Re: [PATCH v2 10/19] mm, swap: consolidate cluster reclaim and check logic

[YoungJun Park <youngjun.park@lge.com>]

On Mon, Nov 17, 2025 at 02:11:51AM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> Swap cluster cache reclaim requires releasing the lock, so some extra
> checks are needed after the reclaim. To prepare for checking swap cache
> using the swap table directly, consolidate the swap cluster reclaim and
> check the logic.
> 
> Also, adjust it very slightly. By moving the cluster empty and usable
> check into the reclaim helper, it will avoid a redundant scan of the
> slots if the cluster is empty.
> 
> And 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. And besides, if the
> scan offset is not aligned with the size of the reclaimed folio, we are
> skipping some existing caches.
> 
> There should be no observable behavior change, which might slightly
> improve the fragmentation issue or performance.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  mm/swapfile.c | 45 +++++++++++++++++++++++----------------------
>  1 file changed, 23 insertions(+), 22 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index d2e60734ce8f..d57e83a4d0a7 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -778,42 +778,50 @@ static int swap_cluster_setup_bad_slot(struct swap_cluster_info *cluster_info,
>  	return 0;
>  }
>  
> -static bool cluster_reclaim_range(struct swap_info_struct *si,
> -				  struct swap_cluster_info *ci,
> -				  unsigned long start, unsigned long end)
> +static unsigned int cluster_reclaim_range(struct swap_info_struct *si,
> +					  struct swap_cluster_info *ci,
> +					  unsigned long start, unsigned int order)
>  {
> +	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);
>  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))
> +		return SWAP_ENTRY_INVALID;
> +	if (cluster_is_empty(ci))
> +		return cluster_offset(si, ci);
> +
>  	/*
>  	 * Recheck the range no matter reclaim succeeded or not, the slot
>  	 * could have been be freed while we are not holding the lock.
>  	 */
>  	for (offset = start; offset < end; offset++)
>  		if (READ_ONCE(map[offset]))
> -			return false;
> +			return SWAP_ENTRY_INVALID;
>  
> -	return true;
> +	return start;
>  }
>  
>  static bool cluster_scan_range(struct swap_info_struct *si,
> @@ -901,7 +909,7 @@ 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);

The Original code. I'm wondering if there's an off-by-one error here. Looking at the code
below, it seems the design allows the end offset to go through the
logic as well. Shouldn't it be 'start + SWAPFILE_CLUSTER - 1' and
'si->max - 1'?

>  	unsigned int nr_pages = 1 << order;
> -	bool need_reclaim, ret;
> +	bool need_reclaim;
>  
>  	lockdep_assert_held(&ci->lock);
>  
> @@ -913,20 +921,13 @@ 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.
> -			 */
> +			found = cluster_reclaim_range(si, ci, offset, order);
>  			if (!cluster_is_usable(ci, order))
>  				goto out;

This check resolves the issue I mentioned in my previous review.

> -			if (cluster_is_empty(ci))
> -				offset = start;
>  			/* Reclaim failed but cluster is usable, try next */
> -			if (!ret)
> +			if (!found)
>  				continue;
> +			offset = found;
>  		}
>  		if (!cluster_alloc_range(si, ci, offset, usage, order))
>  			break;

I think the reason cluster_is_usable() is checked redundantly here is
because cluster_reclaim_range() returns an unsigned int (offset), making
it impossible to distinguish error values.

What if we make offset an output parameter (satisfying the assumption
that it can be changed in reclaim_range) and return an error value
instead? This would eliminate the redundant cluster_is_usable() check
and simplify the logic. Also, the consecutive "offset = found, found =
offset" is a bit confusing, and this approach could eliminate that as
well.

What do you think?

Thnaks!

Youngjun Park
> -- 
> 2.51.2
> 
>