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

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Message-ID: <877c1kcdi1.fsf@DESKTOP-5N7EMDA>
Date: Tue, 10 Jun 2025 14:33:10 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
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

Li Zhijian <lizhijian@...itsu.com> writes:

> 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>

Good catch!  Feel free to add

Reviewed-by: Huang Ying <ying.huang@...ux.alibaba.com>

in the future version.

---
Best Regards,
Huang, Ying

[snip]