[PATCH v4 1/3] mm, swap: speed up hibernation allocation and writeout

[Kairui Song via B4 Relay]

From: Kairui Song <kasong@xxxxxxxxxxx>

Since commit 0ff67f990bd4 ("mm, swap: remove swap slot cache"),
hibernation has been using the swap slot slow allocation path for
simplification, which turns out might cause regression for some
devices because the allocator now rotates clusters too often, leading to
slower allocation and more random distribution of data.

Fast allocation is not complex, so implement hibernation support as
well.

Test result with Samsung SSD 830 Series (SATA II, 3.0 Gbps) shows the
performance is several times better [1]:
6.19:               324 seconds
After this series:  35 seconds

Fixes: 0ff67f990bd4 ("mm, swap: remove swap slot cache")
Reported-by: Carsten Grohmann <mail@xxxxxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/linux-mm/20260206121151.dea3633d1f0ded7bbf49c22e@xxxxxxxxxxxxxxxxxxxx/
Link: https://lore.kernel.org/linux-mm/8b4bdcfa-ce3f-4e23-839f-31367df7c18f@xxxxxx/ [1]
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/swapfile.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index c6863ff7152c..32e0e7545ab8 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1926,8 +1926,9 @@ void swap_put_entries_direct(swp_entry_t entry, int nr)
 /* Allocate a slot for hibernation */
 swp_entry_t swap_alloc_hibernation_slot(int type)
 {
-	struct swap_info_struct *si = swap_type_to_info(type);
-	unsigned long offset;
+	struct swap_info_struct *pcp_si, *si = swap_type_to_info(type);
+	unsigned long pcp_offset, offset = SWAP_ENTRY_INVALID;
+	struct swap_cluster_info *ci;
 	swp_entry_t entry = {0};
 
 	if (!si)
@@ -1937,11 +1938,21 @@ swp_entry_t swap_alloc_hibernation_slot(int type)
 	if (get_swap_device_info(si)) {
 		if (si->flags & SWP_WRITEOK) {
 			/*
-			 * Grab the local lock to be compliant
-			 * with swap table allocation.
+			 * Try the local cluster first if it matches the device. If
+			 * not, try grab a new cluster and override local cluster.
 			 */
 			local_lock(&percpu_swap_cluster.lock);
-			offset = cluster_alloc_swap_entry(si, NULL);
+			pcp_si = this_cpu_read(percpu_swap_cluster.si[0]);
+			pcp_offset = this_cpu_read(percpu_swap_cluster.offset[0]);
+			if (pcp_si == si && pcp_offset) {
+				ci = swap_cluster_lock(si, pcp_offset);
+				if (cluster_is_usable(ci, 0))
+					offset = alloc_swap_scan_cluster(si, ci, NULL, pcp_offset);
+				else
+					swap_cluster_unlock(ci);
+			}
+			if (!offset)
+				offset = cluster_alloc_swap_entry(si, NULL);
 			local_unlock(&percpu_swap_cluster.lock);
 			if (offset)
 				entry = swp_entry(si->type, offset);

-- 
2.52.0