[PATCH v2 1/2] mm/damon/core: preserve the quota passed to damon_new_scheme()

[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.

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.

Fixes: 60bd24f272d0 ("mm/damon/sysfs: test commit input against realistic destination")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20260702212143.0CB6D1F00A3D@smtp.kernel.org/ [1]
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Changes in v2:
 - Explain the running-context validation before the constructor side effect.
 - Clarify the quota parameter and use "commit" for the context operation.
 - Point Fixes to commit 60bd24f272d0 ("mm/damon/sysfs: test commit input
   against realistic destination") and drop the stable version comment.
 - Rebase onto mm-new. No changes to the fix logic.

 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 4687b909d42c9..f76333f26de22 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);
 
-- 
2.53.0