Re: [PATCH v7 11/12] zsmalloc: page migration support

[Chulmin Kim <cmlaika.kim@samsung.com>]

On 01/18/2017 09:44 PM, Minchan Kim wrote:
> Hello Chulmin,
>
> On Wed, Jan 18, 2017 at 07:13:21PM -0500, Chulmin Kim wrote:
>> Hello. Minchan, and all zsmalloc guys.
>>
>> I have a quick question.
>> Is zsmalloc considering memory barrier things correctly?
>>
>> AFAIK, in ARM64,
>> zsmalloc relies on dmb operation in bit_spin_unlock only.
>> (It seems that dmb operations in spinlock functions are being prepared,
>> but let is be aside as it is not merged yet.)
>>
>> If I am correct,
>> migrating a page in a zspage filled with free objs
>> may cause the corruption cause bit_spin_unlock will not be executed at all.
>>
>> I am not sure this is enough memory barrier for zsmalloc operations.
>>
>> Can you enlighten me?
>
> Do you mean bit_spin_unlock is broken or zsmalloc locking scheme broken?
> Could you please describe what you are concerning in detail?
> It would be very helpful if you say it with a example!

Sorry for ambiguous expressions. :)

Recently,
I found multiple zsmalloc corruption cases which have garbage idx values 
in in zspage->freeobj. (not ffffffff (-1) value.)

Honestly, I have no clue yet.

I suspect the case when zspage migrate a zs sub page filled with free 
objects (so that never calls unpin_tag() which has memory barrier).


Assume the page (zs subpage) being migrated has no allocated zs object.

S : zs subpage
D : free page


CPU A : zs_page_migrate()		CPU B : zs_malloc()
---------------------			-----------------------------


migrate_write_lock()
spin_lock()

memcpy(D, S, PAGE_SIZE)   -> (1)
replace_sub_page()

putback_zspage()
spin_unlock()
migrate_write_unlock()
					
					spin_lock()
					obj_malloc()
					--> (2-a) allocate obj in D
					--> (2-b) set freeobj using
      						the first 8 bytes of
  						the allocated obj
					record_obj()
					spin_unlock



I think the locking has no problem, but memory ordering.
I doubt whether (2-b) in CPU B really loads the data stored by (1).

If it doesn't, set_freeobj in (2-b) will corrupt zspage->freeobj.
After then, we will see corrupted object sooner or later.


According to the below link,
(https://patchwork.kernel.org/patch/9313493/)
spin lock in a specific arch (arm64 maybe) seems not guaranteeing memory 
ordering.

===
+/*
+ * Accesses appearing in program order before a spin_lock() operation
+ * can be reordered with accesses inside the critical section, by virtue
+ * of arch_spin_lock being constructed using acquire semantics.
+ *
+ * In cases where this is problematic (e.g. try_to_wake_up), an
+ * smp_mb__before_spinlock() can restore the required ordering.
+ */
+#define smp_mb__before_spinlock()	smp_mb()
===



