[PATCH] frontswap: allow multiple backends

[PATCH] frontswap: allow multiple backends

[Dan Streetman]

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>
---
 drivers/xen/tmem.c        |   8 +--
 include/linux/frontswap.h |   4 +-
 mm/frontswap.c            | 159 +++++++++++++++++++++++++---------------------
 3 files changed, 88 insertions(+), 83 deletions(-)

diff --git a/drivers/xen/tmem.c b/drivers/xen/tmem.c
index c4211a3..d88f367 100644
--- a/drivers/xen/tmem.c
+++ b/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);
 	}
diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
index 8293262..9b3fc53 100644
--- a/include/linux/frontswap.h
+++ b/include/linux/frontswap.h
@@ -11,11 +11,11 @@ struct frontswap_ops {
 	int (*load)(unsigned, pgoff_t, struct page *);
 	void (*invalidate_page)(unsigned, pgoff_t);
 	void (*invalidate_area)(unsigned);
+	struct frontswap_ops *next;
 };
 
 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);
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 8d82809..46f21f8 100644
--- a/mm/frontswap.c
+++ b/mm/frontswap.c
@@ -21,8 +21,8 @@
 #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.
  */
 static struct frontswap_ops *frontswap_ops __read_mostly;
 
@@ -79,15 +79,6 @@ 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 +97,51 @@ 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.
+ * ignorning 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;
-
-	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);
-			ops->init(i);
+	DECLARE_BITMAP(a, MAX_SWAPFILES);
+	DECLARE_BITMAP(b, MAX_SWAPFILES);
+	struct swap_info_struct *si;
+	unsigned int i;
+
+	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);
+
+	for (i = find_first_bit(a, MAX_SWAPFILES);
+	     i < MAX_SWAPFILES;
+	     i = find_next_bit(a, MAX_SWAPFILES, i + 1))
+		ops->init(i);
+
+	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);
+
+	if (!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);
 		}
 	}
-	/*
-	 * 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.
-	 */
-	barrier();
-	frontswap_ops = ops;
-	return old;
 }
 EXPORT_SYMBOL(frontswap_register_ops);
 
@@ -164,6 +169,7 @@ 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);
 
@@ -179,23 +185,18 @@ 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 (ops = frontswap_ops; ops; ops = ops->next)
+		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);
 
@@ -215,24 +216,25 @@ static inline void __frontswap_clear(struct swap_info_struct *sis,
  */
 int __frontswap_store(struct page *page)
 {
-	int ret = -1, dup = 0;
+	int ret, dup;
 	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);
+	dup = __frontswap_test(sis, offset);
+	for (ops = frontswap_ops, ret = -1; ops && ret; ops = ops->next)
+		ret = ops->store(type, offset, page);
 	if (ret == 0) {
 		set_bit(offset, sis->frontswap_map);
 		inc_frontswap_succ_stores();
@@ -263,19 +265,22 @@ EXPORT_SYMBOL(__frontswap_store);
  */
 int __frontswap_load(struct page *page)
 {
-	int ret = -1;
+	int ret;
 	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;
+
+	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;
+	for (ops = frontswap_ops, ret = -1; ops && ret; ops = ops->next)
+		ret = ops->load(type, offset, page);
 	if (ret == 0) {
 		inc_frontswap_loads();
 		if (frontswap_tmem_exclusive_gets_enabled) {
@@ -294,16 +299,19 @@ 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);
-		__frontswap_clear(sis, offset);
-		inc_frontswap_invalidates();
-	}
+	if (!__frontswap_test(sis, offset))
+		return;
+
+	for (ops = frontswap_ops; ops; ops = ops->next)
+		ops->invalidate_page(type, offset);
+	__frontswap_clear(sis, offset);
+	inc_frontswap_invalidates();
 }
 EXPORT_SYMBOL(__frontswap_invalidate_page);
 
