linux-kernel - [PATCH v4 5/5] mm: rmap: support batched unmapping for file large folios

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Message-ID: <d869ae45a7be322f342ace62ffcdd4b47f4154fc.1766455378.git.baolin.wang@linux.alibaba.com>
Date: Tue, 23 Dec 2025 13:48:39 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: akpm@...ux-foundation.org,
	david@...nel.org,
	catalin.marinas@....com,
	will@...nel.org
Cc: lorenzo.stoakes@...cle.com,
	ryan.roberts@....com,
	Liam.Howlett@...cle.com,
	vbabka@...e.cz,
	rppt@...nel.org,
	surenb@...gle.com,
	mhocko@...e.com,
	riel@...riel.com,
	harry.yoo@...cle.com,
	jannh@...gle.com,
	willy@...radead.org,
	baohua@...nel.org,
	dev.jain@....com,
	baolin.wang@...ux.alibaba.com,
	linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v4 5/5] mm: rmap: support batched unmapping for file large folios

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.

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
Acked-by: Barry Song <baohua@...nel.org>
Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
---
 mm/rmap.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index a0fc05f5966f..7482121d4e92 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1862,9 +1862,10 @@ 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;
 
@@ -2230,7 +2231,7 @@ 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