[PATCH] mm/damon/core: fix wasteful CPU calls by skipping non-existent targets

[Enze Li <lienze@kylinos.cn>]

Currently, DAMON does not proactively clean up invalid monitoring
targets during its runtime.  When some monitored processes exit, DAMON
continues to make the following unnecessary function calls,

  --damon_for_each_target--
  --damon_for_each_region--
      damon_do_apply_schemes
        damos_apply_scheme
          damon_va_apply_scheme
            damos_madvise
              damon_get_mm

it is only in the damon_get_mm() function that it may finally discover
the target no longer exists, which wastes CPU resources. A simple idea
is to check for the existence of monitoring targets within the
kdamond_need_stop() function and promptly clean up non-existent targets.

However, SJ pointed out that this approach is problematic because the
online commit logic incorrectly uses list indices to update the
monitoring state.  This can lead to data loss if the target list is
changed concurrently.  Meanwhile, SJ suggests checking for target
existence at the damon_for_each_target level, and if a target does not
exist, simply skip it and proceed to the next one.

Suggested-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Enze Li <lienze@kylinos.cn>
---
 mm/damon/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index f9fc0375890a..e43bdf0b41a8 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2266,6 +2266,9 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 
 	mutex_lock(&c->walk_control_lock);
 	damon_for_each_target(t, c) {
+		if (c->ops.target_valid && c->ops.target_valid(t) == false)
+			continue;
+
 		damon_for_each_region_safe(r, next_r, t)
 			damon_do_apply_schemes(c, t, r);
 	}

base-commit: c9b47175e9131118e6f221cc8fb81397d62e7c91
-- 
2.52.0