@@ -314,16 +322,19 @@ 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) {
-		BUG_ON(sis == NULL);
-		if (sis->frontswap_map == NULL)
-			return;
-		frontswap_ops->invalidate_area(type);
-		atomic_set(&sis->frontswap_pages, 0);
-		bitmap_zero(sis->frontswap_map, sis->max);
-	}
-	clear_bit(type, need_init);
+	if (!frontswap_ops)
+		return;
+
+	BUG_ON(sis == NULL);
+	if (sis->frontswap_map == NULL)
+		return;
+
+	for (ops = frontswap_ops; ops; ops = ops->next)
+		ops->invalidate_area(type);
+	atomic_set(&sis->frontswap_pages, 0);
+	bitmap_zero(sis->frontswap_map, sis->max);
 }
 EXPORT_SYMBOL(__frontswap_invalidate_area);
 
-- 
2.1.0

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Re: [PATCH] frontswap: allow multiple backends

[Andrew Morton]

On Thu, 28 May 2015 16:28:37 -0400 Dan Streetman <ddstreet@ieee.org> wrote:

> 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.

Looks OK to me.  The "you can never deregister" thing makes life
simpler.

But we need to have a fight over style issues.  Just because you *can*
do something doesn't mean you should.  Don't make you poor readers sit
there going crosseyed at elaborate `for' statements.  Try to keep the
code as simple and straightforward as possible.

> ...
>
>  /*
> - * 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;
> -
> -	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);
> -			ops->init(i);
> +	DECLARE_BITMAP(a, MAX_SWAPFILES);
> +	DECLARE_BITMAP(b, MAX_SWAPFILES);
> +	struct swap_info_struct *si;
> +	unsigned int i;
> +
> +	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);
> +
> +	for (i = find_first_bit(a, MAX_SWAPFILES);
> +	     i < MAX_SWAPFILES;
> +	     i = find_next_bit(a, MAX_SWAPFILES, i + 1))
> +		ops->init(i);

	i = find_first_bit(a, MAX_SWAPFILES);
	while (i < MAX_SWAPFILES) {
		ops->init(i);
		i = find_next_bit(a, MAX_SWAPFILES, i + 1);
	}

> +	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);
> +
> +	if (!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);
>  		}
> ...
>
> @@ -215,24 +216,25 @@ static inline void __frontswap_clear(struct swap_info_struct *sis,
>   */
>  int __frontswap_store(struct page *page)
>  {
> -	int ret = -1, dup = 0;
> +	int ret, dup;
>  	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);
> +	dup = __frontswap_test(sis, offset);
> +	for (ops = frontswap_ops, ret = -1; ops && ret; ops = ops->next)
> +		ret = ops->store(type, offset, page);

	ret = -1;
	for (ops = frontswap_ops; ops; ops = ops->next) {
		ret = ops->store(type, offset, page);
		if (!ret)
			break;
	}

One advantage of doing it this way is that it leaves room for comments.
And this code would benefit from a comment above the "if (!ret)". 
What's going on here?  What could cause ->store to return zero and is
this an error?  We should explain this somewhere;  `struct
frontswap_ops' is cheerily undocumented, so where?


Is the `ret = -1' really needed?  Can this function ever be called if
there aren't any registered frontswap_ops?


Also, __frontswap_store() disturbs me:

: int __frontswap_store(struct page *page)
: {
: 	int ret, dup;
: 	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 -1;
: 
: 	BUG_ON(!PageLocked(page));
: 	BUG_ON(sis == NULL);
: 	dup = __frontswap_test(sis, offset);
: 	ret = -1;
: 	for (ops = frontswap_ops; ops; ops = ops->next) {
: 		ret = ops->store(type, offset, page);
: 		if (!ret)
: 			break;
: 	}

Here we've just iterated through all the registered operations.

: 	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
: 		 */
: 		inc_frontswap_failed_stores();
: 		if (dup) {
: 			__frontswap_clear(sis, offset);
: 			frontswap_ops->invalidate_page(type, offset);

But here we call ->invalidate_page on just one of teh registered
operations.  Seems wrong.

