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

[Ryan Roberts <ryan.roberts@arm.com>]

On 23/12/2025 05:48, 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
> Acked-by: Barry Song <baohua@kernel.org>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Reviewed-by: Ryan Roberts <ryan.roberts@arm.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))) {