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Message-Id: <20250610140228.7836d333cea973a8171686fb@linux-foundation.org> Date: Tue, 10 Jun 2025 14:02:28 -0700 From: Andrew Morton <akpm@...ux-foundation.org> To: Li Zhijian <lizhijian@...itsu.com> Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Ying Huang <huang.ying.caritas@...il.com>, Balbir Singh <bsingharora@...il.com>, Donet Tom <donettom@...ux.ibm.com>, Oscar Salvador <osalvador@...e.de> Subject: Re: [PATCH] mm/memory-tier: Fix abstract distance calculation overflow On Tue, 10 Jun 2025 14:27:51 +0800 Li Zhijian <lizhijian@...itsu.com> 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.
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