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Thu, 15 Oct 2026 07:52:57 -0700 (PDT) Received: from [10.131.15.42] ([115.200.102.144]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-536127c62d3sm27173261cf.15.2026.10.15.07.52.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 15 Oct 2026 07:52:57 -0700 (PDT) From: Aveline Noir To: SJ Park Cc: Andrew Morton , damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH] mm/damon/stat: avoid 32-bit overflow in idle percentiles Date: Thu, 15 Oct 2026 22:52:04 +0800 Message-ID: <179207592479.3.11595453180868898703@gmail.com> X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 73A5D100003 X-Stat-Signature: apud66sgmksp3mrc4qpp9gjkf7sxwnkf X-HE-Tag: 1792075978-797946 X-HE-Meta: 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 RoYZYb4o YEDFMQinbaAD0WJmmgR1ZKKnAJwhzhGuOQMPcuiE7PlRPWeNwm79eINsJJTL84X0VEV87411OcoTwp9d1e8kxeOdukP+fk1p62dLkRTg+WCzfbwgMUinA3mbAqIAhHrgoGDaFhI6Z5ctydmhspxSLITgdasRvfsMtoKXn2a0ciLVGWGZud1mhNtLq25yr1XDnK7D5S5DkUdRsuLyBHiNnRBEN5MAWmdEgia9oTPr3KHS1S/ZsoaPWyYmiBDqG8QABqAuCi+LKKJZY23AWrGWn6ErPIFIZRD3pxQJlcs0aDNOVCPURUJxXT8gr138kzoTLd981aSzj50gPg87NJ1Ukdp+OVtsEDIUURzgl9t5z2Ele1591HAx8yIPhZxw4O0JBd/fW8LMysEHgU3RFhXi2fO6hRzJ86iuZo9+T6tmGUX73MlEOhmvewxHK3Z33CZlLCaBIQ1lIFrpdez/tHSFTH6BGlQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 32-bit, accounted_bytes * 100 can wrap before division by total_sz. Some entries in memory_idle_ms_percentiles then keep their old values instead of being updated. For example, a 256 MiB i386 QEMU guest reported p0=19200 ms, but p50 and p100 were both zero. Cast accounted_bytes to u64 before multiplying and use div64_ul() for the division. Compute the percentage once per region. Link: https://lore.kernel.org/r/CANOyQmHad1vamnZT12M5yPrJ69==Pt1U0+MGw6u0MuNtr-DRCw@mail.gmail.com Signed-off-by: Aveline Noir --- This is against mm-new ad9f2d463666. With the fix, the original test reports p0=p50=p100=19200 ms. The i386 kernel build, x86_64 W=1 stat.o build and external arithmetic checks passed. I also ran 38 before/after VM tests and 20 same-image controls. Throughput was close overall, but median kdamond CPU rose by 0.0053 and 0.0221 percentage points of one guest CPU for stream and sparse, respectively. Some pairs had 5.91% lower throughput or 15.49% higher scan P99. So I can't yet tell whether these differences come from the patch. mm/damon/stat.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/mm/damon/stat.c b/mm/damon/stat.c index b05b68f73e10..eccd372810d0 100644 --- a/mm/damon/stat.c +++ b/mm/damon/stat.c @@ -114,7 +114,7 @@ static void damon_stat_set_idletime_percentiles(struct damon_ctx *c) { struct damon_region **sorted_regions, *region; int nr_regions; - unsigned long total_sz, accounted_bytes = 0; + unsigned long total_sz, accounted_bytes = 0, percentile; int err, i, next_percentile = 0; err = damon_stat_sort_regions(c, &sorted_regions, &nr_regions, @@ -124,7 +124,8 @@ static void damon_stat_set_idletime_percentiles(struct damon_ctx *c) for (i = 0; i < nr_regions; i++) { region = sorted_regions[i]; accounted_bytes += region->ar.end - region->ar.start; - while (next_percentile <= accounted_bytes * 100 / total_sz) + percentile = div64_ul((u64)accounted_bytes * 100, total_sz); + while (next_percentile <= percentile) memory_idle_ms_percentiles[next_percentile++] = damon_stat_idletime(region) * (long)c->attrs.aggr_interval / USEC_PER_MSEC;