Re: [PATCH -V3 2/5] swap, __read_swap_cache_async(): enlarge get/put_swap_device protection range

[Chris Li <chrisl@kernel.org>]

On Mon, May 29, 2023 at 02:13:52PM +0800, Huang Ying wrote:
> This makes the function a little easier to be understood because we
> don't need to consider swapoff.  And this makes it possible to remove
> get/put_swap_device() calling in some functions called by
> __read_swap_cache_async().
> 
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Chris Li <chrisl@kernel.org>
> Cc: Yosry Ahmed <yosryahmed@google.com>
> ---
>  mm/swap_state.c | 31 +++++++++++++++++++++----------
>  1 file changed, 21 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index b76a65ac28b3..a8450b4a110c 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -417,9 +417,13 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  {
>  	struct swap_info_struct *si;
>  	struct folio *folio;
> +	struct page *page;
>  	void *shadow = NULL;
>  
>  	*new_page_allocated = false;
> +	si = get_swap_device(entry);
> +	if (!si)
> +		return NULL;
>  
>  	for (;;) {
>  		int err;
> @@ -428,14 +432,12 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 * called after swap_cache_get_folio() failed, re-calling
>  		 * that would confuse statistics.
>  		 */
> -		si = get_swap_device(entry);
> -		if (!si)
> -			return NULL;
>  		folio = filemap_get_folio(swap_address_space(entry),
>  						swp_offset(entry));
> -		put_swap_device(si);
> -		if (!IS_ERR(folio))
> -			return folio_file_page(folio, swp_offset(entry));
> +		if (!IS_ERR(folio)) {
> +			page = folio_file_page(folio, swp_offset(entry));
> +			goto got_page;
> +		}
>  
>  		/*
>  		 * Just skip read ahead for unused swap slot.
> @@ -446,7 +448,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 * else swap_off will be aborted if we return NULL.
>  		 */
>  		if (!__swp_swapcount(entry) && swap_slot_cache_enabled)
> -			return NULL;
> +			goto fail_put_swap;
>  
>  		/*
>  		 * Get a new page to read into from swap.  Allocate it now,
> @@ -455,7 +457,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 */
>  		folio = vma_alloc_folio(gfp_mask, 0, vma, addr, false);
>  		if (!folio)
> -			return NULL;
> +                        goto fail_put_swap;
>  
>  		/*
>  		 * Swap entry may have been freed since our caller observed it.
> @@ -466,7 +468,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  
>  		folio_put(folio);
>  		if (err != -EEXIST)
> -			return NULL;
> +			goto fail_put_swap;
>  
>  		/*
>  		 * We might race against __delete_from_swap_cache(), and
> @@ -500,12 +502,17 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  	/* Caller will initiate read into locked folio */
>  	folio_add_lru(folio);
>  	*new_page_allocated = true;
> -	return &folio->page;
> +	page = &folio->page;
> +got_page:
> +	put_swap_device(si);
> +	return page;
>  
>  fail_unlock:
>  	put_swap_folio(folio, entry);
>  	folio_unlock(folio);
>  	folio_put(folio);
> +fail_put_swap:
> +	put_swap_device(si);
>  	return NULL;
>  }
>  
> @@ -514,6 +521,10 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>   * and reading the disk if it is not already cached.
>   * A failure return means that either the page allocation failed or that
>   * the swap entry is no longer in use.
> + *
> + * get/put_swap_device() aren't needed to call this function, because
> + * __read_swap_cache_async() call them and swap_readpage() holds the
> + * swap cache folio lock.
>   */
>  struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  				   struct vm_area_struct *vma,
> -- 
> 2.39.2
>

Reviewed-by: Chris Li (Google) <chrisl@kernel.org>

Chris