linux-kernel - Re: [PATCH] kmalloc_index optimization(add kmalloc max size check)

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Message-ID: <20200417113928.GL26707@dhcp22.suse.cz>
Date:   Fri, 17 Apr 2020 13:39:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Bernard Zhao <bernard@...o.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel@...o.com
Subject: Re: [PATCH] kmalloc_index optimization(add kmalloc max size check)

On Fri 17-04-20 00:09:35, Bernard Zhao wrote:
> kmalloc size should never exceed KMALLOC_MAX_SIZE.
> kmalloc_index realise if size is exceed KMALLOC_MAX_SIZE, e.g 64M,
> kmalloc_index just return index 26, but never check with OS`s max
> kmalloc config KMALLOC_MAX_SIZE. This index`s kmalloc caches maybe
> not create in function create_kmalloc_caches.
> We can throw an warninginfo in kmalloc at the beginning, instead of
> being guaranteed by the buddy alloc behind.

I am sorry but I do not follow. What does this patch optimizes? AFAICS,
it adds a branch for everybody for something that is highly unlikely
usage. Btw. we already do handle those impossible cases. We could argue
that BUG() is a bit harsh reaction but a lack of reports suggests this
is not a real problem in fact.

So what exactly do you want to achieve here?

> Signed-off-by: Bernard Zhao <bernard@...o.com>
> ---
>  include/linux/slab.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 6d45488..59b60d2 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -351,6 +351,10 @@ static __always_inline unsigned int kmalloc_index(size_t size)
>  	if (!size)
>  		return 0;
>  
> +	/* size should never exceed KMALLOC_MAX_SIZE. */
> +	if (size > KMALLOC_MAX_SIZE)
> +		WARN(1, "size exceed max kmalloc size!\n");
> +
>  	if (size <= KMALLOC_MIN_SIZE)
>  		return KMALLOC_SHIFT_LOW;
>  
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs