Re: [PATCH RFC 2/6] mm: swap: introduce swap_nr_free() for batched swap_free()

[Chris Li]

On Thu, Jan 18, 2024 at 3:11 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> From: Chuanhua Han <hanchuanhua@xxxxxxxx>
>
> While swapping in a large folio, we need to free swaps related to the whole
> folio. To avoid frequently acquiring and releasing swap locks, it is better
> to introduce an API for batched free.
>
> Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
> Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
>  include/linux/swap.h |  6 ++++++
>  mm/swapfile.c        | 29 +++++++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 4db00ddad261..31a4ee2dcd1c 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -478,6 +478,7 @@ extern void swap_shmem_alloc(swp_entry_t);
>  extern int swap_duplicate(swp_entry_t);
>  extern int swapcache_prepare(swp_entry_t);
>  extern void swap_free(swp_entry_t);
> +extern void swap_nr_free(swp_entry_t entry, int nr_pages);
>  extern void swapcache_free_entries(swp_entry_t *entries, int n);
>  extern int free_swap_and_cache(swp_entry_t);
>  int swap_type_of(dev_t device, sector_t offset);
> @@ -553,6 +554,11 @@ static inline void swap_free(swp_entry_t swp)
>  {
>  }
>
> +void swap_nr_free(swp_entry_t entry, int nr_pages)
> +{
> +
> +}
> +
>  static inline void put_swap_folio(struct folio *folio, swp_entry_t swp)
>  {
>  }
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 556ff7347d5f..6321bda96b77 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1335,6 +1335,35 @@ void swap_free(swp_entry_t entry)
>                 __swap_entry_free(p, entry);
>  }
>
> +void swap_nr_free(swp_entry_t entry, int nr_pages)
> +{
> +       int i;
> +       struct swap_cluster_info *ci;
> +       struct swap_info_struct *p;
> +       unsigned type = swp_type(entry);
> +       unsigned long offset = swp_offset(entry);
> +       DECLARE_BITMAP(usage, SWAPFILE_CLUSTER) = { 0 };
> +
> +       VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > SWAPFILE_CLUSTER);

BUG_ON here seems a bit too developer originated. Maybe warn once and
roll back to free one by one?

How big is your typical SWAPFILE_CUSTER and nr_pages typically in arm?

I ask this question because nr_ppages > 64, that is a totally
different game, we can completely  bypass the swap cache slots.

> +
> +       if (nr_pages == 1) {
> +               swap_free(entry);
> +               return;
> +       }
> +
> +       p = _swap_info_get(entry);
> +
> +       ci = lock_cluster(p, offset);
> +       for (i = 0; i < nr_pages; i++) {
> +               if (__swap_entry_free_locked(p, offset + i, 1))
> +                       __bitmap_set(usage, i, 1);
> +       }
> +       unlock_cluster(ci);
> +
> +       for_each_clear_bit(i, usage, nr_pages)
> +               free_swap_slot(swp_entry(type, offset + i));

Notice that free_swap_slot() internal has per CPU cache batching as
well. Every free_swap_slot will get some per_cpu swap slot cache and
cache->lock. There is double batching here.
If the typical batch size here is bigger than 64 entries, we can go
directly to batching swap_entry_free and avoid the free_swap_slot()
batching altogether. Unlike free_swap_slot_entries(), here swap slots
are all from one swap device, there is no need to sort and group the
swap slot by swap devices.

Chris


Chris

> +}
> +
>  /*
>   * Called after dropping swapcache to decrease refcnt to swap entries.
>   */
> --
> 2.34.1
>
>