linux-kernel - Re: [PATCH] mm/zsmalloc: avoid duplicate assignment of prev

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Date:	Thu, 20 Nov 2014 22:22:19 -0500
From:	Dan Streetman <ddstreet@...e.org>
To:	Mahendran Ganesh <opensource.ganesh@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>, Nitin Gupta <ngupta@...are.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/zsmalloc: avoid duplicate assignment of prev_class

On Thu, Nov 20, 2014 at 8:08 AM, Mahendran Ganesh
<opensource.ganesh@...il.com> wrote:
> In zs_create_pool(), prev_class is assigned (ZS_SIZE_CLASSES - 1)
> times. And the prev_class only references to the previous alloc
> size_class. So we do not need unnecessary assignement.
>
> This patch modifies *prev_class* to *prev_alloc_class*. And the
> *prev_alloc_class* will only be assigned when a new size_class
> structure is allocated.
>
> Signed-off-by: Mahendran Ganesh <opensource.ganesh@...il.com>
> ---
>  mm/zsmalloc.c |    9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index b3b57ef..ac2b396 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -970,7 +970,7 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>                 int size;
>                 int pages_per_zspage;
>                 struct size_class *class;
> -               struct size_class *prev_class;
> +               struct size_class *uninitialized_var(prev_alloc_class);

+               struct size_class *prev_class = NULL;

Use the fact it's unset below, so set it to NULL here

>
>                 size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
>                 if (size > ZS_MAX_ALLOC_SIZE)
> @@ -987,9 +987,8 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>                  * previous size_class if possible.
>                  */
>                 if (i < ZS_SIZE_CLASSES - 1) {
> -                       prev_class = pool->size_class[i + 1];
> -                       if (can_merge(prev_class, size, pages_per_zspage)) {
> -                               pool->size_class[i] = prev_class;
> +                       if (can_merge(prev_alloc_class, size, pages_per_zspage)) {
> +                               pool->size_class[i] = prev_alloc_class;

simplify more, we can check if this is the first iteration by looking
at prev_class, e.g.:

                if (prev_class) {
                       if (can_merge(prev_class, size, pages_per_zspage)) {
                               pool->size_class[i] = prev_class;


>                                 continue;
>                         }
>                 }
> @@ -1003,6 +1002,8 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>                 class->pages_per_zspage = pages_per_zspage;
>                 spin_lock_init(&class->lock);
>                 pool->size_class[i] = class;
> +
> +               prev_alloc_class = class;
>         }
>
>         pool->flags = flags;
> --
> 1.7.9.5
>
> --
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