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Subject: Re: [PATCH] kmalloc_index optimization(add kmalloc max size check)
To:     Bernard Zhao <bernard@vivo.com>, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Cc:     kernel@vivo.com
References: <1587107376-111722-1-git-send-email-bernard@vivo.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <05bedc84-3672-975c-87ee-f094ca766ee8@suse.cz>
Date:   Tue, 21 Apr 2020 16:15:33 +0200
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On 4/17/20 9:09 AM, 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.
> 
> Signed-off-by: Bernard Zhao <bernard@vivo.com>

kmalloc_index() is only called from kmalloc() and kmalloc_node() for
compile-time constant sizes up to KMALLOC_MAX_CACHE_SIZE, which is smaller
(SLUB, SLOB) or equal (SLAB) than KMALLOC_MAX_SIZE. So your patch is effectively
a no-op and we better shouldn't tease the compiler too much, so that
kmalloc_index() stays fully compile-time evaluated.

> ---
>  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;
>  
>