THanks.
CHulmin Kim





>
> Thanks.
>
>>
>>
>> THanks!
>> CHulmin KIm
>>
>>
>>
>> On 05/31/2016 07:21 PM, Minchan Kim wrote:
>>> This patch introduces run-time migration feature for zspage.
>>>
>>> For migration, VM uses page.lru field so it would be better to not use
>>> page.next field which is unified with page.lru for own purpose.
>>> For that, firstly, we can get first object offset of the page via
>>> runtime calculation instead of using page.index so we can use
>>> page.index as link for page chaining instead of page.next.
>>> 	
>>> In case of huge object, it stores handle to page.index instead of
>>> next link of page chaining because huge object doesn't need to next
>>> link for page chaining. So get_next_page need to identify huge
>>> object to return NULL. For it, this patch uses PG_owner_priv_1 flag
>>> of the page flag.
>>>
>>> For migration, it supports three functions
>>>
>>> * zs_page_isolate
>>>
>>> It isolates a zspage which includes a subpage VM want to migrate
>> >from class so anyone cannot allocate new object from the zspage.
>>>
>>> We could try to isolate a zspage by the number of subpage so
>>> subsequent isolation trial of other subpage of the zpsage shouldn't
>>> fail. For that, we introduce zspage.isolated count. With that,
>>> zs_page_isolate can know whether zspage is already isolated or not
>>> for migration so if it is isolated for migration, subsequent
>>> isolation trial can be successful without trying further isolation.
>>>
>>> * zs_page_migrate
>>>
>>> First of all, it holds write-side zspage->lock to prevent migrate other
>>> subpage in zspage. Then, lock all objects in the page VM want to migrate.
>>> The reason we should lock all objects in the page is due to race between
>>> zs_map_object and zs_page_migrate.
>>>
>>> zs_map_object				zs_page_migrate
>>>
>>> pin_tag(handle)
>>> obj = handle_to_obj(handle)
>>> obj_to_location(obj, &page, &obj_idx);
>>>
>>> 					write_lock(&zspage->lock)
>>> 					if (!trypin_tag(handle))
>>> 						goto unpin_object
>>>
>>> zspage = get_zspage(page);
>>> read_lock(&zspage->lock);
>>>
>>> If zs_page_migrate doesn't do trypin_tag, zs_map_object's page can
>>> be stale by migration so it goes crash.
>>>
>>> If it locks all of objects successfully, it copies content from
>>> old page to new one, finally, create new zspage chain with new page.
>>> And if it's last isolated subpage in the zspage, put the zspage back
>>> to class.
>>>
>>> * zs_page_putback
>>>
>>> It returns isolated zspage to right fullness_group list if it fails to
>>> migrate a page. If it find a zspage is ZS_EMPTY, it queues zspage
>>> freeing to workqueue. See below about async zspage freeing.
>>>
>>> This patch introduces asynchronous zspage free. The reason to need it
>>> is we need page_lock to clear PG_movable but unfortunately,
>>> zs_free path should be atomic so the apporach is try to grab page_lock.
>>> If it got page_lock of all of pages successfully, it can free zspage
>>> immediately. Otherwise, it queues free request and free zspage via
>>> workqueue in process context.
>>>
>>> If zs_free finds the zspage is isolated when it try to free zspage,
>>> it delays the freeing until zs_page_putback finds it so it will free
>>> free the zspage finally.
>>>
>>> In this patch, we expand fullness_list from ZS_EMPTY to ZS_FULL.
>>> First of all, it will use ZS_EMPTY list for delay freeing.
>>> And with adding ZS_FULL list, it makes to identify whether zspage is
>>> isolated or not via list_empty(&zspage->list) test.
>>>
>>> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
>>> Signed-off-by: Minchan Kim <minchan@kernel.org>
>>> ---
>>> include/uapi/linux/magic.h |   1 +
>>> mm/zsmalloc.c              | 793 ++++++++++++++++++++++++++++++++++++++-------
>>> 2 files changed, 672 insertions(+), 122 deletions(-)
>>>
>>> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
>>> index d829ce63529d..e398beac67b8 100644
>>> --- a/include/uapi/linux/magic.h
>>> +++ b/include/uapi/linux/magic.h
>>> @@ -81,5 +81,6 @@
>>> /* Since UDF 2.01 is ISO 13346 based... */
>>> #define UDF_SUPER_MAGIC		0x15013346
>>> #define BALLOON_KVM_MAGIC	0x13661366
>>> +#define ZSMALLOC_MAGIC		0x58295829
>>>
>>> #endif /* __LINUX_MAGIC_H__ */
>>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>>> index c6fb543cfb98..a80100db16d6 100644
>>> --- a/mm/zsmalloc.c
>>> +++ b/mm/zsmalloc.c
>>> @@ -17,14 +17,14 @@
>>>  *
>>>  * Usage of struct page fields:
>>>  *	page->private: points to zspage
>>> - *	page->index: offset of the first object starting in this page.
>>> - *		For the first page, this is always 0, so we use this field
>>> - *		to store handle for huge object.
>>> - *	page->next: links together all component pages of a zspage
>>> + *	page->freelist(index): links together all component pages of a zspage
>>> + *		For the huge page, this is always 0, so we use this field
>>> + *		to store handle.
>>>  *
>>>  * Usage of struct page flags:
>>>  *	PG_private: identifies the first component page
>>>  *	PG_private2: identifies the last component page
>>> + *	PG_owner_priv_1: indentifies the huge component page
>>>  *
>>>  */
>>>
>>> @@ -49,6 +49,11 @@
>>> #include <linux/debugfs.h>
>>> #include <linux/zsmalloc.h>
>>> #include <linux/zpool.h>
>>> +#include <linux/mount.h>
>>> +#include <linux/compaction.h>
>>> +#include <linux/pagemap.h>
>>> +
>>> +#define ZSPAGE_MAGIC	0x58
>>>
>>> /*
