linux-ext4 - Re: [PATCH 5/7] dax: lock based on slot instead of [mapping, index]

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Date:	Thu, 18 Aug 2016 16:15:50 +0200
From:	Jan Kara <jack@...e.cz>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	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>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 5/7] dax: lock based on slot instead of [mapping, index]

On Wed 17-08-16 14:25:56, Ross Zwisler wrote:
> On Tue, Aug 16, 2016 at 11:28:16AM +0200, Jan Kara wrote:
> > On Mon 15-08-16 13:09:16, Ross Zwisler wrote:
> > > DAX radix tree locking currently locks entries based on the unique
> > > combination of the 'mapping' pointer and the pgoff_t 'index' for the entry.
> > > This works for PTEs, but as we move to PMDs we will need to have all the
> > > offsets within the range covered by the PMD to map to the same bit lock.
> > > To accomplish this, lock based on the 'slot' pointer in the radix tree
> > > instead of [mapping, index].
> > 
> > I'm not convinced this is safe. What makes the slot pointer still valid
> > after you drop tree_lock? At least radix_tree_shrink() or
> > radix_tree_expand() could move your slot without letting the waiter know
> > and he would be never woken.
> > 
> > 								Honza
> 
> Yep, you're right, thanks for catching that.
> 
> Given that we can't rely on 'slot' being stable, my next idea is to use a
> combination of [mapping, index], but tweak 'index' so that it's always the
> beginning of the entry.  So for 4k entries we'd leave it alone, but for 2MiB
> entries we'd mask it down to the appropriate 2MiB barrier.
> 
> Let me hack on that for a bit, unless you've a better idea.

No, that's what I'd do as well.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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