Maybe some careful code commentary would clear this up.

: 		}
: 	}
: 	if (frontswap_writethrough_enabled)
: 		/* report failure so swap also writes to swap device */
: 		ret = -1;
: 	return ret;
: }

Please review:

--- a/mm/frontswap.c~frontswap-allow-multiple-backends-fix
+++ a/mm/frontswap.c
@@ -97,7 +97,7 @@ static inline void inc_frontswap_invalid
  *
  * 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.  However, there is currently no way
+ * ignoring or failing the requests.  However, there is currently no way
  * to unload a backend once it is registered.
  */
 
@@ -118,10 +118,11 @@ void frontswap_register_ops(struct front
 	}
 	spin_unlock(&swap_lock);
 
-	for (i = find_first_bit(a, MAX_SWAPFILES);
-	     i < MAX_SWAPFILES;
-	     i = find_next_bit(a, MAX_SWAPFILES, i + 1))
+	i = find_first_bit(a, MAX_SWAPFILES);
+	while (i < MAX_SWAPFILES) {
 		ops->init(i);
+		i = find_next_bit(a, MAX_SWAPFILES, i + 1);
+	}
 
 	do {
 		ops->next = frontswap_ops;
@@ -233,8 +234,12 @@ int __frontswap_store(struct page *page)
 	BUG_ON(!PageLocked(page));
 	BUG_ON(sis == NULL);
 	dup = __frontswap_test(sis, offset);
-	for (ops = frontswap_ops, ret = -1; ops && ret; ops = ops->next)
+	ret = -1;
+	for (ops = frontswap_ops; ops; ops = ops->next) {
 		ret = ops->store(type, offset, page);
+		if (!ret)
+			break;
+	}
 	if (ret == 0) {
 		set_bit(offset, sis->frontswap_map);
 		inc_frontswap_succ_stores();
@@ -279,8 +284,12 @@ int __frontswap_load(struct page *page)
 	BUG_ON(sis == NULL);
 	if (!__frontswap_test(sis, offset))
 		return -1;
-	for (ops = frontswap_ops, ret = -1; ops && ret; ops = ops->next)
+	ret = -1;
+	for (ops = frontswap_ops; ops; ops = ops->next) {
 		ret = ops->load(type, offset, page);
+		if (!ret)
+			break;
+	}
 	if (ret == 0) {
 		inc_frontswap_loads();
 		if (frontswap_tmem_exclusive_gets_enabled) {
_

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Re: [PATCH] frontswap: allow multiple backends

[Dan Streetman]

On Fri, May 29, 2015 at 6:07 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Thu, 28 May 2015 16:28:37 -0400 Dan Streetman <ddstreet@ieee.org> wrote:
>
>> 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.
>
> Looks OK to me.  The "you can never deregister" thing makes life
> simpler.
>
> But we need to have a fight over style issues.  Just because you *can*
> do something doesn't mean you should.  Don't make you poor readers sit
> there going crosseyed at elaborate `for' statements.  Try to keep the
> code as simple and straightforward as possible.
>
>> ...
>>
>>  /*
>> - * 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;
>> -
>> -     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);
>> -                     ops->init(i);
>> +     DECLARE_BITMAP(a, MAX_SWAPFILES);
>> +     DECLARE_BITMAP(b, MAX_SWAPFILES);
>> +     struct swap_info_struct *si;
>> +     unsigned int i;
>> +
>> +     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);
>> +
>> +     for (i = find_first_bit(a, MAX_SWAPFILES);
>> +          i < MAX_SWAPFILES;
>> +          i = find_next_bit(a, MAX_SWAPFILES, i + 1))
>> +             ops->init(i);
>
>         i = find_first_bit(a, MAX_SWAPFILES);
>         while (i < MAX_SWAPFILES) {
>                 ops->init(i);
>                 i = find_next_bit(a, MAX_SWAPFILES, i + 1);
>         }

i was looking for a for_each_set_bit(), but didn't find it, until now
when i looked in bitops.h (why is that in bitops instead of bitmap?).

