[PATCH v2] mm/damon/core: fix unconditionally skip last region

[Liew Rui Yan <aethernet65535@gmail.com>]

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: <stable@vger.kernel.org> # v5.16.x
Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
---

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 644daf5a1656..fb76b530bbc6 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2342,36 +2342,39 @@ 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,
-- 
2.55.0