Re: [PATCH v2 1/5] mm: swap: introduce swap_free_nr() for batched swap_free()

[Huang, Ying]

Barry Song <21cnbao@xxxxxxxxx> writes:

> 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 |  5 +++++
>  mm/swapfile.c        | 51 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 56 insertions(+)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 11c53692f65f..b7a107e983b8 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -483,6 +483,7 @@ 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_free_nr(swp_entry_t entry, int nr_pages);
>  extern void swapcache_free_entries(swp_entry_t *entries, int n);
>  extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
>  int swap_type_of(dev_t device, sector_t offset);
> @@ -564,6 +565,10 @@ static inline void swap_free(swp_entry_t swp)
>  {
>  }
>  
> +void swap_free_nr(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 28642c188c93..f4c65aeb088d 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1356,6 +1356,57 @@ void swap_free(swp_entry_t entry)
>  		__swap_entry_free(p, entry);
>  }
>  
> +/*
> + * Free up the maximum number of swap entries at once to limit the
> + * maximum kernel stack usage.
> + */
> +#define SWAP_BATCH_NR (SWAPFILE_CLUSTER > 512 ? 512 : SWAPFILE_CLUSTER)
> +
> +/*
> + * Called after swapping in a large folio,

IMHO, it's not good to document the caller in the function definition.
Because this will discourage function reusing.

> batched free swap entries
> + * for this large folio, entry should be for the first subpage and
> + * its offset is aligned with nr_pages

Why do we need this?

> + */
> +void swap_free_nr(swp_entry_t entry, int nr_pages)
> +{
> +	int i, j;
> +	struct swap_cluster_info *ci;
> +	struct swap_info_struct *p;
> +	unsigned int type = swp_type(entry);
> +	unsigned long offset = swp_offset(entry);
> +	int batch_nr, remain_nr;
> +	DECLARE_BITMAP(usage, SWAP_BATCH_NR) = { 0 };
> +
> +	/* all swap entries are within a cluster for mTHP */
> +	VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > SWAPFILE_CLUSTER);
> +
> +	if (nr_pages == 1) {
> +		swap_free(entry);
> +		return;
> +	}

Is it possible to unify swap_free() and swap_free_nr() into one function
with acceptable performance?  IIUC, the general rule in mTHP effort is
to avoid duplicate functions between mTHP and normal small folio.
Right?

> +
> +	remain_nr = nr_pages;
> +	p = _swap_info_get(entry);
> +	if (p) {
> +		for (i = 0; i < nr_pages; i += batch_nr) {
> +			batch_nr = min_t(int, SWAP_BATCH_NR, remain_nr);
> +
> +			ci = lock_cluster_or_swap_info(p, offset);
> +			for (j = 0; j < batch_nr; j++) {
> +				if (__swap_entry_free_locked(p, offset + i * SWAP_BATCH_NR + j, 1))
> +					__bitmap_set(usage, j, 1);
> +			}
> +			unlock_cluster_or_swap_info(p, ci);
> +
> +			for_each_clear_bit(j, usage, batch_nr)
> +				free_swap_slot(swp_entry(type, offset + i * SWAP_BATCH_NR + j));
> +
> +			bitmap_clear(usage, 0, SWAP_BATCH_NR);
> +			remain_nr -= batch_nr;
> +		}
> +	}
> +}
> +
>  /*
>   * Called after dropping swapcache to decrease refcnt to swap entries.
>   */

put_swap_folio() implements batching in another method.  Do you think
that it's good to use the batching method in that function here?  It
avoids to use bitmap operations and stack space.

--
Best Regards,
Huang, Ying