mm/damon/core: reset invalid quota->charge_target_from

DAMOS can suddenly stop working if a target process that the quota is just
fully charged on is terminated.  Fix by catching and processing the corner
case.

When DAMOS quota is fully charged, the target and the region to continue
applying the action in the next round is saved in
damos_quota->charge_{target,addr}_from.  In the next round, DAMOS iterates
targets and regions from the beginning.  It skips applying the action to
the regions until it visits and skips the saved target/region.

Virtual address space targets become invalid if the process is terminated.
Trying to apply the scheme to invalid target is just a waste of time. 
Hence commit 6e4930e33329 ("mm/damon/core: fix wasteful CPU calls by
skipping non-existent targets") made the logic to skip invalid targets. 
However, it does skip before the charged target/region skipping/updating.

Let's suppose the user runs DAMOS for multiple virtual address spaces with
a quota.  The quota exceeded in the middle of a virtual address space. 
And the process of the address space is terminated.  Then the
charge_target_from points to the invalid target.  The pointer update logic
is skipped for the invalid target, so the charge_target_from is never
updated.  DAMOS action to every target/region is skipped.  From the user's
perspective, it would look like suddenly DAMOS has stopped working.

No critical leak or crash can happen.  The user could reinstall the
scheme.  But this makes use of DAMOS under certain setups quite
unreliable.

When the invalid target is found, further check the corner case and reset
the pointer.

This issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260910142846.172957-1-sj@kernel.org
Link: https://lore.kernel.org/20260830064708.40CA61F000E9@smtp.kernel.org [1]
Fixes: 6e4930e33329 ("mm/damon/core: fix wasteful CPU calls by skipping non-existent targets")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Enze Li <lienze@kylinos.cn>
Cc: <stable@vger.kernel.org> # 7.0.x

1 files changed, 8 insertions, 1 deletions

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 06a253df8d4b..1764620903f2 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3245,8 +3245,15 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 	max_region_sz = damon_region_sz_limit(c);
 	mutex_lock(&c->walk_control_lock);
 	damon_for_each_target(t, c) {
-		if (c->ops.target_valid && c->ops.target_valid(t) == false)
+		if (c->ops.target_valid && c->ops.target_valid(t) == false) {
+			damon_for_each_scheme(s, c) {
+				if (s->quota.charge_target_from != t)
+					continue;
+				s->quota.charge_target_from = NULL;
+				s->quota.charge_addr_from = 0;
+			}
 			continue;
+		}
 		damos_apply_target(c, t, max_region_sz);
 	}