[PATCH] mm/damon/stat: avoid 32-bit overflow in idle percentiles

[Aveline Noir <jm5905938@gmail.com>]

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 <jm5905938@gmail.com>
---
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;