Anyway, this can be replaced with a simple for_each_set_bit() which
should be much clearer.

>
>> +     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);
>> +
>> +     if (!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);
>>               }
>> ...
>>
>> @@ -215,24 +216,25 @@ static inline void __frontswap_clear(struct swap_info_struct *sis,
>>   */
>>  int __frontswap_store(struct page *page)
>>  {
>> -     int ret = -1, dup = 0;
>> +     int ret, dup;
>>       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);
>> +     dup = __frontswap_test(sis, offset);
>> +     for (ops = frontswap_ops, ret = -1; ops && ret; ops = ops->next)
>> +             ret = ops->store(type, offset, page);
>
>         ret = -1;
>         for (ops = frontswap_ops; ops; ops = ops->next) {
>                 ret = ops->store(type, offset, page);
>                 if (!ret)
>                         break;
>         }
>
> One advantage of doing it this way is that it leaves room for comments.
> And this code would benefit from a comment above the "if (!ret)".
> What's going on here?  What could cause ->store to return zero and is
> this an error?  We should explain this somewhere;  `struct
> frontswap_ops' is cheerily undocumented, so where?

you're right of course, i think i was sleep-deprived when i wrote that
for loop.  I forgot the rule about what's most important: clarity not
brevity (or, clarity not cleverity...).  ;-)

I'll add some comments, both to the frontswap_ops struct as well as in
functions like here for clarification.

>
>
> Is the `ret = -1' really needed?  Can this function ever be called if
> there aren't any registered frontswap_ops?

in this case, the ret = -1 initialization was only needed because of
the ops && ret check (which would fail if ret started at 0).  Putting
the ret check into the for loop both makes it clearer and removes the
need to initialize ret.

The (!frontswap_ops) above the for loop will ensure the for loop is
always called with at least 1 ops.

>
>
> Also, __frontswap_store() disturbs me:
>
> : int __frontswap_store(struct page *page)
> : {
> :       int ret, dup;
> :       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 -1;
> :
> :       BUG_ON(!PageLocked(page));
> :       BUG_ON(sis == NULL);
> :       dup = __frontswap_test(sis, offset);
> :       ret = -1;
> :       for (ops = frontswap_ops; ops; ops = ops->next) {
> :               ret = ops->store(type, offset, page);
> :               if (!ret)
> :                       break;
> :       }
>
> Here we've just iterated through all the registered operations.
>
> :       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
> :                */
> :               inc_frontswap_failed_stores();
> :               if (dup) {
> :                       __frontswap_clear(sis, offset);
> :                       frontswap_ops->invalidate_page(type, offset);
>
> But here we call ->invalidate_page on just one of teh registered
> operations.  Seems wrong.

oops, yep it is.  I missed updating this to invalidate all the
registered ops.  Will fix.

>
> Maybe some careful code commentary would clear this up.
>
> :               }
> :       }
> :       if (frontswap_writethrough_enabled)
> :               /* report failure so swap also writes to swap device */
> :               ret = -1;
> :       return ret;
> : }
>
> Please review:

yep, plus the additional comments and changes i mentioned above.  I'll
try re-sending the patch with updates.

Thanks!


