[PATCH v2 1/1] mm, swap: only prefer nonfull clusters on synchronous devices

[Alexandre Ghiti]

Commit 9a42aed48421 ("mm, swap: prefer nonfull over free clusters") made
allocations fill the nonfull clusters of their order before taking a
free one, on every device without discard. Since commit 3cedb745d354
("mm: clarify the folio_free_swap() for do_swap_page()"), an exclusive
write fault frees the swap slot, so a workload that rewrites swapped out
memory leaves holes in full clusters, which then go back on the nonfull
list. Reclaim allocates slots one folio at a time, so a batch of folios
gets scattered over those holes, its writes can't be merged and it is
written with many small requests.

So partially revert commit 9a42aed48421 ("mm, swap: prefer nonfull over
free clusters") and prefer free clusters again, except on synchronous
devices like zram: they don't merge I/O, so the contiguity of the slots
doesn't matter to them, and filling the holes first keeps the free
clusters for large folios.

Fixes: 9a42aed48421 ("mm, swap: prefer nonfull over free clusters")
Suggested-by: Kairui Song <kasong@xxxxxxxxxxx>
Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
---
 mm/swapfile.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 189f04d8838f..1c1b5d0b724a 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -973,6 +973,17 @@ static bool __swap_cluster_alloc_entries(struct swap_info_struct *si,
 	return true;
 }
 
+/*
+ * Prefer a new cluster over a nonfull one if the device needs discard, to
+ * spread out the writes, or if it is not synchronous, to keep the slots
+ * of a swapout batch contiguous so that its writes can be merged.
+ */
+static bool swap_prefer_free_cluster(struct swap_info_struct *si)
+{
+	return (si->flags & SWP_PAGE_DISCARD) ||
+	       !(si->flags & SWP_SYNCHRONOUS_IO);
+}
+
 /* Try use a new cluster for current CPU and allocate from it. */
 static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
 					    struct swap_cluster_info *ci,
@@ -1143,11 +1154,7 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 	}
 
 new_cluster:
-	/*
-	 * If the device need discard, prefer new cluster over nonfull
-	 * to spread out the writes.
-	 */
-	if (si->flags & SWP_PAGE_DISCARD) {
+	if (swap_prefer_free_cluster(si)) {
 		found = alloc_swap_scan_list(si, &si->free_clusters, folio, false);
 		if (found)
 			goto done;
@@ -1159,7 +1166,7 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 			goto done;
 	}
 
-	if (!(si->flags & SWP_PAGE_DISCARD)) {
+	if (!swap_prefer_free_cluster(si)) {
 		found = alloc_swap_scan_list(si, &si->free_clusters, folio, false);
 		if (found)
 			goto done;
-- 
2.53.0-Meta