mm/memory-tier: fix abstract distance calculation overflow

JIRA: https://issues.redhat.com/browse/RHEL-100684

commit cce3510
Author: Li Zhijian <lizhijian@fujitsu.com>
Date:   Tue Jun 10 14:27:51 2025 +0800

    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.

    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: 3718c02 ("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: Rafael Aquini <raquini@redhat.com>

Author: Rafael Aquini

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 {