On Mon, Sep 23, 2024 at 4:11 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> The SWAP_MAP_SHMEM state was introduced in the commit aaa468653b4a
> ("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a swap entry
> belongs to shmem during swapoff.
>
> However, swapoff has since been rewritten in the commit b56a2d8af914
> ("mm: rid swapoff of quadratic complexity"). Now having swap count ==
> SWAP_MAP_SHMEM value is basically the same as having swap count == 1,
> and swap_shmem_alloc() behaves analogously to swap_duplicate().
It's probably useful to point out that swap_shmem_alloc() is
equivalent to swap_duplicate() because __swap_duplicate() should never
return -ENOMEM for shmem, as we only ever increment the swap count by
1 (for each entry).
>
> Remove this state and the associated helper to simplify the state
> machine (both mentally and in terms of actual code). We will also have
> an extra state/special value that can be repurposed (for swap entries
> that never gets re-duplicated).
>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
> include/linux/swap.h | 6 ------
> mm/shmem.c | 2 +-
> mm/swapfile.c | 15 ---------------
> 3 files changed, 1 insertion(+), 22 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index e6ab234be7be..017f3c03ff7a 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -232,7 +232,6 @@ enum {
> /* Special value in first swap_map */
> #define SWAP_MAP_MAX 0x3e /* Max count */
> #define SWAP_MAP_BAD 0x3f /* Note page is bad */
> -#define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs */
>
> /* Special value in each swap_map continuation */
> #define SWAP_CONT_MAX 0x7f /* Max count */
> @@ -482,7 +481,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry);
> extern swp_entry_t get_swap_page_of_type(int);
> extern int get_swap_pages(int n, swp_entry_t swp_entries[], int order);
> extern int add_swap_count_continuation(swp_entry_t, gfp_t);
> -extern void swap_shmem_alloc(swp_entry_t, int);
> extern int swap_duplicate_nr(swp_entry_t, int);
> extern int swapcache_prepare(swp_entry_t entry, int nr);
> extern void swap_free_nr(swp_entry_t entry, int nr_pages);
> @@ -549,10 +547,6 @@ static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
> return 0;
> }
>
> -static inline void swap_shmem_alloc(swp_entry_t swp, int nr)
> -{
> -}
> -
> static inline int swap_duplicate_nr(swp_entry_t swp, int nr)
> {
> return 0;
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 361affdf3990..1875f2521dc6 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1559,7 +1559,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN,
> NULL) == 0) {
> shmem_recalc_inode(inode, 0, nr_pages);
> - swap_shmem_alloc(swap, nr_pages);
> + swap_duplicate_nr(swap, nr_pages);
> shmem_delete_from_page_cache(folio, swp_to_radix_entry(swap));
>
> mutex_unlock(&shmem_swaplist_mutex);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 47a2cd5f590d..cebc244ee60f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1381,12 +1381,6 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *si,
> if (usage == SWAP_HAS_CACHE) {
> VM_BUG_ON(!has_cache);
> has_cache = 0;
> - } else if (count == SWAP_MAP_SHMEM) {
> - /*
> - * Or we could insist on shmem.c using a special
> - * swap_shmem_free() and free_shmem_swap_and_cache()...
> - */
> - count = 0;
> } else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
> if (count == COUNT_CONTINUED) {
> if (swap_count_continued(si, offset, count))
> @@ -3686,15 +3680,6 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
> return err;
> }
>
> -/*
> - * Help swapoff by noting that swap entry belongs to shmem/tmpfs
> - * (in which case its reference count is never incremented).
> - */
> -void swap_shmem_alloc(swp_entry_t entry, int nr)
> -{
> - __swap_duplicate(entry, SWAP_MAP_SHMEM, nr);
> -}
> -
> /**
> * swap_duplicate_nr() - Increase reference count of nr contiguous swap entries
> * by 1.
> --
> 2.43.5
