From: Dave Chinner <david@fromorbit.com>
To: Ross Zwisler <ross.zwisler@linux.intel.com>,
Jan Kara <jack@suse.cz>,
linux-kernel@vger.kernel.org,
"J. Bruce Fields" <bfields@fieldses.org>,
Theodore Ts'o <tytso@mit.edu>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Andrew Morton <akpm@linux-foundation.org>,
Dan Williams <dan.j.williams@intel.com>, Jan Kara <jack@suse.com>,
Jeff Layton <jlayton@poochiereds.net>,
Jens Axboe <axboe@kernel.dk>,
Matthew Wilcox <willy@linux.intel.com>,
linux-block@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
linux-nvdimm@lists.01.org, xfs@oss.sgi.com
Subject: Re: [PATCH v3 3/6] ext4: Online defrag not supported with DAX
Date: Thu, 18 Feb 2016 11:12:23 +1100 [thread overview]
Message-ID: <20160218001223.GJ19486@dastard> (raw)
In-Reply-To: <20160217215037.GB30126@linux.intel.com>
On Wed, Feb 17, 2016 at 02:50:37PM -0700, Ross Zwisler wrote:
> On Tue, Feb 16, 2016 at 08:34:16PM -0700, Ross Zwisler wrote:
> > Online defrag operations for ext4 are hard coded to use the page cache.
> > See ext4_ioctl() -> ext4_move_extents() -> move_extent_per_page()
> >
> > When combined with DAX I/O, which circumvents the page cache, this can
> > result in data corruption. This was observed with xfstests ext4/307 and
> > ext4/308.
> >
> > Fix this by only allowing online defrag for non-DAX files.
>
> Jan,
>
> Thinking about this a bit more, it's probably the case that the data
> corruption I was observing was due to us skipping the writeback of the dirty
> page cache pages because S_DAX was set.
>
> I do think we have a problem with defrag because it is doing the extent
> swapping using the page cache, and we won't flush the dirty pages due to
> S_DAX being set.
>
> This patch is the quick and easy answer, and is perhaps appropriate for v4.5.
>
> Looking forward, though, what do you think the correct solution is? Making an
> extent swapper that doesn't use the page cache (as I believe XFS has? see
> xfs_swap_extents()),
XFS does the data copy in userspace using direct IO so we don't
care about whether DAX is enabled or not on either the source or
destination inode. i.e. xfs_swap_extents() is a pure
metadata operation, swapping the entire extent tree between two
inodes if the source data has not changed while the copy was in
progress.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2016-02-18 0:12 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-17 3:34 [PATCH v3 0/6] DAX fixes, move flushing calls to FS Ross Zwisler
2016-02-17 3:34 ` [PATCH v3 1/6] block: disable block device DAX by default Ross Zwisler
2016-02-17 21:55 ` Jan Kara
2016-02-17 3:34 ` [PATCH v3 2/6] ext2, ext4: only set S_DAX for regular inodes Ross Zwisler
2016-02-17 21:33 ` Jan Kara
2016-02-17 3:34 ` [PATCH v3 3/6] ext4: Online defrag not supported with DAX Ross Zwisler
2016-02-17 21:34 ` Jan Kara
2016-02-17 21:50 ` Ross Zwisler
2016-02-17 22:10 ` Jan Kara
2016-02-18 0:12 ` Dave Chinner [this message]
2016-02-17 3:34 ` [PATCH v3 4/6] dax: give DAX clearing code correct bdev Ross Zwisler
2016-02-17 21:37 ` Jan Kara
2016-02-17 3:34 ` [PATCH v3 5/6] dax: move writeback calls into the filesystems Ross Zwisler
2016-02-17 3:34 ` [PATCH v3 6/6] block: use dax_do_io() if blkdev_dax_capable() Ross Zwisler
2016-02-17 21:54 ` Jan Kara
2016-02-17 22:18 ` Dan Williams
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