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mm, swap: prefer nonfull over free clusters

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We prefer a free cluster over a nonfull cluster whenever a CPU local
cluster is drained to respect the SSD discard behavior [1].  It's not a
best practice for non-discarding devices.  And this is causing a higher
fragmentation rate.

So for a non-discarding device, prefer nonfull over free clusters.  This
reduces the fragmentation issue by a lot.

Testing with make -j96, defconfig, using 64k mTHP, 8G ZRAM:

Before: sys time: 6176.34s  64kB/swpout: 1659757  64kB/swpout_fallback: 139503
After:  sys time: 6194.11s  64kB/swpout: 1689470  64kB/swpout_fallback: 56147

Testing with make -j96, defconfig, using 64k mTHP, 10G ZRAM:

After:  sys time: 5531.49s  64kB/swpout: 1791142  64kB/swpout_fallback: 17676
After:  sys time: 5587.53s  64kB/swpout: 1811598  64kB/swpout_fallback: 0

Performance is basically unchanged, and the large allocation failure rate
is lower. Enabling all mTHP sizes showed a more significant result.

Using the same test setup with 10G ZRAM and enabling all mTHP sizes:

128kB swap failure rate:
Before: swpout:451599 swpout_fallback:54525
After:  swpout:502710 swpout_fallback:870

256kB swap failure rate:
Before: swpout:63652  swpout_fallback:2708
After:  swpout:65913  swpout_fallback:20

512kB swap failure rate:
Before: swpout:11663  swpout_fallback:1767
After:  swpout:14480  swpout_fallback:6

2M swap failure rate:
Before: swpout:24     swpout_fallback:1442
After:  swpout:1329   swpout_fallback:7

The success rate of large allocations is much higher.

Link: https://lore.kernel.org/linux-mm/87v8242vng.fsf@yhuang6-desk2.ccr.corp.intel.com/ [1]
Link: https://lkml.kernel.org/r/20250806161748.76651-4-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Kairui Song 2025-08-07 00:17:48 +08:00 committed by Andrew Morton
parent 913fff3145
commit 9a42aed484
1 changed files with 28 additions and 10 deletions
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38
mm/swapfile.c
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@ -908,18 +908,20 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
}
new_cluster:
ci = isolate_lock_cluster(si, &si->free_clusters);
if (ci) {
found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
order, usage);
if (found)
goto done;
/*
* If the device need discard, prefer new cluster over nonfull
* to spread out the writes.
*/
if (si->flags & SWP_PAGE_DISCARD) {
ci = isolate_lock_cluster(si, &si->free_clusters);
if (ci) {
found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
order, usage);
if (found)
goto done;
}
}
/* Try reclaim from full clusters if free clusters list is drained */
if (vm_swap_full())
swap_reclaim_full_clusters(si, false);
if (order < PMD_ORDER) {
while ((ci = isolate_lock_cluster(si, &si->nonfull_clusters[order]))) {
found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
@ -927,7 +929,23 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
if (found)
goto done;
}
}
if (!(si->flags & SWP_PAGE_DISCARD)) {
ci = isolate_lock_cluster(si, &si->free_clusters);
if (ci) {
found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
order, usage);
if (found)
goto done;
}
}
/* Try reclaim full clusters if free and nonfull lists are drained */
if (vm_swap_full())
swap_reclaim_full_clusters(si, false);
if (order < PMD_ORDER) {
/*
* Scan only one fragment cluster is good enough. Order 0
* allocation will surely success, and large allocation