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@vger.kernel.org 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 Signed-off-by: Li Zhijian lizhijian@fujitsu.com --- V2: Document the 'User-visible impact' # Andrew Morton akpm@linux-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 {