[PATCH 5.15.y] mm/damon/ops-common: call folio_test_lru() after folio_get()

[Sasha Levin <sashal@kernel.org>]

From: SeongJae Park <sj@kernel.org>

[ Upstream commit d6b8b02a27b3dd09ec12144322b3dac46d9bc9ef ]

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

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

The race should be rare.  Also the problem can happen only if the kernel
has enabled CONFIG_DEBUG_VM_PGFLAGS.  No real world report of this issue
has been made so far.  This fix is based on only theoretical analysis.
That said, a bug is a bug.  A similar issue was also fixed via commit
3203b3ab0fcf ("mm/filemap: don't call folio_test_locked() without a
reference in next_uptodate_folio()").  I don't expect this change will
make a meaningful impact to DAMON performance in the real world, though I
will be happy to be corrected from the real world reports.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260525162256.8317-1-sj@kernel.org
Link: https://lore.kernel.org/20260517234112.89245-1-sj@kernel.org [1]
Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Fernand Sieber <sieberf@amazon.com>
Cc: Leonard Foerster <foersleo@amazon.de>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/damon/vaddr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 6d8036671e60e5..dbb0f0fb2e598c 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -383,10 +383,10 @@ static struct page *damon_get_page(unsigned long pfn)
 {
 	struct page *page = pfn_to_online_page(pfn);
 
-	if (!page || !PageLRU(page) || !get_page_unless_zero(page))
+	if (!page || !get_page_unless_zero(page))
 		return NULL;
 
-	if (unlikely(!PageLRU(page))) {
+	if (!PageLRU(page)) {
 		put_page(page);
 		page = NULL;
 	}
-- 
2.53.0