6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: SJ Park commit 0250dbe08c730d003ef9f484da56ae09a1ea0c4c upstream. The number of DAMON regions could temporarily exceed the user-defined maximum number of regions limit for corner cases. For example, users could lower the limit via runtime parameters update. For such a case, kdamond_merge_regions() repeats merging regions in the case doubling the merge threshold. The repeated merge operation could update the age of regions multiple times. This corrupts the monitoring results. Fix the issue by asking the merge operation to skip aging for the corner case. The user impact is degradation of the monitoring quality. The impact should be mild, since the degradation is only temporal, and it is not common to happen in realistic setups. The issue was discovered [1,2] by Sashiko. Link: https://lore.kernel.org/20260712165432.87609-1-sj@kernel.org Link: https://lore.kernel.org/20260621203548.10718-1-sj@kernel.org [1] Link: https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org [2] Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet") Signed-off-by: SJ Park Cc: # 6.10 Signed-off-by: Andrew Morton Signed-off-by: SJ Park Signed-off-by: Greg Kroah-Hartman --- mm/damon/core.c | 21 +++++++++++++-------- mm/damon/tests/core-kunit.h | 2 +- 2 files changed, 14 insertions(+), 9 deletions(-) --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2364,17 +2364,19 @@ static void damon_merge_two_regions(stru * sz_limit size upper limit of each region */ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres, - unsigned long sz_limit) + unsigned long sz_limit, bool count_age) { struct damon_region *r, *prev = NULL, *next; damon_for_each_region_safe(r, next, t) { - if (abs(r->nr_accesses - r->last_nr_accesses) > thres) - r->age = 0; - else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0)) - r->age = 0; - else - r->age++; + if (count_age) { + if (abs(r->nr_accesses - r->last_nr_accesses) > thres) + r->age = 0; + else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0)) + r->age = 0; + else + r->age++; + } if (prev && prev->ar.end == r->ar.start && abs(prev->nr_accesses - r->nr_accesses) <= thres && @@ -2408,15 +2410,18 @@ static void kdamond_merge_regions(struct struct damon_target *t; unsigned int nr_regions; unsigned int max_thres; + bool count_age = true; max_thres = c->attrs.aggr_interval / (c->attrs.sample_interval ? c->attrs.sample_interval : 1); do { nr_regions = 0; damon_for_each_target(t, c) { - damon_merge_regions_of(t, threshold, sz_limit); + damon_merge_regions_of(t, threshold, sz_limit, + count_age); nr_regions += damon_nr_regions(t); } + count_age = false; threshold = max(1, threshold * 2); } while (nr_regions > c->attrs.max_nr_regions && threshold / 2 < max_thres); --- a/mm/damon/tests/core-kunit.h +++ b/mm/damon/tests/core-kunit.h @@ -266,7 +266,7 @@ static void damon_test_merge_regions_of( damon_add_region(r, t); } - damon_merge_regions_of(t, 9, 9999); + damon_merge_regions_of(t, 9, 9999, true); /* 0-112, 114-130, 130-156, 156-170 */ KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 5u); if (damon_nr_regions(t) != 5)