frontswap: allow multiple backends

Change frontswap single pointer to a singly linked list of frontswap
implementations.  Update Xen tmem implementation as register no longer
returns anything.

Frontswap only keeps track of a single implementation; any
implementation that registers second (or later) will replace the
previously registered implementation, and gets a pointer to the previous
implementation that the new implementation is expected to pass all
frontswap functions to if it can't handle the function itself.  However
that method doesn't really make much sense, as passing that work on to
every implementation adds unnecessary work to implementations; instead,
frontswap should simply keep a list of all registered implementations
and try each implementation for any function.  Most importantly, neither
of the two currently existing frontswap implementations in the kernel
actually do anything with any previous frontswap implementation that
they replace when registering.

This allows frontswap to successfully manage multiple implementations by
keeping a list of them all.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/xen/tmem.c

@@ -381,15 +381,9 @@ static int __init xen_tmem_init(void)
 #ifdef CONFIG_FRONTSWAP
 	if (tmem_enabled && frontswap) {
 		char *s = "";
-		struct frontswap_ops *old_ops;
 
 		tmem_frontswap_poolid = -1;
-		old_ops = frontswap_register_ops(&tmem_frontswap_ops);
-		if (IS_ERR(old_ops) || old_ops) {
-			if (IS_ERR(old_ops))
-				return PTR_ERR(old_ops);
-			s = " (WARNING: frontswap_ops overridden)";
-		}
+		frontswap_register_ops(&tmem_frontswap_ops);
 		pr_info("frontswap enabled, RAM provided by Xen Transcendent Memory%s\n",
 			s);
 	}

include/linux/frontswap.h

@@ -6,16 +6,16 @@
 #include <linux/bitops.h>
 
 struct frontswap_ops {
-	void (*init)(unsigned);
-	int (*store)(unsigned, pgoff_t, struct page *);
-	int (*load)(unsigned, pgoff_t, struct page *);
-	void (*invalidate_page)(unsigned, pgoff_t);
-	void (*invalidate_area)(unsigned);
+	void (*init)(unsigned); /* this swap type was just swapon'ed */
+	int (*store)(unsigned, pgoff_t, struct page *); /* store a page */
+	int (*load)(unsigned, pgoff_t, struct page *); /* load a page */
+	void (*invalidate_page)(unsigned, pgoff_t); /* page no longer needed */
+	void (*invalidate_area)(unsigned); /* swap type just swapoff'ed */
+	struct frontswap_ops *next; /* private pointer to next ops */
 };
 
 extern bool frontswap_enabled;
-extern struct frontswap_ops *
-	frontswap_register_ops(struct frontswap_ops *ops);
+extern void frontswap_register_ops(struct frontswap_ops *ops);
 extern void frontswap_shrink(unsigned long);
 extern unsigned long frontswap_curr_pages(void);
 extern void frontswap_writethrough(bool);

mm/frontswap.c

@@ -21,11 +21,16 @@
 #include <linux/swapfile.h>
 
 /*
- * frontswap_ops is set by frontswap_register_ops to contain the pointers
- * to the frontswap "backend" implementation functions.
+ * frontswap_ops are added by frontswap_register_ops, and provide the
+ * frontswap "backend" implementation functions.  Multiple implementations
+ * may be registered, but implementations can never deregister.  This
+ * is a simple singly-linked list of all registered implementations.
  */
 static struct frontswap_ops *frontswap_ops __read_mostly;
 
+#define for_each_frontswap_ops(ops)		\
+	for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)
+
 /*
  * If enabled, frontswap_store will return failure even on success.  As
  * a result, the swap subsystem will always write the page to swap, in
@@ -79,15 +84,6 @@ static inline void inc_frontswap_invalidates(void) { }
  * on all frontswap functions to not call the backend until the backend
  * has registered.
  *
- * Specifically when no backend is registered (nobody called
- * frontswap_register_ops) all calls to frontswap_init (which is done via
- * swapon -> enable_swap_info -> frontswap_init) are registered and remembered
- * (via the setting of need_init bitmap) but fail to create tmem_pools. When a
- * backend registers with frontswap at some later point the previous
- * calls to frontswap_init are executed (by iterating over the need_init
- * bitmap) to create tmem_pools and set the respective poolids. All of that is
- * guarded by us using atomic bit operations on the 'need_init' bitmap.
- *
  * This would not guards us against the user deciding to call swapoff right as
  * we are calling the backend to initialize (so swapon is in action).
  * Fortunatly for us, the swapon_mutex has been taked by the callee so we are
@@ -106,37 +102,64 @@ static inline void inc_frontswap_invalidates(void) { }
  *
  * Obviously the opposite (unloading the backend) must be done after all
  * the frontswap_[store|load|invalidate_area|invalidate_page] start
- * ignorning or failing the requests - at which point frontswap_ops
- * would have to be made in some fashion atomic.
+ * ignoring or failing the requests.  However, there is currently no way
+ * to unload a backend once it is registered.
  */
-static DECLARE_BITMAP(need_init, MAX_SWAPFILES);
 
