linux-kernel - Re: [PATCH v3 4/4] mm, slab_common: Make the loop for initializing KMALLOC

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Message-ID: <23cb75f5-4a05-5901-2085-8aeabc78c100@suse.cz>
Date:   Tue, 10 Sep 2019 12:26:14 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Pengfei Li <lpf.vector@...il.com>, akpm@...ux-foundation.org
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, guro@...com
Subject: Re: [PATCH v3 4/4] mm, slab_common: Make the loop for initializing
 KMALLOC_DMA start from 1

On 9/10/19 3:26 AM, Pengfei Li wrote:
> KMALLOC_DMA will be initialized only if KMALLOC_NORMAL with
> the same index exists.
> 
> And kmalloc_caches[KMALLOC_NORMAL][0] is always NULL.
> 
> Therefore, the loop that initializes KMALLOC_DMA should start
> at 1 instead of 0, which will reduce 1 meaningless attempt.

IMHO the saving of one iteration isn't worth making the code more 
subtle. KMALLOC_SHIFT_LOW would be nice, but that would skip 1 + 2 which 
are special.

Since you're doing these cleanups, have you considered reordering 
kmalloc_info, size_index, kmalloc_index() etc so that sizes 96 and 192 
are ordered naturally between 64, 128 and 256? That should remove 
various special casing such as in create_kmalloc_caches(). I can't 
guarantee it will be possible without breaking e.g. constant folding 
optimizations etc., but seems to me it should be feasible. (There are 
definitely more places to change than those I listed.)

> Signed-off-by: Pengfei Li <lpf.vector@...il.com>
> ---
>   mm/slab_common.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index af45b5278fdc..c81fc7dc2946 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1236,7 +1236,7 @@ void __init create_kmalloc_caches(slab_flags_t flags)
>   	slab_state = UP;
>   
>   #ifdef CONFIG_ZONE_DMA
> -	for (i = 0; i <= KMALLOC_SHIFT_HIGH; i++) {
> +	for (i = 1; i <= KMALLOC_SHIFT_HIGH; i++) {
>   		struct kmem_cache *s = kmalloc_caches[KMALLOC_NORMAL][i];
>   
>   		if (s) {
>