linux-kernel - Re: [PATCH 7/7] mm/khugepaged: try to free transhuge swapcache when possible

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Date:   Wed, 15 Jun 2022 16:58:18 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Howells <dhowells@...hat.com>, NeilBrown <neilb@...e.de>,
        Alistair Popple <apopple@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Peter Xu <peterx@...hat.com>, Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] mm/khugepaged: try to free transhuge swapcache when possible

On Sat, Jun 11, 2022 at 1:47 AM Miaohe Lin <linmiaohe@...wei.com> wrote:
>
> Transhuge swapcaches won't be freed in __collapse_huge_page_copy().
> It's because release_pte_page() is not called for these pages and
> thus free_page_and_swap_cache can't grab the page lock. These pages
> won't be freed from swap cache even if we are the only user until
> next time reclaim. It shouldn't hurt indeed, but we could try to
> free these pages to save more memory for system.


>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  include/linux/swap.h | 5 +++++
>  mm/khugepaged.c      | 1 +
>  mm/swap.h            | 5 -----
>  3 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 8672a7123ccd..ccb83b12b724 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -456,6 +456,7 @@ static inline unsigned long total_swapcache_pages(void)
>         return global_node_page_state(NR_SWAPCACHE);
>  }
>
> +extern void free_swap_cache(struct page *page);
>  extern void free_page_and_swap_cache(struct page *);
>  extern void free_pages_and_swap_cache(struct page **, int);
>  /* linux/mm/swapfile.c */
> @@ -540,6 +541,10 @@ static inline void put_swap_device(struct swap_info_struct *si)
>  /* used to sanity check ptes in zap_pte_range when CONFIG_SWAP=0 */
>  #define free_swap_and_cache(e) is_pfn_swap_entry(e)
>
> +static inline void free_swap_cache(struct page *page)
> +{
> +}
> +
>  static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
>  {
>         return 0;
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index ee0a719c8be9..52109ad13f78 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -756,6 +756,7 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>         list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
>                 list_del(&src_page->lru);
>                 release_pte_page(src_page);
> +               free_swap_cache(src_page);

Will this really work? The free_swap_cache() will just dec refcounts
without putting the page back to buddy. So the hugepage is not
actually freed at all. Am I missing something?

>         }
>  }
>
> diff --git a/mm/swap.h b/mm/swap.h
> index 0193797b0c92..863f6086c916 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -41,7 +41,6 @@ void __delete_from_swap_cache(struct page *page,
>  void delete_from_swap_cache(struct page *page);
>  void clear_shadow_from_swap_cache(int type, unsigned long begin,
>                                   unsigned long end);
> -void free_swap_cache(struct page *page);
>  struct page *lookup_swap_cache(swp_entry_t entry,
>                                struct vm_area_struct *vma,
>                                unsigned long addr);
> @@ -81,10 +80,6 @@ static inline struct address_space *swap_address_space(swp_entry_t entry)
>         return NULL;
>  }
>
> -static inline void free_swap_cache(struct page *page)
> -{
> -}
> -
>  static inline void show_swap_cache_info(void)
>  {
>  }
> --
> 2.23.0
>
>