[RFC PATCH 21/57] mm/collapse: settle whatever the round reached

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

Fill in the last pass.  A candidate that never froze is recorded as given
up on, with whatever result ended the round.  Anything no pass took
ownership of -- a destination folio, a table meant for deposit -- goes
back.  The count of installed candidates is what the round reports.

Holding no lock here is the point.  Every refusal before this happens
under a page-table lock: the freeze unwinds under the ptl it took, and
the install aborts under that ptl or the pmd lock.

Dropping the last reference to a folio, and the memcg uncharge behind it,
is not spinlock work.  So a refused candidate keeps its folio and its
table until the round is over, and this is where they are released.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 mm/collapse.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/mm/collapse.c b/mm/collapse.c
index f65f413339bf..2da1f8ddcca8 100644
--- a/mm/collapse.c
+++ b/mm/collapse.c
@@ -1500,7 +1500,30 @@ static unsigned int collapse_finish(struct mm_struct *mm,
 				    struct collapse_control *cc,
 				    enum scan_result result)
 {
-	return 0;
+	unsigned int i, nr_installed = 0;
+
+	for (i = 0; i < cc->nr_candidates; i++) {
+		struct collapse_candidate *cand = &cc->candidates[i];
+
+		/* Never froze: the round gave up before it got that far */
+		if (cand->state == CAND_SELECTED) {
+			cand->state = CAND_SKIPPED;
+			cand->result = result;
+		}
+
+		if (cand->new_folio) {
+			folio_put(cand->new_folio);
+			cand->new_folio = NULL;
+		}
+		if (cand->deposit) {
+			pte_free(mm, cand->deposit);
+			cand->deposit = NULL;
+		}
+		if (cand->state == CAND_INSTALLED)
+			nr_installed++;
+	}
+
+	return nr_installed;
 }
 
 /*
-- 
2.54.0