>>>  * This must be power of 2 and greater than of equal to sizeof(link_free).
>>> @@ -136,25 +141,23 @@
>>>  * We do not maintain any list for completely empty or full pages
>>>  */
>>> enum fullness_group {
>>> -	ZS_ALMOST_FULL,
>>> -	ZS_ALMOST_EMPTY,
>>> 	ZS_EMPTY,
>>> -	ZS_FULL
>>> +	ZS_ALMOST_EMPTY,
>>> +	ZS_ALMOST_FULL,
>>> +	ZS_FULL,
>>> +	NR_ZS_FULLNESS,
>>> };
>>>
>>> enum zs_stat_type {
>>> +	CLASS_EMPTY,
>>> +	CLASS_ALMOST_EMPTY,
>>> +	CLASS_ALMOST_FULL,
>>> +	CLASS_FULL,
>>> 	OBJ_ALLOCATED,
>>> 	OBJ_USED,
>>> -	CLASS_ALMOST_FULL,
>>> -	CLASS_ALMOST_EMPTY,
>>> +	NR_ZS_STAT_TYPE,
>>> };
>>>
>>> -#ifdef CONFIG_ZSMALLOC_STAT
>>> -#define NR_ZS_STAT_TYPE	(CLASS_ALMOST_EMPTY + 1)
>>> -#else
>>> -#define NR_ZS_STAT_TYPE	(OBJ_USED + 1)
>>> -#endif
>>> -
>>> struct zs_size_stat {
>>> 	unsigned long objs[NR_ZS_STAT_TYPE];
>>> };
>>> @@ -163,6 +166,10 @@ struct zs_size_stat {
>>> static struct dentry *zs_stat_root;
>>> #endif
>>>
>>> +#ifdef CONFIG_COMPACTION
>>> +static struct vfsmount *zsmalloc_mnt;
>>> +#endif
>>> +
>>> /*
>>>  * number of size_classes
>>>  */
>>> @@ -186,23 +193,36 @@ static const int fullness_threshold_frac = 4;
>>>
>>> struct size_class {
>>> 	spinlock_t lock;
>>> -	struct list_head fullness_list[2];
>>> +	struct list_head fullness_list[NR_ZS_FULLNESS];
>>> 	/*
>>> 	 * Size of objects stored in this class. Must be multiple
>>> 	 * of ZS_ALIGN.
>>> 	 */
>>> 	int size;
>>> 	int objs_per_zspage;
>>> -	unsigned int index;
>>> -
>>> -	struct zs_size_stat stats;
>>> -
>>> 	/* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */
>>> 	int pages_per_zspage;
>>> -	/* huge object: pages_per_zspage == 1 && maxobj_per_zspage == 1 */
>>> -	bool huge;
>>> +
>>> +	unsigned int index;
>>> +	struct zs_size_stat stats;
>>> };
>>>
>>> +/* huge object: pages_per_zspage == 1 && maxobj_per_zspage == 1 */
>>> +static void SetPageHugeObject(struct page *page)
>>> +{
>>> +	SetPageOwnerPriv1(page);
>>> +}
>>> +
>>> +static void ClearPageHugeObject(struct page *page)
>>> +{
>>> +	ClearPageOwnerPriv1(page);
>>> +}
>>> +
>>> +static int PageHugeObject(struct page *page)
>>> +{
>>> +	return PageOwnerPriv1(page);
>>> +}
>>> +
>>> /*
>>>  * Placed within free objects to form a singly linked list.
>>>  * For every zspage, zspage->freeobj gives head of this list.
>>> @@ -244,6 +264,10 @@ struct zs_pool {
>>> #ifdef CONFIG_ZSMALLOC_STAT
>>> 	struct dentry *stat_dentry;
>>> #endif
>>> +#ifdef CONFIG_COMPACTION
>>> +	struct inode *inode;
>>> +	struct work_struct free_work;
>>> +#endif
>>> };
>>>
>>> /*
>>> @@ -252,16 +276,23 @@ struct zs_pool {
>>>  */
>>> #define FULLNESS_BITS	2
>>> #define CLASS_BITS	8
>>> +#define ISOLATED_BITS	3
>>> +#define MAGIC_VAL_BITS	8
>>>
>>> struct zspage {
>>> 	struct {
>>> 		unsigned int fullness:FULLNESS_BITS;
>>> 		unsigned int class:CLASS_BITS;
>>> +		unsigned int isolated:ISOLATED_BITS;
>>> +		unsigned int magic:MAGIC_VAL_BITS;
>>> 	};
>>> 	unsigned int inuse;
>>> 	unsigned int freeobj;
>>> 	struct page *first_page;
>>> 	struct list_head list; /* fullness list */
>>> +#ifdef CONFIG_COMPACTION
>>> +	rwlock_t lock;
>>> +#endif
>>> };
>>>
>>> struct mapping_area {
>>> @@ -274,6 +305,28 @@ struct mapping_area {
>>> 	enum zs_mapmode vm_mm; /* mapping mode */
>>> };
>>>
>>> +#ifdef CONFIG_COMPACTION
>>> +static int zs_register_migration(struct zs_pool *pool);
>>> +static void zs_unregister_migration(struct zs_pool *pool);
>>> +static void migrate_lock_init(struct zspage *zspage);
>>> +static void migrate_read_lock(struct zspage *zspage);
>>> +static void migrate_read_unlock(struct zspage *zspage);
>>> +static void kick_deferred_free(struct zs_pool *pool);
>>> +static void init_deferred_free(struct zs_pool *pool);
>>> +static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage);
>>> +#else
>>> +static int zsmalloc_mount(void) { return 0; }
>>> +static void zsmalloc_unmount(void) {}
>>> +static int zs_register_migration(struct zs_pool *pool) { return 0; }
>>> +static void zs_unregister_migration(struct zs_pool *pool) {}
>>> +static void migrate_lock_init(struct zspage *zspage) {}
>>> +static void migrate_read_lock(struct zspage *zspage) {}
>>> +static void migrate_read_unlock(struct zspage *zspage) {}
>>> +static void kick_deferred_free(struct zs_pool *pool) {}
>>> +static void init_deferred_free(struct zs_pool *pool) {}
>>> +static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage) {}
>>> +#endif
>>> +
>>> static int create_cache(struct zs_pool *pool)
>>> {
>>> 	pool->handle_cachep = kmem_cache_create("zs_handle", ZS_HANDLE_SIZE,
>>> @@ -301,7 +354,7 @@ static void destroy_cache(struct zs_pool *pool)
>>> static unsigned long cache_alloc_handle(struct zs_pool *pool, gfp_t gfp)
>>> {
>>> 	return (unsigned long)kmem_cache_alloc(pool->handle_cachep,
>>> -			gfp & ~__GFP_HIGHMEM);
>>> +			gfp & ~(__GFP_HIGHMEM|__GFP_MOVABLE));
>>> }
>>>
>>> static void cache_free_handle(struct zs_pool *pool, unsigned long handle)
>>> @@ -311,7 +364,8 @@ static void cache_free_handle(struct zs_pool *pool, unsigned long handle)
>>>
>>> static struct zspage *cache_alloc_zspage(struct zs_pool *pool, gfp_t flags)
>>> {
>>> -	return kmem_cache_alloc(pool->zspage_cachep, flags & ~__GFP_HIGHMEM);
>>> +	return kmem_cache_alloc(pool->zspage_cachep,
>>> +			flags & ~(__GFP_HIGHMEM|__GFP_MOVABLE));
>>> };
>>>
>>> static void cache_free_zspage(struct zs_pool *pool, struct zspage *zspage)
>>> @@ -421,11 +475,17 @@ static unsigned int get_maxobj_per_zspage(int size, int pages_per_zspage)
