From: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
Subject: Re: [PATCH v5 15/17] dax: add struct iomap based DAX PMD support
Date: Tue, 11 Oct 2016 15:48:17 -0600
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Cc: Theodore Ts'o <tytso-3s7WtUTddSA@public.gmane.org>, Matthew Wilcox <mawilcox-0li6OtcxBFHby3iVrkZq2A@public.gmane.org>,
 Dave Chinner <david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org>, linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org,
 linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org,
 Andreas Dilger <adilger.kernel-m1MBpc4rdrD3fQ9qLvQP4Q@public.gmane.org>,
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 linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
 Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
To: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
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On Mon, Oct 10, 2016 at 05:59:17PM +0200, Christoph Hellwig wrote:
> On Fri, Oct 07, 2016 at 03:09:02PM -0600, Ross Zwisler wrote:
> > -	if (RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD)
> > +	if ((unsigned long)entry & RADIX_DAX_PMD)
> 
> Please introduce a proper inline helper that mask all the possible type
> bits out of the radix tree entry, and use them wherever you do the
> open cast.

Yea, this is messy.  I tried having temporary flags, but that's basically
where we came from with the old 'type' thing which used to be
RADIX_DAX_PTE|RADIX_DAX_PMD.  

After playing with it a bit it seems like the cleanest way is to have a little
flag test helper, like this:

static int dax_flag_test(void *entry, int flags)
{
	return (unsigned long)entry & flags;
}

...

		/*
		 * Besides huge zero pages the only other thing that gets
		 * downgraded are empty entries which don't need to be
		 * unmapped.
		 */
		if (pmd_downgrade && dax_flag_test(entry, RADIX_DAX_HZP))
			unmap_mapping_range(mapping,
				(index << PAGE_SHIFT) & PMD_MASK, PMD_SIZE, 0);

etc.   Please let me know if this is undesirable for some reason.  Vs keeping
the flags in a local variable, this is good because a) it doesn't require
callers to cast, and b) it makes operator precedence easy because the flags
are a param, so no "flags & (flag1|flag2)" nesting, and c) we don't keep a
local variable that could get out of sync with 'entry'.

> >  restart:
> >  	spin_lock_irq(&mapping->tree_lock);
> >  	entry = get_unlocked_mapping_entry(mapping, index, &slot);
> > +
> > +	if (entry) {
> > +		if (size_flag & RADIX_DAX_PMD) {
> > +			if (!radix_tree_exceptional_entry(entry) ||
> > +			    !((unsigned long)entry & RADIX_DAX_PMD)) {
> > +				entry = ERR_PTR(-EEXIST);
> > +				goto out_unlock;
> > +			}
> > +		} else { /* trying to grab a PTE entry */
> > +			if (radix_tree_exceptional_entry(entry) &&
> > +			    ((unsigned long)entry & RADIX_DAX_PMD) &&
> > +			    ((unsigned long)entry &
> > +			     (RADIX_DAX_HZP|RADIX_DAX_EMPTY))) {
> 
> And when we do these cases N times next to each other we should
> have a local variable the valid flag bits of entry.