[PATCH v5 5/5] mm: rmap: support batched unmapping for file large folios

[Lorenzo Stoakes]

FYI Dev found an issue here, see [0].

[0]: https://lore.kernel.org/linux-mm/20260116082721.275178-1-dev.jain@arm.com/

Cheers, Lorenzo

On Fri, Dec 26, 2025 at 02:07:59PM +0800, Baolin Wang wrote:
> 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
> Reviewed-by: Ryan Roberts <ryan.roberts at arm.com>
> Acked-by: Barry Song <baohua at kernel.org>
> Signed-off-by: Baolin Wang <baolin.wang at linux.alibaba.com>
> ---
>  mm/rmap.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 985ab0b085ba..e1d16003c514 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1863,9 +1863,10 @@ 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;
>
> @@ -2231,7 +2232,7 @@ 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
>