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

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Date:	Tue, 13 Oct 2015 11:33:20 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dave Chinner <david@...morbit.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 Tue, Oct 13, 2015 at 10:07:12AM +0200, Jan Kara wrote:
> On Mon 12-10-15 15:41:35, Ross Zwisler wrote:
> > 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.
> 
> Actually, since ext2 driver doesn't support punch hole, there is no need
> for additional locking in non-DAX paths. So we can save some space in inode
> and locking for that common case. So I'd prefer if we didn't add
> unnecessary locking in those paths and just document that for non-DAX
> faults using page lock and i_size check is enough. After all the main
> usecase of ext2 driver these days is for people with tiny devices...

Based on this comment I'm assuming you'd like the definition of dax_sem in
struct ext2_inode_info to be conditional like ext2_inode_info->xattr_sem,
correct?
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