linux-kernel - Re: [PATCH 1/9] mm, swap: use unified helper for swap cache look up

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Message-ID: <aK6A76NVcV2j+J57@MiWiFi-R3L-srv>
Date: Wed, 27 Aug 2025 11:52:15 +0800
From: Baoquan He <bhe@...hat.com>
To: Kairui Song <kasong@...cent.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	Hugh Dickins <hughd@...gle.com>, Chris Li <chrisl@...nel.org>,
	Barry Song <baohua@...nel.org>, Nhat Pham <nphamcs@...il.com>,
	Kemeng Shi <shikemeng@...weicloud.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Ying Huang <ying.huang@...ux.alibaba.com>,
	Johannes Weiner <hannes@...xchg.org>,
	David Hildenbrand <david@...hat.com>,
	Yosry Ahmed <yosryahmed@...gle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Zi Yan <ziy@...dia.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/9] mm, swap: use unified helper for swap cache look up

On 08/23/25 at 03:20am, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>
......snip...
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 99513b74b5d8..ff9eb761a103 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -69,6 +69,21 @@ void show_swap_cache_info(void)
>  	printk("Total swap = %lukB\n", K(total_swap_pages));
>  }
>  
> +/*
> + * Lookup a swap entry in the swap cache. A found folio will be returned

Lookup is a noun, we should use 'look up' which is a verb here instead?

And all other places in swap code, even though they are not introduced
by this patchset. Just a nitpick.

> + * unlocked and with its refcount incremented.
> + *
> + * Caller must lock the swap device or hold a reference to keep it valid.
> + */
> +struct folio *swap_cache_get_folio(swp_entry_t entry)
> +{
> +	struct folio *folio = filemap_get_folio(swap_address_space(entry),
> +						swap_cache_index(entry));
> +	if (!IS_ERR(folio))
> +		return folio;
> +	return NULL;
> +}
> +
>  void *get_shadow_from_swap_cache(swp_entry_t entry)
>  {
>  	struct address_space *address_space = swap_address_space(entry);
> @@ -273,54 +288,40 @@ static inline bool swap_use_vma_readahead(void)
>  }
>  
>  /*
> - * Lookup a swap entry in the swap cache. A found folio will be returned
> - * unlocked and with its refcount incremented - we rely on the kernel
> - * lock getting page table operations atomic even if we drop the folio
> - * lock before returning.
> - *
> - * Caller must lock the swap device or hold a reference to keep it valid.
> + * Update the readahead statistics of a vma or globally.
>   */
> -struct folio *swap_cache_get_folio(swp_entry_t entry,
> -		struct vm_area_struct *vma, unsigned long addr)
> +void swap_update_readahead(struct folio *folio,
> +			   struct vm_area_struct *vma,
> +			   unsigned long addr)
>  {
> -	struct folio *folio;
> -
> -	folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry));
> -	if (!IS_ERR(folio)) {
> -		bool vma_ra = swap_use_vma_readahead();
> -		bool readahead;
> +	bool readahead, vma_ra = swap_use_vma_readahead();
>  
> -		/*
> -		 * At the moment, we don't support PG_readahead for anon THP
> -		 * so let's bail out rather than confusing the readahead stat.
> -		 */
> -		if (unlikely(folio_test_large(folio)))
> -			return folio;
> -
> -		readahead = folio_test_clear_readahead(folio);
> -		if (vma && vma_ra) {
> -			unsigned long ra_val;
> -			int win, hits;
> -
> -			ra_val = GET_SWAP_RA_VAL(vma);
> -			win = SWAP_RA_WIN(ra_val);
> -			hits = SWAP_RA_HITS(ra_val);
> -			if (readahead)
> -				hits = min_t(int, hits + 1, SWAP_RA_HITS_MAX);
> -			atomic_long_set(&vma->swap_readahead_info,
> -					SWAP_RA_VAL(addr, win, hits));
> -		}
> -
> -		if (readahead) {
> -			count_vm_event(SWAP_RA_HIT);
> -			if (!vma || !vma_ra)
> -				atomic_inc(&swapin_readahead_hits);
> -		}
> -	} else {
> -		folio = NULL;
> +	/*
> +	 * At the moment, we don't support PG_readahead for anon THP
> +	 * so let's bail out rather than confusing the readahead stat.
> +	 */
> +	if (unlikely(folio_test_large(folio)))
> +		return;
> +
> +	readahead = folio_test_clear_readahead(folio);
> +	if (vma && vma_ra) {
> +		unsigned long ra_val;
> +		int win, hits;
> +
> +		ra_val = GET_SWAP_RA_VAL(vma);
> +		win = SWAP_RA_WIN(ra_val);
> +		hits = SWAP_RA_HITS(ra_val);
> +		if (readahead)
> +			hits = min_t(int, hits + 1, SWAP_RA_HITS_MAX);
> +		atomic_long_set(&vma->swap_readahead_info,
> +				SWAP_RA_VAL(addr, win, hits));
>  	}
>  
> -	return folio;
> +	if (readahead) {
> +		count_vm_event(SWAP_RA_HIT);
> +		if (!vma || !vma_ra)
> +			atomic_inc(&swapin_readahead_hits);
> +	}
>  }
>  
>  struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> @@ -336,14 +337,10 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  	*new_page_allocated = false;
>  	for (;;) {
>  		int err;
> -		/*
> -		 * First check the swap cache.  Since this is normally
> -		 * called after swap_cache_get_folio() failed, re-calling
> -		 * that would confuse statistics.
> -		 */
> -		folio = filemap_get_folio(swap_address_space(entry),
> -					  swap_cache_index(entry));
> -		if (!IS_ERR(folio))
> +
> +		/* Check the swap cache in case the folio is already there */
> +		folio = swap_cache_get_folio(entry);
> +		if (folio)
>  			goto got_folio;
>  
>  		/*
......