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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/18/26 08:36, Dev Jain wrote:
> 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>
> ---

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks!

-- 
Cheers,

David