[PATCH -next v3 2/5] frontswap: invoke ops->init for online swap device in frontswap_register_ops

[Liu Shixin <liushixin2@xxxxxxxxxx>]

Since we are supported to delay zswap initializaton, we need to invoke
ops->init for the swap device which is already online when register
backend.

This patch is a revert of f328c1d16e4c ("frontswap: simplify frontswap_register_ops")
and 633423a09cb5 ("mm: mark swap_lock and swap_active_head static")

Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
---
 include/linux/swapfile.h |  2 ++
 mm/frontswap.c           | 47 ++++++++++++++++++++++++++++++++++++++++
 mm/swapfile.c            |  4 ++--
 3 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
index 2fbcc9afd814..75fc069594a5 100644
--- a/include/linux/swapfile.h
+++ b/include/linux/swapfile.h
@@ -6,6 +6,8 @@
  * these were static in swapfile.c but frontswap.c needs them and we don't
  * want to expose them to the dozens of source files that include swap.h
  */
+extern spinlock_t swap_lock;
+extern struct plist_head swap_active_head;
 extern struct swap_info_struct *swap_info[];
 extern unsigned long generic_max_swapfile_size(void);
 /* Maximum swapfile size supported for the arch (not inclusive). */
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 620f95af81dd..449e6f499b88 100644
--- a/mm/frontswap.c
+++ b/mm/frontswap.c
@@ -96,11 +96,58 @@ static inline void inc_frontswap_invalidates(void) { }
  */
 int frontswap_register_ops(const struct frontswap_ops *ops)
 {
+	DECLARE_BITMAP(a, MAX_SWAPFILES);
+	DECLARE_BITMAP(b, MAX_SWAPFILES);
+	struct swap_info_struct *si;
+	unsigned int i;
+
 	if (frontswap_ops)
 		return -EINVAL;
 
+	bitmap_zero(a, MAX_SWAPFILES);
+	bitmap_zero(b, MAX_SWAPFILES);
+
+	spin_lock(&swap_lock);
+	plist_for_each_entry(si, &swap_active_head, list) {
+		if (!WARN_ON(!si->frontswap_map))
+			__set_bit(si->type, a);
+	}
+	spin_unlock(&swap_lock);
+
+	/* the new ops needs to know the currently active swap devices */
+	for_each_set_bit(i, a, MAX_SWAPFILES) {
+		pr_err("init frontswap_ops\n");
+		ops->init(i);
+	}
+
 	frontswap_ops = ops;
 	static_branch_inc(&frontswap_enabled_key);
+
+	spin_lock(&swap_lock);
+	plist_for_each_entry(si, &swap_active_head, list) {
+		if (si->frontswap_map)
+			__set_bit(si->type, b);
+	}
+	spin_unlock(&swap_lock);
+
+	/*
+	 * On the very unlikely chance that a swap device was added or
+	 * removed between setting the "a" list bits and the ops init
+	 * calls, we re-check and do init or invalidate for any changed
+	 * bits.
+	 */
+	if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
+		for (i = 0; i < MAX_SWAPFILES; i++) {
+			if (!test_bit(i, a) && test_bit(i, b)) {
+				pr_err("init frontswap_ops re\n");
+				ops->init(i);
+			} else if (test_bit(i, a) && !test_bit(i, b)) {
+				pr_err("inval frontswap_ops re\n");
+				ops->invalidate_area(i);
+			}
+		}
+	}
+
 	return 0;
 }
 
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 469d9af86be2..d383b282f269 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -51,7 +51,7 @@ static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
 				 unsigned char);
 static void free_swap_count_continuations(struct swap_info_struct *);
 
-static DEFINE_SPINLOCK(swap_lock);
+DEFINE_SPINLOCK(swap_lock);
 static unsigned int nr_swapfiles;
 atomic_long_t nr_swap_pages;
 /*
@@ -77,7 +77,7 @@ static const char Unused_offset[] = "Unused swap offset entry ";
  * all active swap_info_structs
  * protected with swap_lock, and ordered by priority.
  */
-static PLIST_HEAD(swap_active_head);
+PLIST_HEAD(swap_active_head);
 
 /*
  * all available (active, not full) swap_info_structs
-- 
2.25.1