Re: [PATCH v5 1/6] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

["Huang, Ying" <ying.huang@intel.com>]

Ryan Roberts <ryan.roberts@arm.com> writes:

> 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
> shortcut a 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 functionally equivalent.
>
> That said, removing the fast path means that we will take a reference
> and trylock a large folio much more often, which we would like to avoid.
> The next patch will solve this.
>
> 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@arm.com>

LGTM, Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

> ---
>  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 a211a0383425..f6f78198f000 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
> @@ -590,15 +589,6 @@ 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 b49fcb8a16cc..8c1f3393994a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2961,9 +2961,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>  		shmem_uncharge(folio->mapping->host, nr_dropped);
>  	remap_page(folio, nr);
>  
> -	if (folio_test_swapcache(folio))
> -		split_swap_cluster(folio->swap);
> -
>  	/*
>  	 * set page to its compound_head when split to non order-0 pages, so
>  	 * we can skip unlocking it below, since PG_locked is transferred to
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 5e6d2304a2a4..0d44ee2b4f9c 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -343,18 +343,6 @@ 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)
>  {
> @@ -1027,7 +1015,7 @@ 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(ci, SWAPFILE_CLUSTER);
>  
>  	memset(si->swap_map + offset, SWAP_HAS_CACHE, SWAPFILE_CLUSTER);
>  	unlock_cluster(ci);
> @@ -1365,7 +1353,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
>  
>  	ci = lock_cluster_or_swap_info(si, offset);
>  	if (size == SWAPFILE_CLUSTER) {
> -		VM_BUG_ON(!cluster_is_huge(ci));
>  		map = si->swap_map + offset;
>  		for (i = 0; i < SWAPFILE_CLUSTER; i++) {
>  			val = map[i];
> @@ -1373,7 +1360,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
>  			if (val == SWAP_HAS_CACHE)
>  				free_entries++;
>  		}
> -		cluster_clear_huge(ci);
>  		if (free_entries == SWAPFILE_CLUSTER) {
>  			unlock_cluster_or_swap_info(si, ci);
>  			spin_lock(&si->lock);
> @@ -1395,23 +1381,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
>  	unlock_cluster_or_swap_info(si, ci);
>  }
>  
> -#ifdef CONFIG_THP_SWAP
> -int split_swap_cluster(swp_entry_t entry)
> -{
> -	struct swap_info_struct *si;
> -	struct swap_cluster_info *ci;
> -	unsigned long offset = swp_offset(entry);
> -
> -	si = _swap_info_get(entry);
> -	if (!si)
> -		return -EBUSY;
> -	ci = lock_cluster(si, offset);
> -	cluster_clear_huge(ci);
> -	unlock_cluster(ci);
> -	return 0;
> -}
> -#endif
> -
>  static int swp_entry_cmp(const void *ent1, const void *ent2)
>  {
>  	const swp_entry_t *e1 = ent1, *e2 = ent2;
> @@ -1519,22 +1488,23 @@ int swp_swapcount(swp_entry_t entry)
>  }
>  
>  static bool swap_page_trans_huge_swapped(struct swap_info_struct *si,
> -					 swp_entry_t entry)
> +					 swp_entry_t entry,
> +					 unsigned int nr_pages)
>  {
>  	struct swap_cluster_info *ci;
>  	unsigned char *map = si->swap_map;
>  	unsigned long roffset = swp_offset(entry);
> -	unsigned long offset = round_down(roffset, SWAPFILE_CLUSTER);
> +	unsigned long offset = round_down(roffset, nr_pages);
>  	int i;
>  	bool ret = false;
>  
>  	ci = lock_cluster_or_swap_info(si, offset);
> -	if (!ci || !cluster_is_huge(ci)) {
> +	if (!ci || nr_pages == 1) {
>  		if (swap_count(map[roffset]))
>  			ret = true;
>  		goto unlock_out;
>  	}
> -	for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> +	for (i = 0; i < nr_pages; i++) {
>  		if (swap_count(map[offset + i])) {
>  			ret = true;
>  			break;
> @@ -1556,7 +1526,7 @@ static bool folio_swapped(struct folio *folio)
>  	if (!IS_ENABLED(CONFIG_THP_SWAP) || likely(!folio_test_large(folio)))
>  		return swap_swapcount(si, entry) != 0;
>  
> -	return swap_page_trans_huge_swapped(si, entry);
> +	return swap_page_trans_huge_swapped(si, entry, folio_nr_pages(folio));
>  }
>  
>  /**
> @@ -1622,8 +1592,7 @@ int free_swap_and_cache(swp_entry_t entry)
>  		}
>  
>  		count = __swap_entry_free(p, entry);
> -		if (count == SWAP_HAS_CACHE &&
> -		    !swap_page_trans_huge_swapped(p, entry))
> +		if (count == SWAP_HAS_CACHE)
>  			__try_to_reclaim_swap(p, swp_offset(entry),
>  					      TTRS_UNMAPPED | TTRS_FULL);
>  		put_swap_device(p);