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. [1] https://lore.kernel.org/20260517234112.89245-1-sj@kernel.org Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces") Cc: # 5.15.x Signed-off-by: SeongJae Park --- Changes from RFC v1.1 - RFC v1.1: https://lore.kernel.org/20260524174608.81112-1-sj@kernel.org - Drop RFC tag. Changes from RFC v1 - RFC v1: https://lore.kernel.org/20260523194145.93122-1-sj@kernel.org - Do not change post-folio_try_get() validation order. mm/damon/ops-common.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c index 3a0ddc3ac7196..5c93ef2bb8a97 100644 --- a/mm/damon/ops-common.c +++ b/mm/damon/ops-common.c @@ -32,9 +32,9 @@ struct folio *damon_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; } base-commit: 4901db133abb670b369af93cba9af99c5be0eb0a -- 2.47.3