Re: [PATCH v4 10/19] mm, swap: consolidate cluster reclaim and usability check

[Baoquan He <bhe@redhat.com>]

On 12/05/25 at 03:29am, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> Swap cluster cache reclaim requires releasing the lock, so the cluster
> may become unusable after the reclaim. To prepare for checking swap
> cache using the swap table directly, consolidate the swap cluster
> reclaim and the check logic.
> 
> We will want to avoid touching the cluster's data completely with the
     ~~~~~~~~
    'want to' means 'will'?

> swap table, to avoid RCU overhead here. And by moving the cluster usable
> check into the reclaim helper, it will also help avoid a redundant scan of
> the slots if the cluster is no longer usable, and we will want to avoid
                                                       ~~~~~~~~~~~~
                                                        this place too.
> touching the cluster.
> 
> Also, adjust it very slightly while at it: always scan the whole region
> during reclaim, don't skip slots covered by a reclaimed folio. Because
> the reclaim is lockless, it's possible that new cache lands at any time.
> And for allocation, we want all caches to be reclaimed to avoid
> fragmentation. Besides, if the scan offset is not aligned with the size
> of the reclaimed folio, we might skip some existing cache and fail the
> reclaim unexpectedly.
> 
> There should be no observable behavior change. It might slightly improve
> the fragmentation issue or performance.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  mm/swapfile.c | 45 +++++++++++++++++++++++++++++----------------
>  1 file changed, 29 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 5a766d4fcaa5..2703dfafc632 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -777,33 +777,51 @@ static int swap_cluster_setup_bad_slot(struct swap_cluster_info *cluster_info,
>  	return 0;
>  }
>  
> +/*
> + * Reclaim drops the ci lock, so the cluster may become unusable (freed or
> + * stolen by a lower order). @usable will be set to false if that happens.
> + */
>  static bool cluster_reclaim_range(struct swap_info_struct *si,
>  				  struct swap_cluster_info *ci,
> -				  unsigned long start, unsigned long end)
> +				  unsigned long start, unsigned int order,
> +				  bool *usable)
>  {
> +	unsigned int nr_pages = 1 << order;
> +	unsigned long offset = start, end = start + nr_pages;
>  	unsigned char *map = si->swap_map;
> -	unsigned long offset = start;
>  	int nr_reclaim;
>  
>  	spin_unlock(&ci->lock);
>  	do {
>  		switch (READ_ONCE(map[offset])) {
>  		case 0:
> -			offset++;
>  			break;
>  		case SWAP_HAS_CACHE:
>  			nr_reclaim = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY);
> -			if (nr_reclaim > 0)
> -				offset += nr_reclaim;
> -			else
> +			if (nr_reclaim < 0)
>  				goto out;
>  			break;
>  		default:
>  			goto out;
>  		}
> -	} while (offset < end);
> +	} while (++offset < end);
                 ~~~~~ '++offset' is conflicting with nr_reclaim
                 returned from __try_to_reclaim_swap(). can you explain?
>  out:
>  	spin_lock(&ci->lock);
> +
> +	/*
> +	 * We just dropped ci->lock so cluster could be used by another
> +	 * order or got freed, check if it's still usable or empty.
> +	 */
> +	if (!cluster_is_usable(ci, order)) {
> +		*usable = false;
> +		return false;
> +	}
> +	*usable = true;
> +
> +	/* Fast path, no need to scan if the whole cluster is empty */
> +	if (cluster_is_empty(ci))
> +		return true;
> +
>  	/*
>  	 * Recheck the range no matter reclaim succeeded or not, the slot
>  	 * could have been be freed while we are not holding the lock.
> @@ -900,9 +918,10 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
>  	unsigned long start = ALIGN_DOWN(offset, SWAPFILE_CLUSTER);
>  	unsigned long end = min(start + SWAPFILE_CLUSTER, si->max);
>  	unsigned int nr_pages = 1 << order;
> -	bool need_reclaim, ret;
> +	bool need_reclaim, ret, usable;
>  
>  	lockdep_assert_held(&ci->lock);
> +	VM_WARN_ON(!cluster_is_usable(ci, order));
>  
>  	if (end < nr_pages || ci->count + nr_pages > SWAPFILE_CLUSTER)
>  		goto out;
> @@ -912,14 +931,8 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
>  		if (!cluster_scan_range(si, ci, offset, nr_pages, &need_reclaim))
>  			continue;
>  		if (need_reclaim) {
> -			ret = cluster_reclaim_range(si, ci, offset, offset + nr_pages);
> -			/*
> -			 * Reclaim drops ci->lock and cluster could be used
> -			 * by another order. Not checking flag as off-list
> -			 * cluster has no flag set, and change of list
> -			 * won't cause fragmentation.
> -			 */
> -			if (!cluster_is_usable(ci, order))
> +			ret = cluster_reclaim_range(si, ci, offset, order, &usable);
> +			if (!usable)
>  				goto out;
>  			if (cluster_is_empty(ci))
>  				offset = start;
> 
> -- 
> 2.52.0
>