end_io Matthew Bobrowski
2019-09-12 11:04 ` [PATCH v3 4/6] ext4: reorder map.m_flags checks in ext4_iomap_begin() Matthew Bobrowski
2019-09-16 12:05   ` Christoph Hellwig
2019-09-17 12:48     ` Matthew Bobrowski
2019-09-23 15:08   ` Jan Kara
2019-09-24  9:35     ` Matthew Bobrowski
2019-09-12 11:04 ` [PATCH v3 5/6] ext4: introduce direct IO write path using iomap infrastructure Matthew Bobrowski
2019-09-16  4:37   ` Ritesh Harjani
2019-09-16 10:14     ` Matthew Bobrowski
2019-09-16 12:12   ` Christoph Hellwig
2019-09-16 22:37     ` Matthew Bobrowski
2019-09-17  9:00       ` Ritesh Harjani
2019-09-17  9:02         ` Christoph Hellwig
2019-09-17 10:12           ` Ritesh Harjani
2019-09-17 12:39           ` Matthew Bobrowski [this message]
2019-09-24 10:57         ` Jan Kara
2019-09-17  9:06       ` Christoph Hellwig
2019-09-17 11:31         ` Matthew Bobrowski
2019-09-20 13:24         ` Matthew Bobrowski
2019-09-23 21:10   ` Jan Kara
2019-09-24 10:29     ` Matthew Bobrowski
2019-09-24 14:13       ` Jan Kara
2019-09-25  7:14         ` Matthew Bobrowski
2019-09-25  8:40           ` Jan Kara
2019-09-12 11:05 ` [PATCH v3 6/6] ext4: cleanup legacy buffer_head direct IO code Matthew Bobrowski
2019-09-16 12:06   ` Christoph Hellwig
2019-09-16 12:53     ` Matthew Bobrowski

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