[PATCH 04/19] mm, swap: always try to free swap cache for SWP_SYNCHRONOUS_IO devices

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Now SWP_SYNCHRONOUS_IO devices are also using swap cache. One side
effect is that a folio may stay in swap cache for a longer time due to
lazy freeing (vm_swap_full()). This can help save some CPU / IO if folios
are being swapped out very frequently right after swapin, hence improving
the performance. But the long pinning of swap slots also increases the
fragmentation rate of the swap device significantly, and currently,
all in-tree SWP_SYNCHRONOUS_IO devices are RAM disks, so it also
causes the backing memory to be pinned, increasing the memory pressure.

So drop the swap cache immediately for SWP_SYNCHRONOUS_IO devices
after swapin finishes. Swap cache has served its role as a
synchronization layer to prevent any parallel swapin from wasting
CPU or memory allocation, and the redundant IO is not a major concern
for SWP_SYNCHRONOUS_IO devices.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/memory.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 9a43d4811781..78457347ae60 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4359,12 +4359,21 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
 	return 0;
 }
 
-static inline bool should_try_to_free_swap(struct folio *folio,
+static inline bool should_try_to_free_swap(struct swap_info_struct *si,
+					   struct folio *folio,
 					   struct vm_area_struct *vma,
 					   unsigned int fault_flags)
 {
 	if (!folio_test_swapcache(folio))
 		return false;
+	/*
+	 * Try to free swap cache for SWP_SYNCHRONOUS_IO devices.
+	 * Redundant IO is unlikely to be an issue for them, but a
+	 * slot being pinned by swap cache may cause more fragmentation
+	 * and delayed freeing of swap metadata.
+	 */
+	if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
+		return true;
 	if (mem_cgroup_swap_full(folio) || (vma->vm_flags & VM_LOCKED) ||
 	    folio_test_mlocked(folio))
 		return true;
@@ -4935,7 +4944,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	 * yet.
 	 */
 	swap_free_nr(entry, nr_pages);
-	if (should_try_to_free_swap(folio, vma, vmf->flags))
+	if (should_try_to_free_swap(si, folio, vma, vmf->flags))
 		folio_free_swap(folio);
 
 	add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);

-- 
2.51.1