linux-kernel - Re: [PATCH] mm/memory-tier: Fix abstract distance calculation overflow

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Message-ID: <0b9e1e55-f4c9-4e30-abe6-3b9765ba9ebf@linux.ibm.com>
Date: Tue, 10 Jun 2025 13:44:01 +0530
From: Donet Tom <donettom@...ux.ibm.com>
To: Li Zhijian <lizhijian@...itsu.com>, linux-mm@...ck.org
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        Ying Huang <huang.ying.caritas@...il.com>
Subject: Re: [PATCH] mm/memory-tier: Fix abstract distance calculation
 overflow


On 6/10/25 11:57 AM, Li Zhijian wrote:
> In mt_perf_to_adistance(), the calculation of abstract distance (adist)
> involves multiplying several int values including MEMTIER_ADISTANCE_DRAM.
> ```
> *adist = MEMTIER_ADISTANCE_DRAM *
> 		(perf->read_latency + perf->write_latency) /
> 		(default_dram_perf.read_latency + default_dram_perf.write_latency) *
> 		(default_dram_perf.read_bandwidth + default_dram_perf.write_bandwidth) /
> 		(perf->read_bandwidth + perf->write_bandwidth);
> ```
> Since these values can be large, the multiplication may exceed the maximum
> value of an int (INT_MAX) and overflow (Our platform did), leading to an
> incorrect adist.
>
> Change MEMTIER_ADISTANCE_DRAM to be a long constant by writing it with the
> 'L' suffix. This prevents the overflow because the multiplication will then
> be done in the long type which has a larger range.
>
> Fixes: 3718c02dbd4c ("acpi, hmat: calculate abstract distance with HMAT")
> Cc: Ying Huang <huang.ying.caritas@...il.com>
> Signed-off-by: Li Zhijian <lizhijian@...itsu.com>
> ---
>   include/linux/memory-tiers.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
> index 0dc0cf2863e2..7a805796fcfd 100644
> --- a/include/linux/memory-tiers.h
> +++ b/include/linux/memory-tiers.h
> @@ -18,7 +18,7 @@
>    * adistance value (slightly faster) than default DRAM adistance to be part of
>    * the same memory tier.
>    */
> -#define MEMTIER_ADISTANCE_DRAM	((4 * MEMTIER_CHUNK_SIZE) + (MEMTIER_CHUNK_SIZE >> 1))
> +#define MEMTIER_ADISTANCE_DRAM	((4L * MEMTIER_CHUNK_SIZE) + (MEMTIER_CHUNK_SIZE >> 1))

Hi Li Zhijian

This looks good to me. Feel free to add

Reviewed-byDonet Tom <donettom@...ux.ibm.com>

>   
>   struct memory_tier;
>   struct memory_dev_type {