>
> --- a/mm/frontswap.c~frontswap-allow-multiple-backends-fix
> +++ a/mm/frontswap.c
> @@ -97,7 +97,7 @@ static inline void inc_frontswap_invalid
>   *
>   * 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.  However, there is currently no way
> + * ignoring or failing the requests.  However, there is currently no way
>   * to unload a backend once it is registered.
>   */
>
> @@ -118,10 +118,11 @@ void frontswap_register_ops(struct front
>         }
>         spin_unlock(&swap_lock);
>
> -       for (i = find_first_bit(a, MAX_SWAPFILES);
> -            i < MAX_SWAPFILES;
> -            i = find_next_bit(a, MAX_SWAPFILES, i + 1))
> +       i = find_first_bit(a, MAX_SWAPFILES);
> +       while (i < MAX_SWAPFILES) {
>                 ops->init(i);
> +               i = find_next_bit(a, MAX_SWAPFILES, i + 1);
> +       }
>
>         do {
>                 ops->next = frontswap_ops;
> @@ -233,8 +234,12 @@ int __frontswap_store(struct page *page)
>         BUG_ON(!PageLocked(page));
>         BUG_ON(sis == NULL);
>         dup = __frontswap_test(sis, offset);
> -       for (ops = frontswap_ops, ret = -1; ops && ret; ops = ops->next)
> +       ret = -1;
> +       for (ops = frontswap_ops; ops; ops = ops->next) {
>                 ret = ops->store(type, offset, page);
> +               if (!ret)
> +                       break;
> +       }
>         if (ret == 0) {
>                 set_bit(offset, sis->frontswap_map);
>                 inc_frontswap_succ_stores();
> @@ -279,8 +284,12 @@ int __frontswap_load(struct page *page)
>         BUG_ON(sis == NULL);
>         if (!__frontswap_test(sis, offset))
>                 return -1;
> -       for (ops = frontswap_ops, ret = -1; ops && ret; ops = ops->next)
> +       ret = -1;
> +       for (ops = frontswap_ops; ops; ops = ops->next) {
>                 ret = ops->load(type, offset, page);
> +               if (!ret)
> +                       break;
> +       }
>         if (ret == 0) {
>                 inc_frontswap_loads();
>                 if (frontswap_tmem_exclusive_gets_enabled) {
> _
>

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[PATCHv2] frontswap: allow multiple backends

[Dan Streetman]

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.

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
---
Changes since v1, all per Andrew's suggestions:
  -comments added
  -for loops made simpler/clearer
  -invalidate dup page fixed

 drivers/xen/tmem.c        |   8 +-
 include/linux/frontswap.h |  14 +--
 mm/frontswap.c            | 211 +++++++++++++++++++++++++++-------------------
 3 files changed, 134 insertions(+), 99 deletions(-)

diff --git a/drivers/xen/tmem.c b/drivers/xen/tmem.c
index c4211a3..d88f367 100644
--- a/drivers/xen/tmem.c
+++ b/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);
 	}
diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
index 8293262..e65ef95 100644
--- a/include/linux/frontswap.h
+++ b/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);
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 8d82809..25098e6 100644
--- a/mm/frontswap.c
+++ b/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 +102,59 @@ 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.
+ * ignorning 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;
-
-	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);
-			ops->init(i);
-		}
+	DECLARE_BITMAP(a, MAX_SWAPFILES);
+	DECLARE_BITMAP(b, MAX_SWAPFILES);
+	struct swap_info_struct *si;
+	unsigned int i;
+
+	spin_lock(&swap_lock);
+	plist_for_each_entry(si, &swap_active_head, list) {
+		if (!WARN_ON(!si->frontswap_map))
+			set_bit(si->type, a);
 	}
-	/*
-	 * 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.
+	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);
+
+	/* 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
+	 */
+	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.
 	 */
-	barrier();
-	frontswap_ops = ops;
-	return old;
+	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 +182,7 @@ 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);
 
@@ -179,28 +198,30 @@ 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)
+				     pgoff_t offset)
 {
 	clear_bit(offset, sis->frontswap_map);
 	atomic_dec(&sis->frontswap_pages);
@@ -215,39 +236,45 @@ 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 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.
+	 */
+	if (__frontswap_test(sis, offset)) {
+		__frontswap_clear(sis, 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) {
-		set_bit(offset, sis->frontswap_map);
+		__frontswap_set(sis, offset);
 		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
-		 */
 		inc_frontswap_failed_stores();
-		if (dup) {
-			__frontswap_clear(sis, offset);
-			frontswap_ops->invalidate_page(type, offset);
-		}
 	}
 	if (frontswap_writethrough_enabled)
 		/* report failure so swap also writes to swap device */
@@ -268,14 +295,22 @@ 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) {
@@ -294,16 +329,19 @@ 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);
-		__frontswap_clear(sis, offset);
-		inc_frontswap_invalidates();
-	}
+	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);
 
