[PATCH] mm/damon/core: reset watermark wait state on each iteration

[Aveline Noir]

Found while reviewing the DAMON source code that if an online commit 
removes all schemes while kdamond is waiting for watermark activation,
the scheme loop becomes empty but min_wait_time keeps the previous 
positive value.  kdamond then keeps sleeping instead of resuming 
monitoring without schemes.

Initialize min_wait_time and init_wait_time on each loop iteration so
the wait is calculated from the current schemes and an empty scheme
list resumes monitoring.

Fixes: 6e74d2bf5a26 ("mm/damon/core: add a new callback for watermarks checks")
Assisted-by: LLM
Signed-off-by: Aveline Noir <jm5905938@xxxxxxxxx>
---
An x86-64 KVM test using free-memory watermarks reproduced the stall
with nr_schemes=0 followed by commit.  The same test resumed monitoring
with this fix.  DAMON KUnit and selftests all passed.

 mm/damon/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index eb1fcdabc007..f61ff763d4de 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3976,10 +3976,11 @@ static int kdamond_wait_activation(struct damon_ctx *ctx)
 {
 	struct damos *s;
 	unsigned long wait_time;
-	unsigned long min_wait_time = 0;
-	bool init_wait_time = false;
 
 	while (!kdamond_need_stop(ctx)) {
+		unsigned long min_wait_time = 0;
+		bool init_wait_time = false;
+
 		damon_for_each_scheme(s, ctx) {
 			wait_time = damos_wmark_wait_us(s);
 			if (!init_wait_time || wait_time < min_wait_time) {