From: Liew Rui Yan Once quota set, the charge_{target,addr}_from unconditionally skips and resets at the last region of the tracked target, so the last region can be skipped even when it has not been processed. Example: 1. Target has 2 regions: R1 (0-100 bytes) and R2 (100-200 bytes). 2. Quota is configured to process only 100 bytes per window. 3. Window 1: Processes R1 (0-100). Quota is full. charge_{target, addr}_from is saved at (Target, 100). 4. Window 2: The loop reaches R2. Because R2 is damon_last_region(t), the old code unconditionally returns true, skipping R2 entirely and resetting the charge_{target,addr}_from. Result: R2 is permanently skipped even though it has never been processed. However, it is important to note that this is a very minor issue. This is because it is triggered only when the previous window saved/kept charge_{target,addr}_from, and in the next window, all regions except the last region were skipped by damos_skip_charged_region(). Fix this by only resets the charge_{target,addr}_from when last region is reached, only skips when it applied or cannot split. Fixes: 50585192bc2e ("mm/damon/schemes: skip already charged targets and regions") Cc: # v5.16.x Cc: Andrew Morton Signed-off-by: Liew Rui Yan Reviewed-by: SJ Park Signed-off-by: SJ Park --- Changes from v2: - v2: https://lore.kernel.org/20260830063159.6347-1-aethernet65535@gmail.com - Collect R-b: from SJ. Changes from v1: - Simplify example in commit message. - Add clarification regarding the severity of the bug to avoid causing users unnecessary afraid. - Modify the patch code, each time the last region is reached, only charge_{target,addr}_from is reset, but the return value depends on the situation. - Modify patch's title from 'mm/damon' to 'mm/damon/core', since it only changes core.c. - v1: https://lore.kernel.org/damon/20260828084737.290024-1-aethernet65535@gmail.com Changes from RFC v1: - Minimal fix, only fixes the issue where the last-region is skipped. - Add an example to the commit message to demonstrate that this error occurs very rarely. - RFC v1: https://lore.kernel.org/damon/20260825124616.5129-1-aethernet65535@gmail.com mm/damon/core.c | 25 ++++++++++++++----------- 1 file changed, 14 insertions(+), 11 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index a360c41cda89c..7e6cd405161d4 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2452,36 +2452,39 @@ static bool damos_skip_charged_region(struct damon_target *t, { struct damos_quota *quota = &s->quota; unsigned long sz_to_skip; + bool skip = false; /* Skip previously charged regions */ if (quota->charge_target_from) { if (t != quota->charge_target_from) return true; - if (r == damon_last_region(t)) { - quota->charge_target_from = NULL; - quota->charge_addr_from = 0; - return true; - } if (quota->charge_addr_from && - r->ar.end <= quota->charge_addr_from) - return true; + r->ar.end <= quota->charge_addr_from) { + skip = true; + goto out; + } if (quota->charge_addr_from && r->ar.start < quota->charge_addr_from) { sz_to_skip = ALIGN_DOWN(quota->charge_addr_from - r->ar.start, min_region_sz); if (!sz_to_skip) { - if (damon_sz_region(r) <= min_region_sz) - return true; + if (damon_sz_region(r) <= min_region_sz) { + skip = true; + goto out; + } sz_to_skip = min_region_sz; } damon_split_region_at(t, r, sz_to_skip); - return true; + skip = true; } + } +out: + if (r == damon_last_region(t)) { quota->charge_target_from = NULL; quota->charge_addr_from = 0; } - return false; + return skip; } static void damos_update_stat(struct damos *s, base-commit: ab263dd200a7a9101ce3c1374fd23139f29cdac4 -- 2.47.3