linux-ext4 - Re: [PATCH v5 15/17] dax: add struct iomap based DAX PMD support

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Date:   Tue, 11 Oct 2016 15:48:17 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        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>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-nvdimm@...ts.01.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH v5 15/17] dax: add struct iomap based DAX PMD support

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.
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