[PATCH v2 1/5] mm/swapfile: add percpu_ref support for swap

[Miaohe Lin <linmiaohe@xxxxxxxxxx>]

We will use percpu-refcount to serialize against concurrent swapoff. This
patch adds the percpu_ref support for swap.

Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
---
 include/linux/swap.h |  3 +++
 mm/swapfile.c        | 33 +++++++++++++++++++++++++++++----
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 144727041e78..8be36eb58b7a 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -240,6 +240,7 @@ struct swap_cluster_list {
  * The in-memory structure used to track swap areas.
  */
 struct swap_info_struct {
+	struct percpu_ref users;	/* serialization against concurrent swapoff */
 	unsigned long	flags;		/* SWP_USED etc: see above */
 	signed short	prio;		/* swap priority of this type */
 	struct plist_node list;		/* entry in swap_active_head */
@@ -260,6 +261,8 @@ struct swap_info_struct {
 	struct block_device *bdev;	/* swap device or bdev of swap file */
 	struct file *swap_file;		/* seldom referenced */
 	unsigned int old_block_size;	/* seldom referenced */
+	bool ref_initialized;		/* seldom referenced */
+	struct completion comp;		/* seldom referenced */
 #ifdef CONFIG_FRONTSWAP
 	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
 	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 149e77454e3c..66515a3a2824 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -39,6 +39,7 @@
 #include <linux/export.h>
 #include <linux/swap_slots.h>
 #include <linux/sort.h>
+#include <linux/completion.h>
 
 #include <asm/tlbflush.h>
 #include <linux/swapops.h>
@@ -511,6 +512,14 @@ static void swap_discard_work(struct work_struct *work)
 	spin_unlock(&si->lock);
 }
 
+static void swap_users_ref_free(struct percpu_ref *ref)
+{
+	struct swap_info_struct *si;
+
+	si = container_of(ref, struct swap_info_struct, users);
+	complete(&si->comp);
+}
+
 static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
 {
 	struct swap_cluster_info *ci = si->cluster_info;
@@ -2500,7 +2509,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
 	 * Guarantee swap_map, cluster_info, etc. fields are valid
 	 * between get/put_swap_device() if SWP_VALID bit is set
 	 */
-	synchronize_rcu();
+	percpu_ref_resurrect(&p->users);
 	spin_lock(&swap_lock);
 	spin_lock(&p->lock);
 	_enable_swap_info(p);
@@ -2621,11 +2630,18 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	p->flags &= ~SWP_VALID;		/* mark swap device as invalid */
 	spin_unlock(&p->lock);
 	spin_unlock(&swap_lock);
+
+	percpu_ref_kill(&p->users);
 	/*
-	 * wait for swap operations protected by get/put_swap_device()
-	 * to complete
+	 * We need synchronize_rcu() here to protect the accessing
+	 * to the swap cache data structure.
 	 */
 	synchronize_rcu();
+	/*
+	 * Wait for swap operations protected by get/put_swap_device()
+	 * to complete.
+	 */
+	wait_for_completion(&p->comp);
 
 	flush_work(&p->discard_work);
 
@@ -3132,7 +3148,7 @@ static bool swap_discardable(struct swap_info_struct *si)
 SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 {
 	struct swap_info_struct *p;
-	struct filename *name;
+	struct filename *name = NULL;
 	struct file *swap_file = NULL;
 	struct address_space *mapping;
 	int prio;
@@ -3163,6 +3179,15 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 
 	INIT_WORK(&p->discard_work, swap_discard_work);
 
+	if (!p->ref_initialized) {
+		error = percpu_ref_init(&p->users, swap_users_ref_free,
+					PERCPU_REF_INIT_DEAD, GFP_KERNEL);
+		if (unlikely(error))
+			goto bad_swap;
+		init_completion(&p->comp);
+		p->ref_initialized = true;
+	}
+
 	name = getname(specialfile);
 	if (IS_ERR(name)) {
 		error = PTR_ERR(name);
-- 
2.19.1