>>> /* per-cpu VM mapping areas for zspage accesses that cross page boundaries */
>>> static DEFINE_PER_CPU(struct mapping_area, zs_map_area);
>>>
>>> +static bool is_zspage_isolated(struct zspage *zspage)
>>> +{
>>> +	return zspage->isolated;
>>> +}
>>> +
>>> static int is_first_page(struct page *page)
>>> {
>>> 	return PagePrivate(page);
>>> }
>>>
>>> +/* Protected by class->lock */
>>> static inline int get_zspage_inuse(struct zspage *zspage)
>>> {
>>> 	return zspage->inuse;
>>> @@ -441,20 +501,12 @@ static inline void mod_zspage_inuse(struct zspage *zspage, int val)
>>> 	zspage->inuse += val;
>>> }
>>>
>>> -static inline int get_first_obj_offset(struct page *page)
>>> +static inline struct page *get_first_page(struct zspage *zspage)
>>> {
>>> -	if (is_first_page(page))
>>> -		return 0;
>>> +	struct page *first_page = zspage->first_page;
>>>
>>> -	return page->index;
>>> -}
>>> -
>>> -static inline void set_first_obj_offset(struct page *page, int offset)
>>> -{
>>> -	if (is_first_page(page))
>>> -		return;
>>> -
>>> -	page->index = offset;
>>> +	VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
>>> +	return first_page;
>>> }
>>>
>>> static inline unsigned int get_freeobj(struct zspage *zspage)
>>> @@ -471,6 +523,8 @@ static void get_zspage_mapping(struct zspage *zspage,
>>> 				unsigned int *class_idx,
>>> 				enum fullness_group *fullness)
>>> {
>>> +	VM_BUG_ON(zspage->magic != ZSPAGE_MAGIC);
>>> +
>>> 	*fullness = zspage->fullness;
>>> 	*class_idx = zspage->class;
>>> }
>>> @@ -504,23 +558,19 @@ static int get_size_class_index(int size)
>>> static inline void zs_stat_inc(struct size_class *class,
>>> 				enum zs_stat_type type, unsigned long cnt)
>>> {
>>> -	if (type < NR_ZS_STAT_TYPE)
>>> -		class->stats.objs[type] += cnt;
>>> +	class->stats.objs[type] += cnt;
>>> }
>>>
>>> static inline void zs_stat_dec(struct size_class *class,
>>> 				enum zs_stat_type type, unsigned long cnt)
>>> {
>>> -	if (type < NR_ZS_STAT_TYPE)
>>> -		class->stats.objs[type] -= cnt;
>>> +	class->stats.objs[type] -= cnt;
>>> }
>>>
>>> static inline unsigned long zs_stat_get(struct size_class *class,
>>> 				enum zs_stat_type type)
>>> {
>>> -	if (type < NR_ZS_STAT_TYPE)
>>> -		return class->stats.objs[type];
>>> -	return 0;
>>> +	return class->stats.objs[type];
>>> }
>>>
>>> #ifdef CONFIG_ZSMALLOC_STAT
>>> @@ -664,6 +714,7 @@ static inline void zs_pool_stat_destroy(struct zs_pool *pool)
>>> }
>>> #endif
>>>
>>> +
>>> /*
>>>  * For each size class, zspages are divided into different groups
>>>  * depending on how "full" they are. This was done so that we could
>>> @@ -704,15 +755,9 @@ static void insert_zspage(struct size_class *class,
>>> {
>>> 	struct zspage *head;
>>>
>>> -	if (fullness >= ZS_EMPTY)
>>> -		return;
>>> -
>>> +	zs_stat_inc(class, fullness, 1);
>>> 	head = list_first_entry_or_null(&class->fullness_list[fullness],
>>> 					struct zspage, list);
>>> -
>>> -	zs_stat_inc(class, fullness == ZS_ALMOST_EMPTY ?
>>> -			CLASS_ALMOST_EMPTY : CLASS_ALMOST_FULL, 1);
>>> -
>>> 	/*
>>> 	 * We want to see more ZS_FULL pages and less almost empty/full.
>>> 	 * Put pages with higher ->inuse first.
>>> @@ -734,14 +779,11 @@ static void remove_zspage(struct size_class *class,
>>> 				struct zspage *zspage,
>>> 				enum fullness_group fullness)
>>> {
>>> -	if (fullness >= ZS_EMPTY)
>>> -		return;
>>> -
>>> 	VM_BUG_ON(list_empty(&class->fullness_list[fullness]));
>>> +	VM_BUG_ON(is_zspage_isolated(zspage));
>>>
>>> 	list_del_init(&zspage->list);
>>> -	zs_stat_dec(class, fullness == ZS_ALMOST_EMPTY ?
>>> -			CLASS_ALMOST_EMPTY : CLASS_ALMOST_FULL, 1);
>>> +	zs_stat_dec(class, fullness, 1);
>>> }
>>>
>>> /*
>>> @@ -764,8 +806,11 @@ static enum fullness_group fix_fullness_group(struct size_class *class,
>>> 	if (newfg == currfg)
>>> 		goto out;
>>>
>>> -	remove_zspage(class, zspage, currfg);
>>> -	insert_zspage(class, zspage, newfg);
>>> +	if (!is_zspage_isolated(zspage)) {
>>> +		remove_zspage(class, zspage, currfg);
>>> +		insert_zspage(class, zspage, newfg);
>>> +	}
>>> +
>>> 	set_zspage_mapping(zspage, class_idx, newfg);
>>>
>>> out:
>>> @@ -808,19 +853,45 @@ static int get_pages_per_zspage(int class_size)
>>> 	return max_usedpc_order;
>>> }
>>>
>>> -static struct page *get_first_page(struct zspage *zspage)
>>> +static struct zspage *get_zspage(struct page *page)
>>> {
>>> -	return zspage->first_page;
>>> +	struct zspage *zspage = (struct zspage *)page->private;
>>> +
>>> +	VM_BUG_ON(zspage->magic != ZSPAGE_MAGIC);
>>> +	return zspage;
>>> }
>>>
>>> -static struct zspage *get_zspage(struct page *page)
>>> +static struct page *get_next_page(struct page *page)
>>> {
>>> -	return (struct zspage *)page->private;
>>> +	if (unlikely(PageHugeObject(page)))
>>> +		return NULL;
>>> +
>>> +	return page->freelist;
>>> }
>>>
>>> -static struct page *get_next_page(struct page *page)
>>> +/* Get byte offset of first object in the @page */
>>> +static int get_first_obj_offset(struct size_class *class,
>>> +				struct page *first_page, struct page *page)
>>> {
>>> -	return page->next;
>>> +	int pos;
>>> +	int page_idx = 0;
>>> +	int ofs = 0;
>>> +	struct page *cursor = first_page;
>>> +
>>> +	if (first_page == page)
>>> +		goto out;
>>> +
>>> +	while (page != cursor) {
>>> +		page_idx++;
>>> +		cursor = get_next_page(cursor);
>>> +	}
>>> +
>>> +	pos = class->objs_per_zspage * class->size *
>>> +		page_idx / class->pages_per_zspage;