 /*
- * Register operations for frontswap, returning previous thus allowing
- * detection of multiple backends and possible nesting.
+ * Register operations for frontswap
  */
-struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
+void frontswap_register_ops(struct frontswap_ops *ops)
 {
-	struct frontswap_ops *old = frontswap_ops;
-	int i;
+	DECLARE_BITMAP(a, MAX_SWAPFILES);
+	DECLARE_BITMAP(b, MAX_SWAPFILES);
+	struct swap_info_struct *si;
+	unsigned int i;
 
-	for (i = 0; i < MAX_SWAPFILES; i++) {
-		if (test_and_clear_bit(i, need_init)) {
-			struct swap_info_struct *sis = swap_info[i];
-			/* __frontswap_init _should_ have set it! */
-			if (!sis->frontswap_map)
-				return ERR_PTR(-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)
 		ops->init(i);
-		}
-	}
+
 	/*
-	 * We MUST have frontswap_ops set _after_ the frontswap_init's
-	 * have been called. Otherwise __frontswap_store might fail. Hence
-	 * the barrier to make sure compiler does not re-order us.
+	 * Setting frontswap_ops must happen after the ops->init() calls
+	 * above; cmpxchg implies smp_mb() which will ensure the init is
+	 * complete at this point.
 	 */
-	barrier();
-	frontswap_ops = ops;
-	return old;
+	do {
+		ops->next = frontswap_ops;
+	} while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
+
+	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))
+				ops->init(i);
+			else if (test_bit(i, a) && !test_bit(i, b))
+				ops->invalidate_area(i);
+		}
+	}
 }
 EXPORT_SYMBOL(frontswap_register_ops);
 
@@ -164,6 +187,7 @@ EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
 void __frontswap_init(unsigned type, unsigned long *map)
 {
 	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
 
 	BUG_ON(sis == NULL);
 
@@ -179,26 +203,28 @@ void __frontswap_init(unsigned type, unsigned long *map)
 	 * p->frontswap set to something valid to work properly.
 	 */
 	frontswap_map_set(sis, map);
-	if (frontswap_ops)
-		frontswap_ops->init(type);
-	else {
-		BUG_ON(type >= MAX_SWAPFILES);
-		set_bit(type, need_init);
-	}
+
+	for_each_frontswap_ops(ops)
+		ops->init(type);
 }
 EXPORT_SYMBOL(__frontswap_init);
 
 bool __frontswap_test(struct swap_info_struct *sis,
 				pgoff_t offset)
 {
-	bool ret = false;
-
-	if (frontswap_ops && sis->frontswap_map)
-		ret = test_bit(offset, sis->frontswap_map);
-	return ret;
+	if (sis->frontswap_map)
+		return test_bit(offset, sis->frontswap_map);
+	return false;
 }
 EXPORT_SYMBOL(__frontswap_test);
 
+static inline void __frontswap_set(struct swap_info_struct *sis,
+				   pgoff_t offset)
+{
+	set_bit(offset, sis->frontswap_map);
+	atomic_inc(&sis->frontswap_pages);
+}
+
 static inline void __frontswap_clear(struct swap_info_struct *sis,
 				     pgoff_t offset)
 {
@@ -215,39 +241,46 @@ static inline void __frontswap_clear(struct swap_info_struct *sis,
  */
 int __frontswap_store(struct page *page)
 {
-	int ret = -1, dup = 0;
+	int ret = -1;
 	swp_entry_t entry = { .val = page_private(page), };
 	int type = swp_type(entry);
 	struct swap_info_struct *sis = swap_info[type];
 	pgoff_t offset = swp_offset(entry);
+	struct frontswap_ops *ops;
 
