Re: [RFC PATCH v3 2/5] mm: swap: introduce swap_nr_free() for batched swap_free()

[Ryan Roberts]

On 04/03/2024 08:13, Barry Song 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        | 35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 2955f7a78d8d..d6ab27929458 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -481,6 +481,7 @@ 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);

nit: In my swap-out v4 series, I've created a batched version of
free_swap_and_cache() and called it free_swap_and_cache_nr(). Perhaps it is
preferable to align the naming schemes - i.e. call this swap_free_nr(). Your
scheme doesn't really work when applied to free_swap_and_cache().

>  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);
> @@ -561,6 +562,11 @@ 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 3f594be83b58..244106998a69 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1341,6 +1341,41 @@ void swap_free(swp_entry_t entry)
>  		__swap_entry_free(p, entry);
>  }
>  
> +/*
> + * Called after swapping in a large folio, batched free swap entries
> + * for this large folio, entry should be for the first subpage and
> + * its offset is aligned with nr_pages
> + */
> +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);

nit: checkpatch.py will complain about bare "unsigned", preferring "unsigned
int" or at least it did for me when I did something similar in my swap-out patch
set.

> +	unsigned long offset = swp_offset(entry);
> +	DECLARE_BITMAP(usage, SWAPFILE_CLUSTER) = { 0 };

I don't love this, as it could blow the stack if SWAPFILE_CLUSTER ever
increases. But the only other way I can think of is to explicitly loop over
fixed size chunks, and that's not much better.

> +
> +	/* 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;
> +	}
> +
> +	p = _swap_info_get(entry);

You need to handle this returning NULL, like swap_free() does.

> +
> +	ci = lock_cluster(p, offset);

The existing swap_free() calls lock_cluster_or_swap_info(). So if swap is backed
by rotating media, and clusters are not in use, it will lock the whole swap
info. But your new version only calls lock_cluster() which won't lock anything
if clusters are not in use. So I think this is a locking bug.

> +	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));
> +}
> +
>  /*
>   * Called after dropping swapcache to decrease refcnt to swap entries.
>   */

Thanks,
Ryan