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

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Date:   Fri, 21 Oct 2016 09:33:08 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Johannes Weiner <hannes@...xchg.org>
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

On Wed, Oct 19, 2016 at 01:24:26PM -0400, Johannes Weiner wrote:
> Add an internal tag to identify special entries that are accounted in
> node->special in addition to node->count.
> 
> 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.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> ---
>  include/linux/radix-tree.h | 10 ++++++----
>  lib/radix-tree.c           | 14 ++++++++++----
>  2 files changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
> index 756b2909467e..2e1c9added23 100644
> --- a/include/linux/radix-tree.h
> +++ b/include/linux/radix-tree.h
> @@ -68,7 +68,8 @@ enum radix_tree_tags {
>  	/* Freely allocatable radix tree user tags */
>  	RADIX_TREE_NR_USER_TAGS = 3,
>  	/* Radix tree internal tags */
> -	RADIX_TREE_NR_TAGS = RADIX_TREE_NR_USER_TAGS,
> +	RADIX_TREE_TAG_SPECIAL = RADIX_TREE_NR_USER_TAGS,
> +	RADIX_TREE_NR_TAGS,
>  };
>  
>  #ifndef RADIX_TREE_MAP_SHIFT
> @@ -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 */
.....

Cheers,

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