@@ -314,16 +352,19 @@ 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) {
-		BUG_ON(sis == NULL);
-		if (sis->frontswap_map == NULL)
-			return;
-		frontswap_ops->invalidate_area(type);
-		atomic_set(&sis->frontswap_pages, 0);
-		bitmap_zero(sis->frontswap_map, sis->max);
-	}
-	clear_bit(type, need_init);
+	if (!frontswap_ops)
+		return;
+
+	BUG_ON(sis == NULL);
+	if (sis->frontswap_map == NULL)
+		return;
+
+	for_each_frontswap_ops(ops)
+		ops->invalidate_area(type);
+	atomic_set(&sis->frontswap_pages, 0);
+	bitmap_zero(sis->frontswap_map, sis->max);
 }
 EXPORT_SYMBOL(__frontswap_invalidate_area);
 
-- 
2.1.0

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Re: [PATCHv2] frontswap: allow multiple backends

[Andrew Morton]

On Mon,  1 Jun 2015 10:22:24 -0400 Dan Streetman <ddstreet@ieee.org> wrote:

> 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.
> 
> ...
>
> -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;
> -
> -	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);
> -			ops->init(i);
> -		}
> +	DECLARE_BITMAP(a, MAX_SWAPFILES);
> +	DECLARE_BITMAP(b, MAX_SWAPFILES);
> +	struct swap_info_struct *si;
> +	unsigned int i;
> +
> +	spin_lock(&swap_lock);
> +	plist_for_each_entry(si, &swap_active_head, list) {
> +		if (!WARN_ON(!si->frontswap_map))
> +			set_bit(si->type, a);

umm, DECLARE_BITMAP() doesn't initialise the storage.  Either this
patch wasn't tested very well or you should buy me a lottery ticket!

>  	}
> -	/*
> -	 * 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.
> +	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);
> +
> +	/* 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
> +	 */

Like this, please:

	/*
	 * Setting ...

and sentences start with capital letters ;)


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Re: [PATCHv2] frontswap: allow multiple backends

[Dan Streetman]

On Tue, Jun 2, 2015 at 5:06 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Mon,  1 Jun 2015 10:22:24 -0400 Dan Streetman <ddstreet@ieee.org> wrote:
>
>> 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.
>>
>> ...
>>
>> -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;
>> -
>> -     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);
>> -                     ops->init(i);
>> -             }
>> +     DECLARE_BITMAP(a, MAX_SWAPFILES);
>> +     DECLARE_BITMAP(b, MAX_SWAPFILES);
>> +     struct swap_info_struct *si;
>> +     unsigned int i;
>> +
>> +     spin_lock(&swap_lock);
>> +     plist_for_each_entry(si, &swap_active_head, list) {
>> +             if (!WARN_ON(!si->frontswap_map))
>> +                     set_bit(si->type, a);
>
> umm, DECLARE_BITMAP() doesn't initialise the storage.  Either this
> patch wasn't tested very well or you should buy me a lottery ticket!

Doh!  I'll fix and resend.

