Re: [RFC PATCH v1 1/2] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

[Kefeng Wang]

On 2023/10/10 22:21, Ryan Roberts wrote:
> As preparation for supporting small-sized THP in the swap-out path,
> without first needing to split to order-0, Remove the CLUSTER_FLAG_HUGE,
> which, when present, always implies PMD-sized THP, which is the same as
> the cluster size.
>
> The only use of the flag was to determine whether a swap entry refers to
> a single page or a PMD-sized THP in swap_page_trans_huge_swapped().
> Instead of relying on the flag, we now pass in nr_pages, which
> originates from the folio's number of pages. This allows the logic to
> work for folios of any order.
>
> The one snag is that one of the swap_page_trans_huge_swapped() call
> sites does not have the folio. But it was only being called there to
> avoid bothering to call __try_to_reclaim_swap() in some cases.
> __try_to_reclaim_swap() gets the folio and (via some other functions)
> calls swap_page_trans_huge_swapped(). So I've removed the problematic
> call site and believe the new logic should be equivalent.
>
> Removing CLUSTER_FLAG_HUGE also means we can remove split_swap_cluster()
> which used to be called during folio splitting, since
> split_swap_cluster()'s only job was to remove the flag.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
>   include/linux/swap.h | 10 ----------
>   mm/huge_memory.c     |  3 ---
>   mm/swapfile.c        | 47 ++++++++------------------------------------
>   3 files changed, 8 insertions(+), 52 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 19f30a29e1f1..a073366a227c 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -259,7 +259,6 @@ struct swap_cluster_info {
>   };
>   #define CLUSTER_FLAG_FREE 1 /* This cluster is free */
>   #define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
> -#define CLUSTER_FLAG_HUGE 4 /* This cluster is backing a transparent huge page */
>
>   /*
>    * We assign a cluster to each CPU, so each CPU can allocate swap entry from
> @@ -595,15 +594,6 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
>   }
>   #endif /* CONFIG_SWAP */
>
> -#ifdef CONFIG_THP_SWAP
> -extern int split_swap_cluster(swp_entry_t entry);
> -#else
> -static inline int split_swap_cluster(swp_entry_t entry)
> -{
> -	return 0;
> -}
> -#endif
> -
>   #ifdef CONFIG_MEMCG
>   static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
>   {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index c9cbcbf6697e..46b3fb943207 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2597,9 +2597,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>   		shmem_uncharge(head->mapping->host, nr_dropped);
>   	remap_page(folio, nr);
>
> -	if (folio_test_swapcache(folio))
> -		split_swap_cluster(folio->swap);
> -
>   	for (i = 0; i < nr; i++) {
>   		struct page *subpage = head + i;
>   		if (subpage == page)
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index e52f486834eb..c668838fa660 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -342,18 +342,6 @@ static inline void cluster_set_null(struct swap_cluster_info *info)
>   	info->data = 0;
>   }
>
> -static inline bool cluster_is_huge(struct swap_cluster_info *info)
> -{
> -	if (IS_ENABLED(CONFIG_THP_SWAP))
> -		return info->flags & CLUSTER_FLAG_HUGE;
> -	return false;
> -}
> -
> -static inline void cluster_clear_huge(struct swap_cluster_info *info)
> -{
> -	info->flags &= ~CLUSTER_FLAG_HUGE;
> -}
> -
>   static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
>   						     unsigned long offset)
>   {
> @@ -1021,7 +1009,7 @@ static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
>   	offset = idx * SWAPFILE_CLUSTER;
>   	ci = lock_cluster(si, offset);
>   	alloc_cluster(si, idx);
> -	cluster_set_count_flag(ci, SWAPFILE_CLUSTER, CLUSTER_FLAG_HUGE);
> +	cluster_set_count_flag(ci, SWAPFILE_CLUSTER, 0);

Maybe just use cluster_set_count() and kill cluster_set_count_flag().