Re: [PATCH 6/7] mm, swap: remove swap slot cache

[Baoquan He]

Hi Kairui,

On 02/15/25 at 01:57am, Kairui Song wrote:
......snip....  
> -int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order)
> +swp_entry_t folio_alloc_swap(struct folio *folio)
>  {
> -	int order = swap_entry_order(entry_order);
> -	unsigned long size = 1 << order;
> +	unsigned int order = folio_order(folio);
> +	unsigned int size = 1 << order;
>  	struct swap_info_struct *si, *next;
> -	int n_ret = 0;
> +	swp_entry_t entry = {};
> +	unsigned long offset;
>  	int node;
>  
> +	if (order) {
> +		/*
> +		 * Should not even be attempting large allocations when huge
> +		 * page swap is disabled. Warn and fail the allocation.
> +		 */
> +		if (!IS_ENABLED(CONFIG_THP_SWAP) || size > SWAPFILE_CLUSTER) {
> +			VM_WARN_ON_ONCE(1);
> +			return entry;
> +		}
> +	}
> +
>  	/* Fast path using percpu cluster */
>  	local_lock(&percpu_swap_cluster.lock);
> -	n_ret = swap_alloc_fast(swp_entries,
> -				SWAP_HAS_CACHE,
> -				order, n_goal);
> -	if (n_ret == n_goal)
> -		goto out;
> +	if (swap_alloc_fast(&entry, SWAP_HAS_CACHE, order))
> +		goto out_alloced;
>  
> -	n_goal = min_t(int, n_goal - n_ret, SWAP_BATCH);
>  	/* Rotate the device and switch to a new cluster */
>  	spin_lock(&swap_avail_lock);
>  start_over:
> @@ -1268,11 +1236,14 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order)
>  		plist_requeue(&si->avail_lists[node], &swap_avail_heads[node]);
>  		spin_unlock(&swap_avail_lock);
>  		if (get_swap_device_info(si)) {
> -			n_ret += scan_swap_map_slots(si, SWAP_HAS_CACHE, n_goal,
> -					swp_entries + n_ret, order);
> +			offset = cluster_alloc_swap_entry(si, order, SWAP_HAS_CACHE);
>  			put_swap_device(si);
> -			if (n_ret || size > 1)
> -				goto out;
> +			if (offset) {
> +				entry = swp_entry(si->type, offset);
> +				goto out_alloced;
> +			}
> +			if (order)
> +				goto out_failed;

This is not related to this patch, do you know why non order-0 case
can't start over on different devices?

>  		}
>  
>  		spin_lock(&swap_avail_lock);
> @@ -1291,10 +1262,20 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order)
>  			goto start_over;
>  	}
>  	spin_unlock(&swap_avail_lock);
> -out:
> +out_failed:
> +	local_unlock(&percpu_swap_cluster.lock);
> +	return entry;
> +
> +out_alloced:
>  	local_unlock(&percpu_swap_cluster.lock);
> -	atomic_long_sub(n_ret * size, &nr_swap_pages);
> -	return n_ret;
> +	if (mem_cgroup_try_charge_swap(folio, entry)) {
> +		put_swap_folio(folio, entry);
> +		entry.val = 0;
> +	} else {
> +		atomic_long_sub(size, &nr_swap_pages);
> +	}
> +
> +	return entry;
>  }
>  
>  static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
......snip....
> @@ -2623,16 +2591,6 @@ static bool __has_usable_swap(void)
>  	return !plist_head_empty(&swap_active_head);
>  }

seems the __has_usable_swap() function need be moved into the ifdeffery
scope where __folio_throttle_swaprate() is located to fix the lkp
warning.

>  
> -bool has_usable_swap(void)
> -{
> -	bool ret;
> -
> -	spin_lock(&swap_lock);
> -	ret = __has_usable_swap();
> -	spin_unlock(&swap_lock);
> -	return ret;
> -}
> -
>  /*
>   * Called after clearing SWP_WRITEOK, ensures cluster_alloc_range
>   * see the updated flags, so there will be no more allocations.

Other than the test robot reported warning, this patch looks good to me.
Thanks.