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d6b8b02a27
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 commit3203b3ab0f("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>