Re: [PATCH 0/6] mm: batch rmap walks during large folio migration

[Garg, Shivank]

On 7/14/2026 4:25 PM, David Hildenbrand (Arm) wrote:
> On 7/12/26 09:38, Shivank Garg wrote:
>> Migrating a PTE-mapped large folio currently walks the folio's rmap
>> one page at a time. It does so twice per migration:
>> 1. try_to_migrate_one(): Replace present PTEs with migration entries
>> 2. remove_migration_pte(): Restore migration entries to working PTEs.
>>
>> For a folio with N contiguous PTEs, each walk repeats page_vma_mapped_walk()
>> and the associated operation per subpage of the folio. (256 iterations
>> for 1M folio).
>>
>> This series enables both walks to process a batch of consecutive PTEs
>> that map the same large folio and do some refactoring and clean up on
>> the way.
>>
>> Performance:
>>
>> AMD EPYC ZEN 3, 2-socket system (1 node per socket).
>> move_pages() migrating 1 GB from node 0 -> node 1, throughput
>> in GB/s:
>> 		         batch-	        batch-restore +
>> folio     vanilla        restore        unmap          restore%  restore+unmap%
>> 4K        2.98±0.09      2.95±0.07      3.06±0.05        -1.0%    +2.6%
>> 64K       6.16±0.09      6.24±0.08      7.80±0.08        +1.3%   +26.6%
>> 256K      6.85±0.06      6.85±0.09      9.54±0.19        +0.0%   +39.3%
>> 1M        7.06±0.09      7.17±0.09      9.89±0.06        +1.5%   +40.1%
>> 2M       10.97±0.17     10.67±0.13     10.69±0.15        -2.7%    -2.6%
>>
>> In plain migration, the folio copy itself dominate the cost, so these rmap walk
>> savings are only partly visible end-to-end.
>> When the folio copy is batched-copy and offloaded to DMA engine [1], the copy is
>> no longer the bottlneck and rmap-walk batching translates into much larger gains:
>>
>> This series + [1], DMA Offload on PTDMA (DCBM), 16 channels:
>>
>> 		     batch-copy
>> folio   vanilla      -offload(dcbm)  dcbm+restore   dcbm+restore+unmap
>> 4K      3.36 ± 0.10   4.16 ± 0.18     4.10 ± 0.14    4.12 ± 0.13
>> 64K     6.16 ± 0.47   9.63 ± 0.23    10.00 ± 0.33   14.63 ± 0.14
>> 256K    6.70 ± 0.09  12.98 ± 0.18    13.52 ± 0.15   32.30 ± 0.65    (2.38x)
>> 1M      6.99 ± 0.02  14.32 ± 0.67    14.84 ± 0.17   38.97 ± 1.03    (2.62x)
>> 2M     10.74 ± 0.29  64.25 ± 1.48    65.36 ± 1.00   64.97 ± 1.75
>>
>> [1] https://lore.kernel.org/linux-mm/20260630-shivank-batch-migrate-offload-v6-0-da95d7e8b8a2@xxxxxxx
>>
>> Applies cleanly on mm-new 10-07-26 (61cccb8363f).
>>
>> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
>> ---
>> Dev Jain (1):
>>       mm/rmap: Add batched version of folio_try_share_anon_rmap_pte
> 
> This would ideally be based on Dev's series instead if including a patch from
> his series.
> 

Sure, make sense. I will rebase on this.

I initially thought that including this patch will make series complete to easily apply and test.

Thanks,
Shivank