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 50 bytes per window. 3. Window 1: Processes R1 (0-50). Quota is full. Cursor is saved at (Target, 50). 4. Window 2: Skips R1 (0-50). Processes R1 (50-100). Quota is full. Cursor is saved at (Target, 100), which is exactly the start of R2. 5. Window 3: The loop reaches R2. Because R2 is damon_last_region(t), the old code unconditionally returns true, skipping R2 entirely and resetting the cursor. Result: R2 is permanently skipped even though it has never been processed. Fix this by only skipping the last region after it has been applied. Fixes: 50585192bc2e ("mm/damon/schemes: skip already charged targets and regions") Cc: # v5.16.x Signed-off-by: Liew Rui Yan --- 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 | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 644daf5a1656..21dc6b086c42 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2347,14 +2347,15 @@ static bool damos_skip_charged_region(struct damon_target *t, 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) + r->ar.end <= quota->charge_addr_from) { + if (r->ar.end == quota->charge_addr_from || + damon_is_last_region(r, t)) { + quota->charge_target_from = NULL; + quota->charge_addr_from = 0; + } return true; + } if (quota->charge_addr_from && r->ar.start < quota->charge_addr_from) { -- 2.55.0