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

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Date:	Tue, 13 Oct 2015 09:02:45 -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 Tue, Oct 13, 2015 at 10:24:28AM +1100, Dave Chinner wrote:
> On Mon, Oct 12, 2015 at 03:41:35PM -0600, 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.
> > 
> > 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?
> 
> xfs_filemap_pfn_mkwrite() should not be called for read faults.
> We've already had to have a fault that maps the page to pfn for us
> to get a pfn based fault, and hence that code is correct.
> 
> Or are you talking about xfs_filemap_pmd_fault()? In which case, I
> refer you to the commit log and it should be obvious that it was
> committed without me even looking at it.  I have another patch in my
> current series for 4.4 that will fix this.

Yep, sorry, I meant xfs_filemap_pmd_fault().  Thanks, I was just making sure the
pattern I'm following in ext2 where I'm only locking the pagefault lock &
updating the file_update_time() on writes was correct.
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