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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Kairui Song <ryncsn@gmail.com>
Cc:     linux-mm@kvack.org, Kairui Song <kasong@tencent.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 09/24] mm/swap: inline __swap_count
In-Reply-To: <20231119194740.94101-10-ryncsn@gmail.com> (Kairui Song's message
        of "Mon, 20 Nov 2023 03:47:25 +0800")
References: <20231119194740.94101-1-ryncsn@gmail.com>
        <20231119194740.94101-10-ryncsn@gmail.com>
Date:   Mon, 20 Nov 2023 15:41:12 +0800
Message-ID: <87il5wc1nr.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Kairui Song <ryncsn@gmail.com> writes:

> From: Kairui Song <kasong@tencent.com>
>
> There is only one caller in swap subsystem now, where it can be inline
> smoothly, avoid the memory access and function call overheads.
>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  include/linux/swap.h | 6 ------
>  mm/swap_state.c      | 6 +++---
>  mm/swapfile.c        | 8 --------
>  3 files changed, 3 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 2401990d954d..64a37819a9b3 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -485,7 +485,6 @@ int swap_type_of(dev_t device, sector_t offset);
>  int find_first_swap(dev_t *device);
>  extern unsigned int count_swap_pages(int, int);
>  extern sector_t swapdev_block(int, pgoff_t);
> -extern int __swap_count(swp_entry_t entry);
>  extern int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry);
>  extern int swp_swapcount(swp_entry_t entry);
>  extern struct swap_info_struct *page_swap_info(struct page *);
> @@ -559,11 +558,6 @@ static inline void put_swap_folio(struct folio *folio, swp_entry_t swp)
>  {
>  }
>  
> -static inline int __swap_count(swp_entry_t entry)
> -{
> -	return 0;
> -}
> -
>  static inline int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
>  {
>  	return 0;
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index fb78f7f18ed7..d87c20f9f7ec 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -316,9 +316,9 @@ void free_pages_and_swap_cache(struct encoded_page **pages, int nr)
>  	release_pages(pages, nr);
>  }
>  
> -static inline bool swap_use_no_readahead(struct swap_info_struct *si, swp_entry_t entry)
> +static inline bool swap_use_no_readahead(struct swap_info_struct *si, pgoff_t offset)
>  {
> -	return data_race(si->flags & SWP_SYNCHRONOUS_IO) && __swap_count(entry) == 1;
> +	return data_race(si->flags & SWP_SYNCHRONOUS_IO) && swap_count(si->swap_map[offset]) == 1;
>  }
>  
>  static inline bool swap_use_vma_readahead(struct swap_info_struct *si)
> @@ -928,7 +928,7 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
>  
>  	si = swp_swap_info(entry);
>  	mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
> -	if (swap_use_no_readahead(si, entry)) {
> +	if (swap_use_no_readahead(si, swp_offset(entry))) {
>  		page = swapin_no_readahead(entry, gfp_mask, mpol, ilx, vmf->vma->vm_mm);
>  		cached = false;
>  	} else if (swap_use_vma_readahead(si)) {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index a8ae472ed2b6..e15a6c464a38 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1431,14 +1431,6 @@ void swapcache_free_entries(swp_entry_t *entries, int n)
>  		spin_unlock(&p->lock);
>  }
>  
> -int __swap_count(swp_entry_t entry)
> -{
> -	struct swap_info_struct *si = swp_swap_info(entry);
> -	pgoff_t offset = swp_offset(entry);
> -
> -	return swap_count(si->swap_map[offset]);
> -}
> -

I'd rather keep __swap_count() in the original place together with other
swap count related functions.  And si->swap_map[] was hided in
swapfile.c before.  I don't think the change will have any real
performance improvement.

>  /*
>   * How many references to @entry are currently swapped out?
>   * This does not give an exact answer when swap count is continued,

--
Best Regards,
Huang, Ying