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

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Message-ID: <aEh9QaU2wUkufK18@localhost.localdomain>
Date: Tue, 10 Jun 2025 20:45:21 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Li Zhijian <lizhijian@...itsu.com>
Cc: linux-mm@...ck.org, 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 Tue, Jun 10, 2025 at 02:27:51PM +0800, 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>

Reviewed-by: Oscar Salvador <osalvador@...e.de>


-- 
Oscar Salvador
SUSE Labs