Re: [PATCH 4/9] mm: swap: remove unneeded VM_BUG_ON(*map != SWAP_HAS_CACHE) in swap_entry_range_free()

[Tim Chen <tim.c.chen@linux.intel.com>]

On Fri, 2025-03-14 at 05:05 +0800, Kemeng Shi wrote:
> As all callers of swap_entry_range_free() have already ensured slots to
> be freed are marked as SWAP_HAS_CACHE while holding the cluster lock,
> the BUG_ON check can be safely removed. After this, the function
> swap_entry_range_free() could drop any kind of last flag, rename it to
> swap_entries_free() and update it's comment accordingly.
> 
> This is a preparation to use swap_entries_free() to drop last 1 and

Probably clearer to say 
drop last ref count 
instead or drop last 1

> SWAP_MAP_SHMEM flag.
> 
> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
> ---
>  mm/swapfile.c | 21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index ba37b9bff586..14b7b37996ff 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -52,9 +52,9 @@
>  static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
>  				 unsigned char);
>  static void free_swap_count_continuations(struct swap_info_struct *);
> -static void swap_entry_range_free(struct swap_info_struct *si,
> -				  struct swap_cluster_info *ci,
> -				  swp_entry_t entry, unsigned int nr_pages);
> +static void swap_entries_free(struct swap_info_struct *si,
> +			      struct swap_cluster_info *ci,
> +			      swp_entry_t entry, unsigned int nr_pages);
>  static void swap_range_alloc(struct swap_info_struct *si,
>  			     unsigned int nr_entries);
>  static bool folio_swapcache_freeable(struct folio *folio);
> @@ -1511,7 +1511,7 @@ static bool swap_entries_put_nr(struct swap_info_struct *si,
>  	for (i = 0; i < nr; i++)
>  		WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
>  	if (!has_cache)
> -		swap_entry_range_free(si, ci, entry, nr);
> +		swap_entries_free(si, ci, entry, nr);
>  	unlock_cluster(ci);
>  
>  	return has_cache;
> @@ -1530,12 +1530,12 @@ static bool swap_entries_put_nr(struct swap_info_struct *si,
>  }
>  
>  /*
> - * Drop the last HAS_CACHE flag of swap entries, caller have to
> - * ensure all entries belong to the same cgroup.
> + * Drop the last flag(1, SWAP_HAS_CACHE or SWAP_MAP_SHMEM) of swap entries,
> + * caller have to ensure all entries belong to the same cgroup.

Will be nice to modify the above comment: 
	all entries belong to the same cgroup and cluster.

Otherwise

Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
 
>  
> -static void swap_entry_range_free(struct swap_info_struct *si,
> -				  struct swap_cluster_info *ci,
> -				  swp_entry_t entry, unsigned int nr_pages)
> +static void swap_entries_free(struct swap_info_struct *si,
> +			      struct swap_cluster_info *ci,
> +			      swp_entry_t entry, unsigned int nr_pages)
>  {
>  	unsigned long offset = swp_offset(entry);
>  	unsigned char *map = si->swap_map + offset;
> @@ -1545,7 +1545,6 @@ static void swap_entry_range_free(struct swap_info_struct *si,
>  	VM_BUG_ON(ci != offset_to_cluster(si, offset + nr_pages - 1));
>  
>  	do {
> -		VM_BUG_ON(*map != SWAP_HAS_CACHE);
>  		*map = 0;
>  	} while (++map < map_end);
>  
> @@ -1605,7 +1604,7 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
>  
>  	ci = lock_cluster(si, offset);
>  	if (swap_only_has_cache(si, offset, size))
> -		swap_entry_range_free(si, ci, entry, size);
> +		swap_entries_free(si, ci, entry, size);
>  	else
>  		for (int i = 0; i < size; i++, entry.val++)
>  			swap_entry_put_locked(si, ci, entry, SWAP_HAS_CACHE);