linux-ext4 - Re: [PATCH 2/2] ext2: Add locking for DAX faults

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Message-ID: <20151012214135.GA24720@linux.intel.com>
Date:	Mon, 12 Oct 2015 15:41:35 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>,
	linux-ext4@...r.kernel.org,
	Dan Williams <dan.j.williams@...el.com>,
	linux-nvdimm@...ts.01.org,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Andreas Dilger <andreas.dilger@...el.com>
Subject: Re: [PATCH 2/2] ext2: Add locking for DAX faults

On Mon, Oct 12, 2015 at 10:14:43AM +1100, Dave Chinner wrote:
> On Fri, Oct 09, 2015 at 04:02:08PM -0600, Ross Zwisler wrote:
> > Add locking to ensure that DAX faults are isolated from ext2 operations
> > that modify the data blocks allocation for an inode.  This is intended to
> > be analogous to the work being done in XFS by Dave Chinner:
> > 
> > http://www.spinics.net/lists/linux-fsdevel/msg90260.html
> > 
> > Compared with XFS the ext2 case is greatly simplified by the fact that ext2
> > already allocates and zeros new blocks before they are returned as part of
> > ext2_get_block(), so DAX doesn't need to worry about getting unmapped or
> > unwritten buffer heads.
> > 
> > This means that the only work we need to do in ext2 is to isolate the DAX
> > faults from inode block allocation changes.  I believe this just means that
> > we need to isolate the DAX faults from truncate operations.
> 
> Why limit this just to DAX page faults?

Yep, I see that XFS uses the same locking to protect both DAX and non-DAX
faults.  I'll add this protection to non-DAX ext2 faults as well.

One quick question - it looks like that dax_pmd_fault() only grabs the
pagefault lock and updates the file_update_time() if the FAULT_WRITE_FLAG is
set. In xfs_filemap_pfn_mkwrite(), though, these two steps are taken for read
faults as well.  Is this intentional?
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