[PATCH] mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

[Dev Jain <dev.jain@arm.com>]

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.

Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation")
Cc: stable@vger.kernel.org
Signed-off-by: Dev Jain <dev.jain@arm.com>
Acked-by: Barry Song <baohua@kernel.org>
---
Applies on mm-unstable. This patch was part of
https://lore.kernel.org/all/20260506094504.2588857-2-dev.jain@arm.com/

 mm/rmap.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index fb3c351f8c45..1c77d5dc06e9 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.
 		 */
-- 
2.43.0