 	/*
 	 * Return if no backend registed.
 	 * Don't need to inc frontswap_failed_stores here.
 	 */
 	if (!frontswap_ops)
-		return ret;
+		return -1;
 
 	BUG_ON(!PageLocked(page));
 	BUG_ON(sis == NULL);
-	if (__frontswap_test(sis, offset))
-		dup = 1;
-	ret = frontswap_ops->store(type, offset, page);
-	if (ret == 0) {
-		set_bit(offset, sis->frontswap_map);
-		inc_frontswap_succ_stores();
-		if (!dup)
-			atomic_inc(&sis->frontswap_pages);
-	} else {
+
 	/*
-		  failed dup always results in automatic invalidate of
-		  the (older) page from frontswap
+	 * If a dup, we must remove the old page first; we can't leave the
+	 * old page no matter if the store of the new page succeeds or fails,
+	 * and we can't rely on the new page replacing the old page as we may
+	 * not store to the same implementation that contains the old page.
 	 */
-		inc_frontswap_failed_stores();
-		if (dup) {
+	if (__frontswap_test(sis, offset)) {
 		__frontswap_clear(sis, offset);
-			frontswap_ops->invalidate_page(type, offset);
+		for_each_frontswap_ops(ops)
+			ops->invalidate_page(type, offset);
 	}
+
+	/* Try to store in each implementation, until one succeeds. */
+	for_each_frontswap_ops(ops) {
+		ret = ops->store(type, offset, page);
+		if (!ret) /* successful store */
+			break;
+	}
+	if (ret == 0) {
+		__frontswap_set(sis, offset);
+		inc_frontswap_succ_stores();
+	} else {
+		inc_frontswap_failed_stores();
 	}
 	if (frontswap_writethrough_enabled)
 		/* report failure so swap also writes to swap device */
@@ -268,14 +301,22 @@ int __frontswap_load(struct page *page)
 	int type = swp_type(entry);
 	struct swap_info_struct *sis = swap_info[type];
 	pgoff_t offset = swp_offset(entry);
+	struct frontswap_ops *ops;
+
+	if (!frontswap_ops)
+		return -1;
 
 	BUG_ON(!PageLocked(page));
 	BUG_ON(sis == NULL);
-	/*
-	 * __frontswap_test() will check whether there is backend registered
-	 */
-	if (__frontswap_test(sis, offset))
-		ret = frontswap_ops->load(type, offset, page);
+	if (!__frontswap_test(sis, offset))
+		return -1;
+
+	/* Try loading from each implementation, until one succeeds. */
+	for_each_frontswap_ops(ops) {
+		ret = ops->load(type, offset, page);
+		if (!ret) /* successful load */
+			break;
+	}
 	if (ret == 0) {
 		inc_frontswap_loads();
 		if (frontswap_tmem_exclusive_gets_enabled) {
@@ -294,17 +335,20 @@ EXPORT_SYMBOL(__frontswap_load);
 void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 {
 	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
+
+	if (!frontswap_ops)
+		return;
 
 	BUG_ON(sis == NULL);
-	/*
-	 * __frontswap_test() will check whether there is backend registered
-	 */
-	if (__frontswap_test(sis, offset)) {
-		frontswap_ops->invalidate_page(type, offset);
+	if (!__frontswap_test(sis, offset))
+		return;
+
+	for_each_frontswap_ops(ops)
+		ops->invalidate_page(type, offset);
 	__frontswap_clear(sis, offset);
 	inc_frontswap_invalidates();
 }
-}
 EXPORT_SYMBOL(__frontswap_invalidate_page);
 
 /*
@@ -314,17 +358,20 @@ EXPORT_SYMBOL(__frontswap_invalidate_page);
 void __frontswap_invalidate_area(unsigned type)
 {
 	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
+
+	if (!frontswap_ops)
+		return;
 
-	if (frontswap_ops) {
 	BUG_ON(sis == NULL);
 	if (sis->frontswap_map == NULL)
 		return;
-		frontswap_ops->invalidate_area(type);
+
+	for_each_frontswap_ops(ops)
+		ops->invalidate_area(type);
 	atomic_set(&sis->frontswap_pages, 0);
 	bitmap_zero(sis->frontswap_map, sis->max);
 }
-	clear_bit(type, need_init);
-}
 EXPORT_SYMBOL(__frontswap_invalidate_area);
 
 static unsigned long __frontswap_curr_pages(void)