Li Zhijian 89237dd4c8 mm/memory-tier: fix abstract distance calculation overflow ...

commit cce3510313 upstream.

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.

Link: https://lkml.kernel.org/r/20250611023439.2845785-1-lizhijian@fujitsu.com
Link: https://lkml.kernel.org/r/20250610062751.2365436-1-lizhijian@fujitsu.com
Fixes: 3718c02dbd ("acpi, hmat: calculate abstract distance with HMAT")
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Reviewed-by: Donet Tom <donettom@linux.ibm.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2025-08-20 18:30:16 +02:00

3.9 KiB

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