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

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Message-ID: <20151012230237.GG27164@dastard>
Date:	Tue, 13 Oct 2015 10:02:37 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	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 11:21:56AM -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:
> <>
> > > +/*
> > > + * The lock ordering for ext2 DAX fault paths is:
> > > + *
> > > + * mmap_sem (MM)
> > > + *   ext2_inode_info->dax_sem
> > > + *     sb_start_pagefault (vfs, freeze - taken in DAX)
> > > + *       address_space->i_mmap_rwsem or page_lock (mutually exclusive in DAX)
> > > + *         ext2_inode_info->truncate_mutex
> > 
> > This is a different lock order to XFS - it puts the i_mmaplock
> > inside sb_start_pagefault(), not outside it. This ordering means the
> > timestamp updates during the page fault are also under
> > ext2_inode_info->dax_sem...
> 
> Yep - I was trying not not open code dax_fault() yet again, but it looks like
> both XFS and ext4 both open-code dax_fault() and call __dax_fault() directly.
> 
> I assume that when we get an analogous lock to i_mmaplock in the ext4 fault
> path we'll end up with the same locking order that is found in XFS.
> 
> I'll do the same for v2 of this patch and I'll kill the then-unused
> dax_fault().

Great!

I'd suggest that you also then rename __dax_fault() to dax_fault()
in that case, and also document the requirements for callers in
terms of sb_start_pagefault() and timestamp updates...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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