linux-ext4 - Re: [PATCH v2 0/2] DAX bdev fixes

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Date:	Fri, 12 Feb 2016 21:59:12 -0700
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	Theodore Ts'o <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Jan Kara <jack@...e.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvdimm@...ts.01.org, xfs@....sgi.com
Subject: Re: [PATCH v2 0/2] DAX bdev fixes - move flushing calls to FS

On Sat, Feb 13, 2016 at 01:38:49PM +1100, Dave Chinner wrote:
> On Fri, Feb 12, 2016 at 12:03:20PM -0700, Ross Zwisler wrote:
> > On Thu, Feb 11, 2016 at 01:43:04PM +0100, Jan Kara wrote:
> > > On Wed 10-02-16 13:48:54, Ross Zwisler wrote:
> > > > 3) In filemap_write_and_wait() and filemap_write_and_wait_range(), continue
> > > > the writeback in the case that DAX is enabled but we only have a nonzero
> > > > mapping->nrpages.  As with 1) and 2), I believe this is necessary to
> > > > properly writeback metadata changes.  If this sounds wrong, please let me
> > > > know and I'll get more info.
> > > 
> > > And I'm surprised here as well. If there are dax_mapping() inodes that have
> > > pagecache pages, then we have issues with radix tree handling as well. So
> > > how come dax_mapping() inodes have pages attached? If it is about block
> > > device inodes, then I find it buggy, that S_DAX gets set for such inodes
> > > when filesystem is mounted on them because in such cases we are IMO asking
> > > for data corruption sooner rather than later...
> > 
> > I think I've figured this one out, at least partially.
> > 
> > For ext2 the issues I was seeing were due to the fact that directory inodes
> > have S_DAX set, but have dirty page cache pages.   In testing with
> > generic/002, I see two ext2 inodes with S_DAX trying to do a writeback while
> > they have dirty page cache pages.  The first has i_ino=2, which is the
> > EXT2_ROOT_INO.
> ....
> > As far as I can see, XFS does not have these issues - returning immediately
> > having done just the DAX writeback in xfs_vm_writepages() lets all my xfstests
> > pass.
> 
> XFS will not have issues because it does not dirty directory inodes
> at the VFS level, nor does it use the page cache for directory data.
> However, looking at the code I think it does still set S_DAX on
> directory inodes, which it shouldn't be doing.
> 
> I've got a couple of fixes I need to do in this area - hopefully
> I'll get it done on Monday.

Cool.  I've got a quick patch that stops S_DAX from being set on everything
but regular inodes for ext2 and ext4.  This solved a lot of my xfstests
failures.

Even after that I'm seeing two last failures with ext4 - I'll keep working on
those.

- Ross
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