From 3f8968e9cbf95d5d87d32218906cab0b9b9eddbe Mon Sep 17 00:00:00 2001 From: Dev Jain Date: Mon, 18 May 2026 12:06:56 +0530 Subject: [PATCH] mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one Initialize nr_pages to 1 at the start of each loop iteration, like folio_referenced_one() does. Without this, nr_pages computed by a previous folio_unmap_pte_batch() call can be reused on a later iteration that does not run folio_unmap_pte_batch() again. mmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call madvise(MADV_FREE), then make the last page device-exclusive via HMM_DMIRROR_EXCLUSIVE. Trigger node reclaim through sysfs. Now, in try_to_unmap_one(), we will first clear the first 15 out of 16 entries mapping the lazyfree folio. This will set nr_pages to 15. In the next pvmw walk, this nr_pages gets reused on a device-exclusive pte, thus potentially corrupting folio refcount/mapcount. At the moment, I have a userspace program which can make the kernel spit out a trace, but the blow up is in folio_referenced_one(), because there are existing bugs in the interaction between device-private and rmap (which too I am investigating). I did a one liner kernel change to avoid going into folio_referenced_one(), and the kernel blows up at folio_remove_rmap_ptes in try_to_unmap_one which is what I wanted. Note that the bug is there not since file folio batching but lazyfree folio batching, since device-exclusive only works for anonymous folios. Userspace visible effect is simply kernel crashing somewhere due to refcount/mapcount corruption. Link: https://lore.kernel.org/20260518063656.3721056-1-dev.jain@arm.com Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation") Signed-off-by: Dev Jain Acked-by: Barry Song Acked-by: David Hildenbrand (Arm) Reviewed-by: Lorenzo Stoakes Cc: Anshuman Khandual Cc: Barry Song Cc: Dev Jain Cc: Harry Yoo Cc: Jann Horn Cc: Liam R. Howlett Cc: Rik van Riel Cc: Ryan Roberts Cc: Vlastimil Babka Cc: Signed-off-by: Andrew Morton --- mm/rmap.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/mm/rmap.c b/mm/rmap.c index 78b7fb5f367c..99e1b3dc390b 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -2030,6 +2030,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, mmu_notifier_invalidate_range_start(&range); while (page_vma_mapped_walk(&pvmw)) { + nr_pages = 1; + /* * If the folio is in an mlock()d vma, we must not swap it out. */