[PATCH] mm/page_idle: call folio_test_lru() after folio_get()

[Jiale Yao]

page_idle_get_folio() speculatively calls folio_test_lru() before
folio_try_get(). The folio can get freed and reallocated to a tail page
in the meantime. In that case, VM_BUG_ON_PGFLAGS() in
const_folio_flags() can be triggered. Remove the speculative call.

Also mark the folio_test_lru() check right after folio_try_get() success
as no more unlikely.

This is a sibling-path bug: damon_get_folio() was copied from this
function with the same flawed pattern. Commit d6b8b02a27b3
("mm/damon/ops-common: call folio_test_lru() after folio_get()") fixed
damon_get_folio(), but page_idle_get_folio() was left unfixed. KCSAN
(strict mode) confirms the data race on the folio flags:

  BUG: KCSAN: data-race in ... / percpu_counter_add_batch
    page_idle_get_folio+0x7a/0x2d0
    page_idle_bitmap_read+0xc9/0x220

Signed-off-by: Jiale Yao <yaojiale02@xxxxxxx>
---
 mm/page_idle.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_idle.c b/mm/page_idle.c
index 9c67cbac2965..29ee18f0e8ce 100644
--- a/mm/page_idle.c
+++ b/mm/page_idle.c
@@ -40,9 +40,9 @@ static struct folio *page_idle_get_folio(unsigned long pfn)
 		return NULL;
 
 	folio = page_folio(page);
-	if (!folio_test_lru(folio) || !folio_try_get(folio))
+	if (!folio_try_get(folio))
 		return NULL;
-	if (unlikely(page_folio(page) != folio || !folio_test_lru(folio))) {
+	if (unlikely(page_folio(page) != folio) || !folio_test_lru(folio)) {
 		folio_put(folio);
 		folio = NULL;
 	}
-- 
2.34.1