[PATCH RFC 1/2] Add notifiers for various swap events

[Nitin Gupta <ngupta@vflare.org>]

Add notifiers for following swap events:
 - Swapon
 - Swapoff
 - When a swap slot is freed

This is required for ramzswap module which implements RAM based block
devices to be used as swap disks. These devices require a notification
on these events to function properly (as shown in patch 2/2).

Currently, I'm not sure if any of these event notifiers have any other
users. However, adding ramzswap specific hooks instead of this generic
approach resulted in a bad/hacky code.

For SWAP_EVENT_SLOT_FREE, callbacks are made under swap_lock. Currently, this
is not a problem since ramzswap is the only user and the callback it registers
can be safely made under this lock. However, if this event finds more users,
we might have to work on reducing contention on this lock.

Signed-off-by: Nitin Gupta <ngupta@vflare.org>

---
 include/linux/swap.h |   12 +++++++++
 mm/swapfile.c        |   67 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+), 0 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 7c15334..2873aad 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -127,6 +127,12 @@ enum {
 	SWP_SCANNING	= (1 << 8),	/* refcount in scan_swap_map */
 };
 
+enum swap_event {
+	SWAP_EVENT_SWAPON,
+	SWAP_EVENT_SWAPOFF,
+	SWAP_EVENT_SLOT_FREE,
+};
+
 #define SWAP_CLUSTER_MAX 32
 
 #define SWAP_MAP_MAX	0x7ffe
@@ -155,6 +161,7 @@ struct swap_info_struct {
 	unsigned int max;
 	unsigned int inuse_pages;
 	unsigned int old_block_size;
+	struct atomic_notifier_head slot_free_notify_list;
 };
 
 struct swap_list_t {
@@ -295,8 +302,13 @@ extern sector_t swapdev_block(int, pgoff_t);
 extern struct swap_info_struct *get_swap_info_struct(unsigned);
 extern int reuse_swap_page(struct page *);
 extern int try_to_free_swap(struct page *);
+extern int register_swap_event_notifier(struct notifier_block *nb,
+                                enum swap_event event, unsigned long val);
+extern int unregister_swap_event_notifier(struct notifier_block *nb,
+                                enum swap_event event, unsigned long val);
 struct backing_dev_info;
 
+
 /* linux/mm/thrash.c */
 extern struct mm_struct *swap_token_mm;
 extern void grab_swap_token(struct mm_struct *);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 74f1102..f63643c 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -52,6 +52,9 @@ static struct swap_list_t swap_list = {-1, -1};
 static struct swap_info_struct swap_info[MAX_SWAPFILES];
 
 static DEFINE_MUTEX(swapon_mutex);
+static BLOCKING_NOTIFIER_HEAD(swapon_notify_list);
+static BLOCKING_NOTIFIER_HEAD(swapoff_notify_list);
+
 
 /* For reference count accounting in swap_map */
 /* enum for swap_map[] handling. internal use only */
@@ -585,6 +588,8 @@ static int swap_entry_free(struct swap_info_struct *p,
 			swap_list.next = p - swap_info;
 		nr_swap_pages++;
 		p->inuse_pages--;
+		atomic_notifier_call_chain(&p->slot_free_notify_list,
+					offset, p->swap_file);
 	}
 	if (!swap_count(count))
 		mem_cgroup_uncharge_swap(ent);
@@ -1626,6 +1631,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	p->swap_map = NULL;
 	p->flags = 0;
 	spin_unlock(&swap_lock);
+	blocking_notifier_call_chain(&swapoff_notify_list, type, swap_file);
 	mutex_unlock(&swapon_mutex);
 	vfree(swap_map);
 	/* Destroy swap account informatin */
@@ -2014,7 +2020,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	} else {
 		swap_info[prev].next = p - swap_info;
 	}
+	ATOMIC_INIT_NOTIFIER_HEAD(&p->slot_free_notify_list);
 	spin_unlock(&swap_lock);
+	blocking_notifier_call_chain(&swapon_notify_list, type, swap_file);
 	mutex_unlock(&swapon_mutex);
 	error = 0;
 	goto out;
@@ -2216,3 +2224,62 @@ int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
 	*offset = ++toff;
 	return nr_pages? ++nr_pages: 0;
 }
+
+int register_swap_event_notifier(struct notifier_block *nb,
+				enum swap_event event, unsigned long val)
+{
+	switch (event) {
+	case SWAP_EVENT_SWAPON:
+		return blocking_notifier_chain_register(
+					&swapon_notify_list, nb);
+	case SWAP_EVENT_SWAPOFF:
+		return blocking_notifier_chain_register(
+					&swapoff_notify_list, nb);
+	case SWAP_EVENT_SLOT_FREE:
+		{
+		struct swap_info_struct *sis;
+
+		if (val > nr_swapfiles)
+			goto out;
+		sis = get_swap_info_struct(val);
+		return atomic_notifier_chain_register(
+				&sis->slot_free_notify_list, nb);
+		}
+	default:
+		pr_err("Invalid swap event: %d\n", event);
+	};
+
+out:
+	return -EINVAL;
+}
+EXPORT_SYMBOL_GPL(register_swap_event_notifier);
+
+int unregister_swap_event_notifier(struct notifier_block *nb,
+				enum swap_event event, unsigned long val)
+{
+	switch (event) {
+	case SWAP_EVENT_SWAPON:
+		return blocking_notifier_chain_unregister(
+					&swapon_notify_list, nb);
+	case SWAP_EVENT_SWAPOFF:
+		return blocking_notifier_chain_unregister(
+					&swapoff_notify_list, nb);
+	case SWAP_EVENT_SLOT_FREE:
+		{
+		struct swap_info_struct *sis;
+
+		if (val > nr_swapfiles)
+			goto out;
+		sis = get_swap_info_struct(val);
+		return atomic_notifier_chain_unregister(
+				&sis->slot_free_notify_list, nb);
+		}
+	default:
+		pr_err("Invalid swap event: %d\n", event);
+	};
+
+out:
+	return -EINVAL;
+}
+EXPORT_SYMBOL_GPL(unregister_swap_event_notifier);
+

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