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

[Andrew Morton]

On Tue, 10 Jun 2025 14:27:51 +0800 Li Zhijian <lizhijian@xxxxxxxxxxx> wrote:

> 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.

Thanks.  The changelog doesn't describe the userspace-visible effects
of this.  Please always include this info.

I'll assume "minor" and it's been this way for a while so I'll add a
cc:stable to this change and shall queue it for 6.17-rc1, so it will be
backported into 6.17.x and earlier kernels at a later time.