I did test it, too, but zswap doesn't care if the swap device actually
exists, it just alloc's a tree for whatever it's told.  So likely it
was allocing some extra trees there :)

>
>>       }
>> -     /*
>> -      * 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.
>> +     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);
>> +
>> +     /* 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
>> +      */
>
> Like this, please:
>
>         /*
>          * Setting ...
>
> and sentences start with capital letters ;)

okay, okay :-)

>
>

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[PATCHv3] frontswap: allow multiple backends

[Dan Streetman]

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>
---
Changes since v2: 
  -initialize bitmaps in frontswap_register_ops
  -fix comment capitalization

 drivers/xen/tmem.c        |   8 +-
 include/linux/frontswap.h |  14 +--
 mm/frontswap.c            | 215 ++++++++++++++++++++++++++++------------------
 3 files changed, 139 insertions(+), 98 deletions(-)

diff --git a/drivers/xen/tmem.c b/drivers/xen/tmem.c
index c4211a3..d88f367 100644
--- a/drivers/xen/tmem.c
+++ b/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);
 	}
diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
index 8293262..e65ef95 100644
--- a/include/linux/frontswap.h
+++ b/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);
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 8d82809..27a9924 100644
--- a/mm/frontswap.c
+++ b/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;
-
-	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);
-			ops->init(i);
-		}
+	DECLARE_BITMAP(a, MAX_SWAPFILES);
+	DECLARE_BITMAP(b, MAX_SWAPFILES);
+	struct swap_info_struct *si;
+	unsigned int i;
+
+	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);
 
@@ -179,28 +203,30 @@ 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)
+				     pgoff_t offset)
 {
 	clear_bit(offset, sis->frontswap_map);
 	atomic_dec(&sis->frontswap_pages);
@@ -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 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.
+	 */
+	if (__frontswap_test(sis, offset)) {
+		__frontswap_clear(sis, 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) {
-		set_bit(offset, sis->frontswap_map);
+		__frontswap_set(sis, offset);
 		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
-		 */
 		inc_frontswap_failed_stores();
-		if (dup) {
-			__frontswap_clear(sis, offset);
-			frontswap_ops->invalidate_page(type, offset);
-		}
 	}
 	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) {
@@ -294,16 +335,19 @@ 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);
-		__frontswap_clear(sis, offset);
-		inc_frontswap_invalidates();
-	}
+	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);
 
@@ -314,16 +358,19 @@ 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) {
-		BUG_ON(sis == NULL);
-		if (sis->frontswap_map == NULL)
-			return;
-		frontswap_ops->invalidate_area(type);
-		atomic_set(&sis->frontswap_pages, 0);
-		bitmap_zero(sis->frontswap_map, sis->max);
-	}
-	clear_bit(type, need_init);
+	if (!frontswap_ops)
+		return;
+
+	BUG_ON(sis == NULL);
+	if (sis->frontswap_map == NULL)
+		return;
+
+	for_each_frontswap_ops(ops)
+		ops->invalidate_area(type);
+	atomic_set(&sis->frontswap_pages, 0);
+	bitmap_zero(sis->frontswap_map, sis->max);
 }
 EXPORT_SYMBOL(__frontswap_invalidate_area);
 
-- 
2.1.0

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Re: [PATCHv3] frontswap: allow multiple backends

[Andrew Morton]

On Tue,  2 Jun 2015 18:08:46 -0400 Dan Streetman <ddstreet@ieee.org> wrote:

> 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.
> 

offtopic trivia: this

--- a/mm/frontswap.c~frontswap-allow-multiple-backends-fix
+++ a/mm/frontswap.c
@@ -111,14 +111,11 @@ static inline void inc_frontswap_invalid
  */
 void frontswap_register_ops(struct frontswap_ops *ops)
 {
-	DECLARE_BITMAP(a, MAX_SWAPFILES);
-	DECLARE_BITMAP(b, MAX_SWAPFILES);
+	DECLARE_BITMAP(a, MAX_SWAPFILES) = { };
+	DECLARE_BITMAP(b, MAX_SWAPFILES) = { };
 	struct swap_info_struct *si;
 	unsigned int i;
 
-	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))

saves 64 bytes of text with my gcc.


It shouldn't be open-coded here, but a new macro in bitmap.h could be
useful, assuming it's a win for other sizes of bitmaps.

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