[PATCH 6.1.y v2 2/2] mm/damon/core: fix unconditionally skip last region

[SJ Park <sj@kernel.org>]

From: 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 resetting the charge_{target,addr}_from when last region
is reached, only skipping when it is applied or cannot split.

Link: https://lore.kernel.org/20260908134739.96919-1-sj@kernel.org
Fixes: 50585192bc2e ("mm/damon/schemes: skip already charged targets and regions")
Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org> # v5.16.x
(cherry picked from commit b3723b596b548c837a766aae3553c14a7b15af2b)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index c65d2cbeea6b..f2f6f125c09b 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -756,6 +756,7 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		unsigned long sz = damon_sz_region(r);
 		struct timespec64 begin, end;
 		unsigned long sz_applied = 0;
+		bool skip = false;
 
 		if (!s->wmarks.activated)
 			continue;
@@ -768,14 +769,11 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (quota->charge_target_from) {
 			if (t != quota->charge_target_from)
 				continue;
-			if (r == damon_last_region(t)) {
-				quota->charge_target_from = NULL;
-				quota->charge_addr_from = 0;
-				continue;
-			}
 			if (quota->charge_addr_from &&
-					r->ar.end <= quota->charge_addr_from)
-				continue;
+					r->ar.end <= quota->charge_addr_from) {
+				skip = true;
+				goto clean_charge_from_skip;
+			}
 
 			if (quota->charge_addr_from && r->ar.start <
 					quota->charge_addr_from) {
@@ -783,17 +781,25 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 						r->ar.start, DAMON_MIN_REGION);
 				if (!sz) {
 					if (damon_sz_region(r) <=
-					    DAMON_MIN_REGION)
-						continue;
+					    DAMON_MIN_REGION) {
+						skip = true;
+						goto clean_charge_from_skip;
+					}
 					sz = DAMON_MIN_REGION;
 				}
 				damon_split_region_at(t, r, sz);
 				r = damon_next_region(r);
 				sz = damon_sz_region(r);
+				skip = false;
 			}
+		}
+clean_charge_from_skip:
+		if (r == damon_last_region(t)) {
 			quota->charge_target_from = NULL;
 			quota->charge_addr_from = 0;
 		}
+		if (skip)
+			continue;
 
 		if (!damos_valid_target(c, t, r, s))
 			continue;
-- 
2.47.3