[PATCH v3 1/2] mm/damon/core: preserve the quota passed to damon_new_scheme()

[SJ Park <sj@kernel.org>]

From: Karl Mehltretter <kmehltretter@gmail.com>

damon_commit_ctx() first commits the running context's parameters to a
temporary context for validating proposed updates. When
damon_commit_schemes() creates the temporary schemes, it passes the
running scheme's quota as the quota parameter of damon_new_scheme().

damon_new_scheme() calls damos_quota_init() on that quota before copying
it to the new scheme. This clears the running scheme's effective quota,
feedback input and charging state. Even an update rejected with -EINVAL
loses the running quota state.

For a size quota, this discards the bytes already charged and allows the
scheme to use a fresh quota before the reset interval has elapsed. For a
goal-driven quota, the consist tuner loses its accumulated input and
restarts from its minimum input. A time quota loses its throughput
estimate and falls back to the initial estimate.

The end users will show DAMOS works more or less aggressively than
expected for online-commit updates of quotas.  DAMON provides best
efforts by default.  DAMON parameters online commit is supposed to be
executed only occasionally.  Hence, the issue wouldn't be critical on
sane setups.  For user_input type quota goals, online commit of the user
input score is expected to be frequent.  But, for the case
commit_schemes_quota_goals command is recommended for optimal execution,
and it doesn't have this bug.

Commit 60bd24f272d0 ("mm/damon/sysfs: test commit input against realistic
destination") introduced this problem in v6.19 when sysfs validation
began committing the running context's parameters to a temporary context.
Commit b90408ef1163 ("mm/damon/core: safely validate src on
damon_commit_ctx()") later moved that validation into the core API,
exposing other callers including DAMON_RECLAIM and DAMON_LRU_SORT.

Sashiko reported the same side effect [1] on the RFC of the core API
change.

Copy the quota to the new scheme first, then initialize that copy. Make
damos_quota_init() return void, since its return value is no longer needed.

Link: https://lore.kernel.org/r/20260702212143.0CB6D1F00A3D@smtp.kernel.org/ [1]
Fixes: 60bd24f272d0 ("mm/damon/sysfs: test commit input against realistic destination")
Cc: <stable@vger.kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Bijan Tabatabai <bijan311@gmail.com>
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 60e4233ed23c..5ecbea5d71e1 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -734,7 +734,7 @@ static bool damos_quota_goals_empty(struct damos_quota *q)
 }
 
 /* initialize fields of @quota that normally API users wouldn't set */
-static struct damos_quota *damos_quota_init(struct damos_quota *quota)
+static void damos_quota_init(struct damos_quota *quota)
 {
 	quota->esz = 0;
 	quota->total_charged_sz = 0;
@@ -744,7 +744,6 @@ static struct damos_quota *damos_quota_init(struct damos_quota *quota)
 	quota->charge_target_from = NULL;
 	quota->charge_addr_from = 0;
 	quota->esz_bp = 0;
-	return quota;
 }
 
 struct damos *damon_new_scheme(struct damos_access_pattern *pattern,
@@ -776,7 +775,8 @@ struct damos *damon_new_scheme(struct damos_access_pattern *pattern,
 	scheme->last_applied = NULL;
 	INIT_LIST_HEAD(&scheme->list);
 
-	scheme->quota = *(damos_quota_init(quota));
+	scheme->quota = *quota;
+	damos_quota_init(&scheme->quota);
 	/* quota.goals should be separately set by caller */
 	INIT_LIST_HEAD(&scheme->quota.goals);
 
-- 
2.47.3