[RFC PATCH v3 2/5] mm: swap: introduce swap_nr_free() for batched swap_free()

[Barry Song]

From: Chuanhua Han <hanchuanhua@xxxxxxxx>

While swapping in a large folio, we need to free swaps related to the whole
folio. To avoid frequently acquiring and releasing swap locks, it is better
to introduce an API for batched free.

Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
 include/linux/swap.h |  6 ++++++
 mm/swapfile.c        | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 2955f7a78d8d..d6ab27929458 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -481,6 +481,7 @@ extern void swap_shmem_alloc(swp_entry_t);
 extern int swap_duplicate(swp_entry_t);
 extern int swapcache_prepare(swp_entry_t);
 extern void swap_free(swp_entry_t);
+extern void swap_nr_free(swp_entry_t entry, int nr_pages);
 extern void swapcache_free_entries(swp_entry_t *entries, int n);
 extern int free_swap_and_cache(swp_entry_t);
 int swap_type_of(dev_t device, sector_t offset);
@@ -561,6 +562,11 @@ static inline void swap_free(swp_entry_t swp)
 {
 }
 
+void swap_nr_free(swp_entry_t entry, int nr_pages)
+{
+
+}
+
 static inline void put_swap_folio(struct folio *folio, swp_entry_t swp)
 {
 }
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3f594be83b58..244106998a69 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1341,6 +1341,41 @@ void swap_free(swp_entry_t entry)
 		__swap_entry_free(p, entry);
 }
 
+/*
+ * Called after swapping in a large folio, batched free swap entries
+ * for this large folio, entry should be for the first subpage and
+ * its offset is aligned with nr_pages
+ */
+void swap_nr_free(swp_entry_t entry, int nr_pages)
+{
+	int i;
+	struct swap_cluster_info *ci;
+	struct swap_info_struct *p;
+	unsigned type = swp_type(entry);
+	unsigned long offset = swp_offset(entry);
+	DECLARE_BITMAP(usage, SWAPFILE_CLUSTER) = { 0 };
+
+	/* all swap entries are within a cluster for mTHP */
+	VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > SWAPFILE_CLUSTER);
+
+	if (nr_pages == 1) {
+		swap_free(entry);
+		return;
+	}
+
+	p = _swap_info_get(entry);
+
+	ci = lock_cluster(p, offset);
+	for (i = 0; i < nr_pages; i++) {
+		if (__swap_entry_free_locked(p, offset + i, 1))
+			__bitmap_set(usage, i, 1);
+	}
+	unlock_cluster(ci);
+
+	for_each_clear_bit(i, usage, nr_pages)
+		free_swap_slot(swp_entry(type, offset + i));
+}
+
 /*
  * Called after dropping swapcache to decrease refcnt to swap entries.
  */
-- 
2.34.1