[PATCH v6 5/5] mm: rmap: support batched unmapping for file large folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Similar to folio_referenced_one(), we can apply batched unmapping for file
large folios to optimize the performance of file folios reclamation.

Barry previously implemented batched unmapping for lazyfree anonymous large
folios[1] and did not further optimize anonymous large folios or file-backed
large folios at that stage. As for file-backed large folios, the batched
unmapping support is relatively straightforward, as we only need to clear
the consecutive (present) PTE entries for file-backed large folios.

Note that it's not ready to support batched unmapping for uffd case, so
let's still fallback to per-page unmapping for the uffd case.

Performance testing:
Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to
reclaim 8G file-backed folios via the memory.reclaim interface. I can observe
75% performance improvement on my Arm64 32-core server (and 50%+ improvement
on my X86 machine) with this patch.

W/o patch:
real    0m1.018s
user    0m0.000s
sys     0m1.018s

W/ patch:
real	0m0.249s
user	0m0.000s
sys	0m0.249s

[1] https://lore.kernel.org/all/20250214093015.51024-4-21cnbao@gmail.com/T/#u
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Barry Song <baohua@kernel.org>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/rmap.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 8807f8a7df28..43cb9ac6f523 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1945,12 +1945,16 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
 	end_addr = pmd_addr_end(addr, vma->vm_end);
 	max_nr = (end_addr - addr) >> PAGE_SHIFT;
 
-	/* We only support lazyfree batching for now ... */
-	if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
+	/* We only support lazyfree or file folios batching for now ... */
+	if (folio_test_anon(folio) && folio_test_swapbacked(folio))
 		return 1;
+
 	if (pte_unused(pte))
 		return 1;
 
+	if (userfaultfd_wp(vma))
+		return 1;
+
 	return folio_pte_batch(folio, pvmw->pte, pte, max_nr);
 }
 
@@ -2313,7 +2317,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			 *
 			 * See Documentation/mm/mmu_notifier.rst
 			 */
-			dec_mm_counter(mm, mm_counter_file(folio));
+			add_mm_counter(mm, mm_counter_file(folio), -nr_pages);
 		}
 discard:
 		if (unlikely(folio_test_hugetlb(folio))) {
-- 
2.47.3