[PATCH] mm/damon: fix unconditionally skip last region

[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 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: <stable@xxxxxxxxxxxxxxx> # v5.16.x
Signed-off-by: Liew Rui Yan <aethernet65535@xxxxxxxxx>
---

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@xxxxxxxxx

---
 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