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
3203b3ab0f ("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: 3f49584b26 ("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>
d6b8b02a27