linux-kernel - Re: [PATCH v2] mm/mempool: Add 'else' to split mutually exclusive case

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Date:   Thu, 24 Sep 2020 19:40:51 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm/mempool: Add 'else' to split mutually exclusive
 case

On Thu, 24 Sep 2020 07:16:41 -0400 Miaohe Lin <linmiaohe@...wei.com> wrote:

> Add else to split mutually exclusive case and avoid some unnecessary check.
> It doesn't seem to change code generation (compiler is smart), but I think
> it helps readability.
> 
> ...
>
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -58,11 +58,10 @@ static void __check_element(mempool_t *pool, void *element, size_t size)
>  static void check_element(mempool_t *pool, void *element)
>  {
>  	/* Mempools backed by slab allocator */
> -	if (pool->free == mempool_free_slab || pool->free == mempool_kfree)
> +	if (pool->free == mempool_free_slab || pool->free == mempool_kfree) {
>  		__check_element(pool, element, ksize(element));
> -
>  	/* Mempools backed by page allocator */
> -	if (pool->free == mempool_free_pages) {
> +	} else if (pool->free == mempool_free_pages) {
>  		int order = (int)(long)pool->pool_data;
>  		void *addr = kmap_atomic((struct page *)element);
>  
> @@ -82,11 +81,10 @@ static void __poison_element(void *element, size_t size)
>  static void poison_element(mempool_t *pool, void *element)
>  {
>  	/* Mempools backed by slab allocator */
> -	if (pool->alloc == mempool_alloc_slab || pool->alloc == mempool_kmalloc)
> +	if (pool->alloc == mempool_alloc_slab || pool->alloc == mempool_kmalloc) {
>  		__poison_element(element, ksize(element));
> -
>  	/* Mempools backed by page allocator */
> -	if (pool->alloc == mempool_alloc_pages) {
> +	} else if (pool->alloc == mempool_alloc_pages) {
>  		int order = (int)(long)pool->pool_data;
>  		void *addr = kmap_atomic((struct page *)element);
>  

OK, I guess.  But the comments are now in the wrong place.

--- a/mm/mempool.c~mm-mempool-add-else-to-split-mutually-exclusive-case-fix
+++ a/mm/mempool.c
@@ -60,8 +60,8 @@ static void check_element(mempool_t *poo
 	/* Mempools backed by slab allocator */
 	if (pool->free == mempool_free_slab || pool->free == mempool_kfree) {
 		__check_element(pool, element, ksize(element));
-	/* Mempools backed by page allocator */
 	} else if (pool->free == mempool_free_pages) {
+		/* Mempools backed by page allocator */
 		int order = (int)(long)pool->pool_data;
 		void *addr = kmap_atomic((struct page *)element);
 
@@ -83,8 +83,8 @@ static void poison_element(mempool_t *po
 	/* Mempools backed by slab allocator */
 	if (pool->alloc == mempool_alloc_slab || pool->alloc == mempool_kmalloc) {
 		__poison_element(element, ksize(element));
-	/* Mempools backed by page allocator */
 	} else if (pool->alloc == mempool_alloc_pages) {
+		/* Mempools backed by page allocator */
 		int order = (int)(long)pool->pool_data;
 		void *addr = kmap_atomic((struct page *)element);
 
_