mm/damon/ops-common: call folio_test_lru() after folio_get()

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
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>

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index 8c6d613425c1314d4d1825ad7661388290d669d3..c3e4c871b0bb29777e9d2439831100da246aa32d 100644 (file)
--- 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;
        }