>>> +
>>> +	ofs = (pos + class->size) % PAGE_SIZE;
>>> +out:
>>> +	return ofs;
>>> }
>>>
>>> /**
>>> @@ -857,16 +928,20 @@ static unsigned long handle_to_obj(unsigned long handle)
>>> 	return *(unsigned long *)handle;
>>> }
>>>
>>> -static unsigned long obj_to_head(struct size_class *class, struct page *page,
>>> -			void *obj)
>>> +static unsigned long obj_to_head(struct page *page, void *obj)
>>> {
>>> -	if (class->huge) {
>>> +	if (unlikely(PageHugeObject(page))) {
>>> 		VM_BUG_ON_PAGE(!is_first_page(page), page);
>>> 		return page->index;
>>> 	} else
>>> 		return *(unsigned long *)obj;
>>> }
>>>
>>> +static inline int testpin_tag(unsigned long handle)
>>> +{
>>> +	return bit_spin_is_locked(HANDLE_PIN_BIT, (unsigned long *)handle);
>>> +}
>>> +
>>> static inline int trypin_tag(unsigned long handle)
>>> {
>>> 	return bit_spin_trylock(HANDLE_PIN_BIT, (unsigned long *)handle);
>>> @@ -884,27 +959,93 @@ static void unpin_tag(unsigned long handle)
>>>
>>> static void reset_page(struct page *page)
>>> {
>>> +	__ClearPageMovable(page);
>>> 	clear_bit(PG_private, &page->flags);
>>> 	clear_bit(PG_private_2, &page->flags);
>>> 	set_page_private(page, 0);
>>> -	page->index = 0;
>>> +	ClearPageHugeObject(page);
>>> +	page->freelist = NULL;
>>> }
>>>
>>> -static void free_zspage(struct zs_pool *pool, struct zspage *zspage)
>>> +/*
>>> + * To prevent zspage destroy during migration, zspage freeing should
>>> + * hold locks of all pages in the zspage.
>>> + */
>>> +void lock_zspage(struct zspage *zspage)
>>> +{
>>> +	struct page *page = get_first_page(zspage);
>>> +
>>> +	do {
>>> +		lock_page(page);
>>> +	} while ((page = get_next_page(page)) != NULL);
>>> +}
>>> +
>>> +int trylock_zspage(struct zspage *zspage)
>>> +{
>>> +	struct page *cursor, *fail;
>>> +
>>> +	for (cursor = get_first_page(zspage); cursor != NULL; cursor =
>>> +					get_next_page(cursor)) {
>>> +		if (!trylock_page(cursor)) {
>>> +			fail = cursor;
>>> +			goto unlock;
>>> +		}
>>> +	}
>>> +
>>> +	return 1;
>>> +unlock:
>>> +	for (cursor = get_first_page(zspage); cursor != fail; cursor =
>>> +					get_next_page(cursor))
>>> +		unlock_page(cursor);
>>> +
>>> +	return 0;
>>> +}
>>> +
>>> +static void __free_zspage(struct zs_pool *pool, struct size_class *class,
>>> +				struct zspage *zspage)
>>> {
>>> 	struct page *page, *next;
>>> +	enum fullness_group fg;
>>> +	unsigned int class_idx;
>>> +
>>> +	get_zspage_mapping(zspage, &class_idx, &fg);
>>> +
>>> +	assert_spin_locked(&class->lock);
>>>
>>> 	VM_BUG_ON(get_zspage_inuse(zspage));
>>> +	VM_BUG_ON(fg != ZS_EMPTY);
>>>
>>> -	next = page = zspage->first_page;
>>> +	next = page = get_first_page(zspage);
>>> 	do {
>>> -		next = page->next;
>>> +		VM_BUG_ON_PAGE(!PageLocked(page), page);
>>> +		next = get_next_page(page);
>>> 		reset_page(page);
>>> +		unlock_page(page);
>>> 		put_page(page);
>>> 		page = next;
>>> 	} while (page != NULL);
>>>
>>> 	cache_free_zspage(pool, zspage);
>>> +
>>> +	zs_stat_dec(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
>>> +			class->size, class->pages_per_zspage));
>>> +	atomic_long_sub(class->pages_per_zspage,
>>> +					&pool->pages_allocated);
>>> +}
>>> +
>>> +static void free_zspage(struct zs_pool *pool, struct size_class *class,
>>> +				struct zspage *zspage)
>>> +{
>>> +	VM_BUG_ON(get_zspage_inuse(zspage));
>>> +	VM_BUG_ON(list_empty(&zspage->list));
>>> +
>>> +	if (!trylock_zspage(zspage)) {
>>> +		kick_deferred_free(pool);
>>> +		return;
>>> +	}
>>> +
>>> +	remove_zspage(class, zspage, ZS_EMPTY);
>>> +	__free_zspage(pool, class, zspage);
>>> }
>>>
>>> /* Initialize a newly allocated zspage */
>>> @@ -912,15 +1053,13 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
>>> {
>>> 	unsigned int freeobj = 1;
>>> 	unsigned long off = 0;
>>> -	struct page *page = zspage->first_page;
>>> +	struct page *page = get_first_page(zspage);
>>>
>>> 	while (page) {
>>> 		struct page *next_page;
>>> 		struct link_free *link;
>>> 		void *vaddr;
>>>
>>> -		set_first_obj_offset(page, off);
>>> -
>>> 		vaddr = kmap_atomic(page);
>>> 		link = (struct link_free *)vaddr + off / sizeof(*link);
>>>
>>> @@ -952,16 +1091,17 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
>>> 	set_freeobj(zspage, 0);
>>> }
>>>
>>> -static void create_page_chain(struct zspage *zspage, struct page *pages[],
>>> -				int nr_pages)
>>> +static void create_page_chain(struct size_class *class, struct zspage *zspage,
>>> +				struct page *pages[])
>>> {
>>> 	int i;
>>> 	struct page *page;
>>> 	struct page *prev_page = NULL;
>>> +	int nr_pages = class->pages_per_zspage;
>>>
>>> 	/*
>>> 	 * Allocate individual pages and link them together as:
>>> -	 * 1. all pages are linked together using page->next
>>> +	 * 1. all pages are linked together using page->freelist
>>> 	 * 2. each sub-page point to zspage using page->private
>>> 	 *
>>> 	 * we set PG_private to identify the first page (i.e. no other sub-page
>>> @@ -970,16 +1110,18 @@ static void create_page_chain(struct zspage *zspage, struct page *pages[],
>>> 	for (i = 0; i < nr_pages; i++) {
>>> 		page = pages[i];
>>> 		set_page_private(page, (unsigned long)zspage);
>>> +		page->freelist = NULL;
>>> 		if (i == 0) {
>>> 			zspage->first_page = page;
>>> 			SetPagePrivate(page);
>>> +			if (unlikely(class->objs_per_zspage == 1 &&
>>> +					class->pages_per_zspage == 1))
>>> +				SetPageHugeObject(page);
>>> 		} else {
>>> -			prev_page->next = page;
