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: 354dffd2 ("mm: support batched unmap for lazyfree large folios during reclamation")
Signed-off-by: Dev Jain <dev.jain@arm.com>
Acked-by: Barry Song <baohua@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Harry Yoo <harry@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>