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Message-ID: <aR65k5Xxl28MvMcf@yjaykim-PowerEdge-T330>
Date: Thu, 20 Nov 2025 15:47:47 +0900
From: YoungJun Park <youngjun.park@....com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>,
Chris Li <chrisl@...nel.org>, Nhat Pham <nphamcs@...il.com>,
Yosry Ahmed <yosry.ahmed@...ux.dev>,
David Hildenbrand <david@...nel.org>,
Johannes Weiner <hannes@...xchg.org>,
Hugh Dickins <hughd@...gle.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
Ying Huang <ying.huang@...ux.alibaba.com>,
Kemeng Shi <shikemeng@...weicloud.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
linux-kernel@...r.kernel.org, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v2 10/19] mm, swap: consolidate cluster reclaim and check
logic
On Mon, Nov 17, 2025 at 02:11:51AM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.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@...cent.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
>
>
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