mm-rmap-initialize-nr_pages-to-1-at-loop-start-in-try_to_unmap_one.patch removed from -mm tree

The quilt patch titled
Subject: mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one
has been removed from the -mm tree. Its filename was
mm-rmap-initialize-nr_pages-to-1-at-loop-start-in-try_to_unmap_one.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Dev Jain <[email protected]>
Subject: mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one
Date: Mon, 18 May 2026 12:06:56 +0530

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.

---

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

@@ -2030,6 +2030,8 @@ static bool try_to_unmap_one(struct foli
 	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.
 		 */

_

Patches currently in -mm which might be from [email protected] are

selftests-mm-simplify-byte-pattern-checking-in-mremap_test.patch mm-khugepaged-generalize-alloc_charge_folio.patch