linux-kernel - [PATCH v2] mm/memory-tier: Fix abstract distance calculation overflow

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Message-ID: <20250611023439.2845785-1-lizhijian@fujitsu.com>
Date: Wed, 11 Jun 2025 10:34:39 +0800
From: Li Zhijian <lizhijian@...itsu.com>
To: linux-mm@...ck.org
Cc: akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Li Zhijian <lizhijian@...itsu.com>,
	stable@...r.kernel.org,
	Huang Ying <ying.huang@...ux.alibaba.com>,
	Balbir Singh <balbirs@...dia.com>,
	Donet Tom <donettom@...ux.ibm.com>,
	Oscar Salvador <osalvador@...e.de>
Subject: [PATCH v2] mm/memory-tier: Fix abstract distance calculation overflow

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.

User-visible impact:
The memory tiering subsystem will misinterpret slow memory (like CXL)
as faster than DRAM, causing inappropriate demotion of pages from
CXL (slow memory) to DRAM (fast memory).

For example, we will see the following demotion chains from the dmesg, where
Node0,1 are DRAM, and Node2,3 are CXL node:
 Demotion targets for Node 0: null
 Demotion targets for Node 1: null
 Demotion targets for Node 2: preferred: 0-1, fallback: 0-1
 Demotion targets for Node 3: preferred: 0-1, fallback: 0-1

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: stable@...r.kernel.org
Reviewed-by: Huang Ying <ying.huang@...ux.alibaba.com>
Acked-by: Balbir Singh <balbirs@...dia.com>
Reviewed-by: Donet Tom <donettom@...ux.ibm.com>
Reviewed-by: Oscar Salvador <osalvador@...e.de>
Signed-off-by: Li Zhijian <lizhijian@...itsu.com>
---
V2:
  Document the 'User-visible impact' # Andrew Morton <akpm@...ux-foundation.org>
---
 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))
 
 struct memory_tier;
 struct memory_dev_type {
-- 
2.41.0