>>> +			prev_page->freelist = page;
>>> 		}
>>> -		if (i == nr_pages - 1) {
>>> +		if (i == nr_pages - 1)
>>> 			SetPagePrivate2(page);
>>> -			page->next = NULL;
>>> -		}
>>> 		prev_page = page;
>>> 	}
>>> }
>>> @@ -999,6 +1141,8 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
>>> 		return NULL;
>>>
>>> 	memset(zspage, 0, sizeof(struct zspage));
>>> +	zspage->magic = ZSPAGE_MAGIC;
>>> +	migrate_lock_init(zspage);
>>>
>>> 	for (i = 0; i < class->pages_per_zspage; i++) {
>>> 		struct page *page;
>>> @@ -1013,7 +1157,7 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
>>> 		pages[i] = page;
>>> 	}
>>>
>>> -	create_page_chain(zspage, pages, class->pages_per_zspage);
>>> +	create_page_chain(class, zspage, pages);
>>> 	init_zspage(class, zspage);
>>>
>>> 	return zspage;
>>> @@ -1024,7 +1168,7 @@ static struct zspage *find_get_zspage(struct size_class *class)
>>> 	int i;
>>> 	struct zspage *zspage;
>>>
>>> -	for (i = ZS_ALMOST_FULL; i <= ZS_ALMOST_EMPTY; i++) {
>>> +	for (i = ZS_ALMOST_FULL; i >= ZS_EMPTY; i--) {
>>> 		zspage = list_first_entry_or_null(&class->fullness_list[i],
>>> 				struct zspage, list);
>>> 		if (zspage)
>>> @@ -1289,6 +1433,10 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
>>> 	obj = handle_to_obj(handle);
>>> 	obj_to_location(obj, &page, &obj_idx);
>>> 	zspage = get_zspage(page);
>>> +
>>> +	/* migration cannot move any subpage in this zspage */
>>> +	migrate_read_lock(zspage);
>>> +
>>> 	get_zspage_mapping(zspage, &class_idx, &fg);
>>> 	class = pool->size_class[class_idx];
>>> 	off = (class->size * obj_idx) & ~PAGE_MASK;
>>> @@ -1309,7 +1457,7 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
>>>
>>> 	ret = __zs_map_object(area, pages, off, class->size);
>>> out:
>>> -	if (!class->huge)
>>> +	if (likely(!PageHugeObject(page)))
>>> 		ret += ZS_HANDLE_SIZE;
>>>
>>> 	return ret;
>>> @@ -1348,6 +1496,8 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
>>> 		__zs_unmap_object(area, pages, off, class->size);
>>> 	}
>>> 	put_cpu_var(zs_map_area);
>>> +
>>> +	migrate_read_unlock(zspage);
>>> 	unpin_tag(handle);
>>> }
>>> EXPORT_SYMBOL_GPL(zs_unmap_object);
>>> @@ -1377,7 +1527,7 @@ static unsigned long obj_malloc(struct size_class *class,
>>> 	vaddr = kmap_atomic(m_page);
>>> 	link = (struct link_free *)vaddr + m_offset / sizeof(*link);
>>> 	set_freeobj(zspage, link->next >> OBJ_ALLOCATED_TAG);
>>> -	if (!class->huge)
>>> +	if (likely(!PageHugeObject(m_page)))
>>> 		/* record handle in the header of allocated chunk */
>>> 		link->handle = handle;
>>> 	else
>>> @@ -1407,6 +1557,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
>>> {
>>> 	unsigned long handle, obj;
>>> 	struct size_class *class;
>>> +	enum fullness_group newfg;
>>> 	struct zspage *zspage;
>>>
>>> 	if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))
>>> @@ -1422,28 +1573,37 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
>>>
>>> 	spin_lock(&class->lock);
>>> 	zspage = find_get_zspage(class);
>>> -
>>> -	if (!zspage) {
>>> +	if (likely(zspage)) {
>>> +		obj = obj_malloc(class, zspage, handle);
>>> +		/* Now move the zspage to another fullness group, if required */
>>> +		fix_fullness_group(class, zspage);
>>> +		record_obj(handle, obj);
>>> 		spin_unlock(&class->lock);
>>> -		zspage = alloc_zspage(pool, class, gfp);
>>> -		if (unlikely(!zspage)) {
>>> -			cache_free_handle(pool, handle);
>>> -			return 0;
>>> -		}
>>>
>>> -		set_zspage_mapping(zspage, class->index, ZS_EMPTY);
>>> -		atomic_long_add(class->pages_per_zspage,
>>> -					&pool->pages_allocated);
>>> +		return handle;
>>> +	}
>>>
>>> -		spin_lock(&class->lock);
>>> -		zs_stat_inc(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
>>> -				class->size, class->pages_per_zspage));
>>> +	spin_unlock(&class->lock);
>>> +
>>> +	zspage = alloc_zspage(pool, class, gfp);
>>> +	if (!zspage) {
>>> +		cache_free_handle(pool, handle);
>>> +		return 0;
>>> 	}
>>>
>>> +	spin_lock(&class->lock);
>>> 	obj = obj_malloc(class, zspage, handle);
>>> -	/* Now move the zspage to another fullness group, if required */
>>> -	fix_fullness_group(class, zspage);
>>> +	newfg = get_fullness_group(class, zspage);
>>> +	insert_zspage(class, zspage, newfg);
>>> +	set_zspage_mapping(zspage, class->index, newfg);
>>> 	record_obj(handle, obj);
>>> +	atomic_long_add(class->pages_per_zspage,
>>> +				&pool->pages_allocated);
>>> +	zs_stat_inc(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
>>> +			class->size, class->pages_per_zspage));
>>> +
>>> +	/* We completely set up zspage so mark them as movable */
>>> +	SetZsPageMovable(pool, zspage);
>>> 	spin_unlock(&class->lock);
>>>
>>> 	return handle;
>>> @@ -1484,6 +1644,7 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
>>> 	int class_idx;
>>> 	struct size_class *class;
>>> 	enum fullness_group fullness;
>>> +	bool isolated;
>>>
>>> 	if (unlikely(!handle))
>>> 		return;
>>> @@ -1493,22 +1654,28 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
>>> 	obj_to_location(obj, &f_page, &f_objidx);
>>> 	zspage = get_zspage(f_page);
>>>
>>> +	migrate_read_lock(zspage);
>>> +
>>> 	get_zspage_mapping(zspage, &class_idx, &fullness);
>>> 	class = pool->size_class[class_idx];
>>>
>>> 	spin_lock(&class->lock);
>>> 	obj_free(class, obj);
>>> 	fullness = fix_fullness_group(class, zspage);
>>> -	if (fullness == ZS_EMPTY) {
>>> -		zs_stat_dec(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
>>> -				class->size, class->pages_per_zspage));
