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

[Baolin Wang]

On 2/9/26 5:38 PM, David Hildenbrand (Arm) wrote:
> On 12/26/25 07:07, 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 at 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;
> 
> Right, the anon folio handling would require a bit more work in the
> 
> 
>      } else if (folio_test_anon(folio)) {
> 
> branch.
> 
> Do you intend to tackle that one as well?
 >> I'll reply to the fixup.

I'm not sure whether Barry has time to continue this work. If he does 
not, I can take over. Barry?