[RFC PATCH v2 3/9] mm/damon/paddr: lock the folio for the page fault primitive rmap walk

[Ravi Jonnalagadda <ravis.opensrc@gmail.com>]

rmap_walk() requires the folio to be locked for every mapping type it
walks, and the page fault access check primitive takes the lock only for
file-backed and KSM folios, so a plain anonymous folio reaches the walk
unlocked and the walk does not install the marker.

Take the lock unconditionally, as the other reverse mapping walks in DAMON
do, and release the folio reference the lookup took on every exit path.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
---
 mm/damon/paddr.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index f4fa7c231e551..5fcef3005c536 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -91,21 +91,25 @@ static void damon_pa_change_protection(unsigned long paddr)
 		.rmap_one = damon_pa_change_protection_one,
 		.anon_lock = folio_lock_anon_vma_read,
 	};
-	bool need_lock;
 
 	if (!folio)
 		return;
 	if (!folio_mapped(folio) || !folio_raw_mapping(folio))
-		return;
+		goto put;
 
-	need_lock = !folio_test_anon(folio) || folio_test_ksm(folio);
-	if (need_lock && !folio_trylock(folio))
-		return;
+	/*
+	 * rmap_walk() requires a locked folio for every mapping type, so the
+	 * lock is taken unconditionally, as the other reverse mapping walks in
+	 * DAMON do.
+	 */
+	if (!folio_trylock(folio))
+		goto put;
 
 	rmap_walk(folio, &rwc);
 
-	if (need_lock)
-		folio_unlock(folio);
+	folio_unlock(folio);
+put:
+	folio_put(folio);
 }
 
 static void damon_pa_prepare_access_checks_faults(struct damon_ctx *ctx)
-- 
2.43.0