[PATCH] mm/damon/core: reject zero sample_interval

From: liyouhong <[email protected]>

damon_set_attrs() allows sample_interval == 0.  Sysfs likewise accepts
sample_us=0.  kdamond_fn() then calls kdamond_usleep(0) every loop
iteration, so kdamond busy-spins and can peg a CPU.

With a zero sampling interval the access-rate accounting also diverges:
nr_accesses_bp no longer matches nr_accesses * 10000, and
kdamond_reset_aggregated() hits:

  WARNING: invalid nr_accesses_bp at reset: ...

Reject sample_interval == 0 in damon_set_attrs(), which covers sysfs
commit and other callers.  damon_lru_sort already refused a zero
sample_interval; align the core validation with that.

Reproduce:
  echo 0 > .../intervals/sample_us
  echo on > .../kdamonds/0/state
  # kdamond ~100% CPU; may WARN in kdamond_reset_aggregated()

Fixes: 4472edf63d66 ("mm/damon/core: use number of passed access sampling as a timer")
Assisted-by: Cursor Grok 4.5
Signed-off-by: liyouhong <[email protected]>
---
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 7e4b9affc5b0..a6d54ec8c4be 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -911,6 +911,8 @@ int damon_set_attrs(struct damon_ctx *ctx, struct damon_attrs *attrs)
 		return -EINVAL;
 	if (attrs->min_nr_regions > attrs->max_nr_regions)
 		return -EINVAL;
+	if (!attrs->sample_interval)
+		return -EINVAL;
 	if (attrs->sample_interval > attrs->aggr_interval)
 		return -EINVAL;
 
@@ -3770,8 +3772,9 @@ void damon_update_region_access_rate(struct damon_region *r, bool accessed,
 	unsigned int len_window = 1;
 
 	/*
-	 * sample_interval can be zero, but cannot be larger than
-	 * aggr_interval, owing to validation of damon_set_attrs().
+	 * sample_interval cannot be zero or larger than aggr_interval,
+	 * owing to validation of damon_set_attrs().  Keep the zero check
+	 * below as defense in depth.
 	 */
 	if (attrs->sample_interval)
 		len_window = damon_max_nr_accesses(attrs);