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

[Yang Shi]

On Thu, Jun 16, 2022 at 12:42 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>
> On 2022/6/16 7:58, Yang Shi wrote:
> > On Sat, Jun 11, 2022 at 1:47 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> 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@xxxxxxxxxx>
> >> ---
> >>  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?
>
> Thanks for catching this! If page is on percpu lru_pvecs cache, page will
> be released when lru_pvecs are drained. But if not, free_swap_cache() won't
> free the page as it assumes the caller has a reference on the page and thus
> only does page_ref_sub(). Does the below change looks sense for you?

THP gets drained immediately so they won't stay in pagevecs.

>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 52109ad13f78..b8c96e33591d 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -755,8 +755,12 @@ 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);
> +               mod_node_page_state(page_pgdat(src_page),
> +                                   NR_ISOLATED_ANON + page_is_file_lru(src_page),
> +                                   -compound_nr(src_page));
> +               unlock_page(src_page);
>                 free_swap_cache(src_page);
> +               putback_lru_page(src_page);

I'm not sure if it is worth it or not for a rare corner case since THP
should not stay in swapcache unless try_to_unmap() in vmscan fails
IIUC. And it is not guaranteed that free_swap_cache() will get the
page lock.

>         }
>  }
>
> Thanks!
>
> >
> >>         }
> >>  }
> >>
> >> 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
> >>
> >>
> > .
> >
>