linux-kernel - Re: [PATCH 3/5] lib: radix-tree: native accounting and tracking of special entries

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Date:   Mon, 24 Oct 2016 12:01:22 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Dave Jones <davej@...emonkey.org.uk>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 3/5] lib: radix-tree: native accounting and tracking of
 special entries

Hi Dave,

On Fri, Oct 21, 2016 at 09:33:08AM +1100, Dave Chinner wrote:
> On Wed, Oct 19, 2016 at 01:24:26PM -0400, Johannes Weiner wrote:
> > With this in place, the next patch can restore refault detection in
> > single-page files. It will also move the shadow count from the upper
> > bits of count to the new special counter, and then shrink count to a
> > char as well; the growth of struct radix_tree_node is temporary.

[...]

> > @@ -90,9 +91,10 @@ enum radix_tree_tags {
> >  #define RADIX_TREE_COUNT_MASK	((1UL << RADIX_TREE_COUNT_SHIFT) - 1)
> >  
> >  struct radix_tree_node {
> > -	unsigned char	shift;	/* Bits remaining in each slot */
> > -	unsigned char	offset;	/* Slot offset in parent */
> > -	unsigned int	count;
> > +	unsigned char	shift;		/* Bits remaining in each slot */
> > +	unsigned char	offset;		/* Slot offset in parent */
> > +	unsigned int	count;		/* Total entry count */
> > +	unsigned char	special;	/* Special entry count */
> 
> How about putting the new char field into the implicit hole between
> offset and count? pahole is your friend here:
> 
> struct radix_tree_node {
>         unsigned char              shift;                /*     0     1 */
>         unsigned char              offset;               /*     1     1 */
> 
>         /* XXX 2 bytes hole, try to pack */
> 
>         unsigned int               count;                /*     4     4 */
> .....

The next patch turns `count' into an unsigned char again, so the hole
is only temporary.

Thanks