>>> -		atomic_long_sub(class->pages_per_zspage,
>>> -				&pool->pages_allocated);
>>> -		free_zspage(pool, zspage);
>>> +	if (fullness != ZS_EMPTY) {
>>> +		migrate_read_unlock(zspage);
>>> +		goto out;
>>> 	}
>>> +
>>> +	isolated = is_zspage_isolated(zspage);
>>> +	migrate_read_unlock(zspage);
>>> +	/* If zspage is isolated, zs_page_putback will free the zspage */
>>> +	if (likely(!isolated))
>>> +		free_zspage(pool, class, zspage);
>>> +out:
>>> +
>>> 	spin_unlock(&class->lock);
>>> 	unpin_tag(handle);
>>> -
>>> 	cache_free_handle(pool, handle);
>>> }
>>> EXPORT_SYMBOL_GPL(zs_free);
>>> @@ -1587,12 +1754,13 @@ static unsigned long find_alloced_obj(struct size_class *class,
>>> 	int offset = 0;
>>> 	unsigned long handle = 0;
>>> 	void *addr = kmap_atomic(page);
>>> +	struct zspage *zspage = get_zspage(page);
>>>
>>> -	offset = get_first_obj_offset(page);
>>> +	offset = get_first_obj_offset(class, get_first_page(zspage), page);
>>> 	offset += class->size * index;
>>>
>>> 	while (offset < PAGE_SIZE) {
>>> -		head = obj_to_head(class, page, addr + offset);
>>> +		head = obj_to_head(page, addr + offset);
>>> 		if (head & OBJ_ALLOCATED_TAG) {
>>> 			handle = head & ~OBJ_ALLOCATED_TAG;
>>> 			if (trypin_tag(handle))
>>> @@ -1684,6 +1852,7 @@ static struct zspage *isolate_zspage(struct size_class *class, bool source)
>>> 		zspage = list_first_entry_or_null(&class->fullness_list[fg[i]],
>>> 							struct zspage, list);
>>> 		if (zspage) {
>>> +			VM_BUG_ON(is_zspage_isolated(zspage));
>>> 			remove_zspage(class, zspage, fg[i]);
>>> 			return zspage;
>>> 		}
>>> @@ -1704,6 +1873,8 @@ static enum fullness_group putback_zspage(struct size_class *class,
>>> {
>>> 	enum fullness_group fullness;
>>>
>>> +	VM_BUG_ON(is_zspage_isolated(zspage));
>>> +
>>> 	fullness = get_fullness_group(class, zspage);
>>> 	insert_zspage(class, zspage, fullness);
>>> 	set_zspage_mapping(zspage, class->index, fullness);
>>> @@ -1711,6 +1882,377 @@ static enum fullness_group putback_zspage(struct size_class *class,
>>> 	return fullness;
>>> }
>>>
>>> +#ifdef CONFIG_COMPACTION
>>> +static struct dentry *zs_mount(struct file_system_type *fs_type,
>>> +				int flags, const char *dev_name, void *data)
>>> +{
>>> +	static const struct dentry_operations ops = {
>>> +		.d_dname = simple_dname,
>>> +	};
>>> +
>>> +	return mount_pseudo(fs_type, "zsmalloc:", NULL, &ops, ZSMALLOC_MAGIC);
>>> +}
>>> +
>>> +static struct file_system_type zsmalloc_fs = {
>>> +	.name		= "zsmalloc",
>>> +	.mount		= zs_mount,
>>> +	.kill_sb	= kill_anon_super,
>>> +};
>>> +
>>> +static int zsmalloc_mount(void)
>>> +{
>>> +	int ret = 0;
>>> +
>>> +	zsmalloc_mnt = kern_mount(&zsmalloc_fs);
>>> +	if (IS_ERR(zsmalloc_mnt))
>>> +		ret = PTR_ERR(zsmalloc_mnt);
>>> +
>>> +	return ret;
>>> +}
>>> +
>>> +static void zsmalloc_unmount(void)
>>> +{
>>> +	kern_unmount(zsmalloc_mnt);
>>> +}
>>> +
>>> +static void migrate_lock_init(struct zspage *zspage)
>>> +{
>>> +	rwlock_init(&zspage->lock);
>>> +}
>>> +
>>> +static void migrate_read_lock(struct zspage *zspage)
>>> +{
>>> +	read_lock(&zspage->lock);
>>> +}
>>> +
>>> +static void migrate_read_unlock(struct zspage *zspage)
>>> +{
>>> +	read_unlock(&zspage->lock);
>>> +}
>>> +
>>> +static void migrate_write_lock(struct zspage *zspage)
>>> +{
>>> +	write_lock(&zspage->lock);
>>> +}
>>> +
>>> +static void migrate_write_unlock(struct zspage *zspage)
>>> +{
>>> +	write_unlock(&zspage->lock);
>>> +}
>>> +
>>> +/* Number of isolated subpage for *page migration* in this zspage */
>>> +static void inc_zspage_isolation(struct zspage *zspage)
>>> +{
>>> +	zspage->isolated++;
>>> +}
>>> +
>>> +static void dec_zspage_isolation(struct zspage *zspage)
>>> +{
>>> +	zspage->isolated--;
>>> +}
>>> +
>>> +static void replace_sub_page(struct size_class *class, struct zspage *zspage,
>>> +				struct page *newpage, struct page *oldpage)
>>> +{
>>> +	struct page *page;
>>> +	struct page *pages[ZS_MAX_PAGES_PER_ZSPAGE] = {NULL, };
>>> +	int idx = 0;
>>> +
>>> +	page = get_first_page(zspage);
>>> +	do {
>>> +		if (page == oldpage)
>>> +			pages[idx] = newpage;
>>> +		else
>>> +			pages[idx] = page;
>>> +		idx++;
>>> +	} while ((page = get_next_page(page)) != NULL);
>>> +
>>> +	create_page_chain(class, zspage, pages);
>>> +	if (unlikely(PageHugeObject(oldpage)))
>>> +		newpage->index = oldpage->index;
>>> +	__SetPageMovable(newpage, page_mapping(oldpage));
>>> +}
>>> +
>>> +bool zs_page_isolate(struct page *page, isolate_mode_t mode)
>>> +{
>>> +	struct zs_pool *pool;
>>> +	struct size_class *class;
>>> +	int class_idx;
>>> +	enum fullness_group fullness;
>>> +	struct zspage *zspage;
>>> +	struct address_space *mapping;
>>> +
>>> +	/*
>>> +	 * Page is locked so zspage couldn't be destroyed. For detail, look at
>>> +	 * lock_zspage in free_zspage.
>>> +	 */
>>> +	VM_BUG_ON_PAGE(!PageMovable(page), page);
>>> +	VM_BUG_ON_PAGE(PageIsolated(page), page);
>>> +
>>> +	zspage = get_zspage(page);
>>> +
>>> +	/*
>>> +	 * Without class lock, fullness could be stale while class_idx is okay
>>> +	 * because class_idx is constant unless page is freed so we should get
>>> +	 * fullness again under class lock.
>>> +	 */
>>> +	get_zspage_mapping(zspage, &class_idx, &fullness);
>>> +	mapping = page_mapping(page);
>>> +	pool = mapping->private_data;
>>> +	class = pool->size_class[class_idx];
>>> +
>>> +	spin_lock(&class->lock);
>>> +	if (get_zspage_inuse(zspage) == 0) {
>>> +		spin_unlock(&class->lock);
