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

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Date:   Wed, 15 Jun 2022 10:13:55 -0700
From:   Zach O'Keefe <zokeefe@...gle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, aarcange@...hat.com,
        willy@...radead.org, vbabka@...e.cz, dhowells@...hat.com,
        neilb@...e.de, apopple@...dia.com, david@...hat.com,
        surenb@...gle.com, peterx@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] mm/khugepaged: try to free transhuge swapcache when
 possible

On 11 Jun 16:47, Miaohe Lin 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);
>  	}
>  }

Aside: in __collapse_huge_page_isolate() (and also here) why can't we just check
PageCompound(page) && page == compound_head(page) to only act on compound pages
once? AFAIK this would alleviate this compound_pagelist business..

Anyways, as-is, free_page_and_swap_cache() won't be able to do
try_to_free_swap(), since it can't grab page lock, put it will call put_page().
I think (?) the last page ref might be dropped in release_pte_page(), so should
free_swap_cache() come before it?

>  
> 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
> 
>