Re: [PATCH v6 1/6] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

[Chris Li]

Hi Ryan,

Sorry for the late reply. I want to review this series but don't have
the chance to do it sooner.

On Wed, Apr 3, 2024 at 4:40 AM Ryan Roberts <ryan.roberts@xxxxxxx> 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
> 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.

Seems necessary to remove the assumption of large folio be PMD size.

Acked-by: Chris Li <chrisl@xxxxxxxxxx>

>
> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> 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 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 ea6d1f09a0b9..3ca9282a0dc9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2844,9 +2844,6 @@ 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);

It is obvious this code only works for powers two nr_pages. The
SWAPFILE_CLSTER is a power of two. If we switch to an API for
nr_pages, we might want to warn/ban passing in the non-power of two
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++) {

Here we assume the swap entry offset is contiguous. That is beyond
your patch's scope. If in the future we want to have non-contiguous
swap entries to swap out large pages, we will need to find out and
change all the places that have the assumption of contiguous swap
entries.

Chris

>                 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);
> --
> 2.25.1
>