>>> +		return false;
>>> +	}
>>> +
>>> +	/* zspage is isolated for object migration */
>>> +	if (list_empty(&zspage->list) && !is_zspage_isolated(zspage)) {
>>> +		spin_unlock(&class->lock);
>>> +		return false;
>>> +	}
>>> +
>>> +	/*
>>> +	 * If this is first time isolation for the zspage, isolate zspage from
>>> +	 * size_class to prevent further object allocation from the zspage.
>>> +	 */
>>> +	if (!list_empty(&zspage->list) && !is_zspage_isolated(zspage)) {
>>> +		get_zspage_mapping(zspage, &class_idx, &fullness);
>>> +		remove_zspage(class, zspage, fullness);
>>> +	}
>>> +
>>> +	inc_zspage_isolation(zspage);
>>> +	spin_unlock(&class->lock);
>>> +
>>> +	return true;
>>> +}
>>> +
>>> +int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>>> +		struct page *page, enum migrate_mode mode)
>>> +{
>>> +	struct zs_pool *pool;
>>> +	struct size_class *class;
>>> +	int class_idx;
>>> +	enum fullness_group fullness;
>>> +	struct zspage *zspage;
>>> +	struct page *dummy;
>>> +	void *s_addr, *d_addr, *addr;
>>> +	int offset, pos;
>>> +	unsigned long handle, head;
>>> +	unsigned long old_obj, new_obj;
>>> +	unsigned int obj_idx;
>>> +	int ret = -EAGAIN;
>>> +
>>> +	VM_BUG_ON_PAGE(!PageMovable(page), page);
>>> +	VM_BUG_ON_PAGE(!PageIsolated(page), page);
>>> +
>>> +	zspage = get_zspage(page);
>>> +
>>> +	/* Concurrent compactor cannot migrate any subpage in zspage */
>>> +	migrate_write_lock(zspage);
>>> +	get_zspage_mapping(zspage, &class_idx, &fullness);
>>> +	pool = mapping->private_data;
>>> +	class = pool->size_class[class_idx];
>>> +	offset = get_first_obj_offset(class, get_first_page(zspage), page);
>>> +
>>> +	spin_lock(&class->lock);
>>> +	if (!get_zspage_inuse(zspage)) {
>>> +		ret = -EBUSY;
>>> +		goto unlock_class;
>>> +	}
>>> +
>>> +	pos = offset;
>>> +	s_addr = kmap_atomic(page);
>>> +	while (pos < PAGE_SIZE) {
>>> +		head = obj_to_head(page, s_addr + pos);
>>> +		if (head & OBJ_ALLOCATED_TAG) {
>>> +			handle = head & ~OBJ_ALLOCATED_TAG;
>>> +			if (!trypin_tag(handle))
>>> +				goto unpin_objects;
>>> +		}
>>> +		pos += class->size;
>>> +	}
>>> +
>>> +	/*
>>> +	 * Here, any user cannot access all objects in the zspage so let's move.
>>> +	 */
>>> +	d_addr = kmap_atomic(newpage);
>>> +	memcpy(d_addr, s_addr, PAGE_SIZE);
>>> +	kunmap_atomic(d_addr);
>>> +
>>> +	for (addr = s_addr + offset; addr < s_addr + pos;
>>> +					addr += class->size) {
>>> +		head = obj_to_head(page, addr);
>>> +		if (head & OBJ_ALLOCATED_TAG) {
>>> +			handle = head & ~OBJ_ALLOCATED_TAG;
>>> +			if (!testpin_tag(handle))
>>> +				BUG();
>>> +
>>> +			old_obj = handle_to_obj(handle);
>>> +			obj_to_location(old_obj, &dummy, &obj_idx);
>>> +			new_obj = (unsigned long)location_to_obj(newpage,
>>> +								obj_idx);
>>> +			new_obj |= BIT(HANDLE_PIN_BIT);
>>> +			record_obj(handle, new_obj);
>>> +		}
>>> +	}
>>> +
>>> +	replace_sub_page(class, zspage, newpage, page);
>>> +	get_page(newpage);
>>> +
>>> +	dec_zspage_isolation(zspage);
>>> +
>>> +	/*
>>> +	 * Page migration is done so let's putback isolated zspage to
>>> +	 * the list if @page is final isolated subpage in the zspage.
>>> +	 */
>>> +	if (!is_zspage_isolated(zspage))
>>> +		putback_zspage(class, zspage);
>>> +
>>> +	reset_page(page);
>>> +	put_page(page);
>>> +	page = newpage;
>>> +
>>> +	ret = 0;
>>> +unpin_objects:
>>> +	for (addr = s_addr + offset; addr < s_addr + pos;
>>> +						addr += class->size) {
>>> +		head = obj_to_head(page, addr);
>>> +		if (head & OBJ_ALLOCATED_TAG) {
>>> +			handle = head & ~OBJ_ALLOCATED_TAG;
>>> +			if (!testpin_tag(handle))
>>> +				BUG();
>>> +			unpin_tag(handle);
>>> +		}
>>> +	}
>>> +	kunmap_atomic(s_addr);
>>> +unlock_class:
>>> +	spin_unlock(&class->lock);
>>> +	migrate_write_unlock(zspage);
>>> +
>>> +	return ret;
>>> +}
>>> +
>>> +void zs_page_putback(struct page *page)
>>> +{
>>> +	struct zs_pool *pool;
>>> +	struct size_class *class;
>>> +	int class_idx;
>>> +	enum fullness_group fg;
>>> +	struct address_space *mapping;
>>> +	struct zspage *zspage;
>>> +
>>> +	VM_BUG_ON_PAGE(!PageMovable(page), page);
>>> +	VM_BUG_ON_PAGE(!PageIsolated(page), page);
>>> +
>>> +	zspage = get_zspage(page);
>>> +	get_zspage_mapping(zspage, &class_idx, &fg);
>>> +	mapping = page_mapping(page);
>>> +	pool = mapping->private_data;
>>> +	class = pool->size_class[class_idx];
>>> +
>>> +	spin_lock(&class->lock);
>>> +	dec_zspage_isolation(zspage);
>>> +	if (!is_zspage_isolated(zspage)) {
>>> +		fg = putback_zspage(class, zspage);
>>> +		/*
>>> +		 * Due to page_lock, we cannot free zspage immediately
>>> +		 * so let's defer.
>>> +		 */
>>> +		if (fg == ZS_EMPTY)
>>> +			schedule_work(&pool->free_work);
>>> +	}
>>> +	spin_unlock(&class->lock);
>>> +}
>>> +
>>> +const struct address_space_operations zsmalloc_aops = {
>>> +	.isolate_page = zs_page_isolate,
>>> +	.migratepage = zs_page_migrate,
>>> +	.putback_page = zs_page_putback,
>>> +};
>>> +
>>> +static int zs_register_migration(struct zs_pool *pool)
>>> +{
>>> +	pool->inode = alloc_anon_inode(zsmalloc_mnt->mnt_sb);
>>> +	if (IS_ERR(pool->inode)) {
>>> +		pool->inode = NULL;
>>> +		return 1;
>>> +	}
>>> +
>>> +	pool->inode->i_mapping->private_data = pool;
>>> +	pool->inode->i_mapping->a_ops = &zsmalloc_aops;
>>> +	return 0;
>>> +}
>>> +
>>> +static void zs_unregister_migration(struct zs_pool *pool)
>>> +{
>>> +	flush_work(&pool->free_work);
>>> +	if (pool->inode)
>>> +		iput(pool->inode);
>>> +}
>>> +
>>> +/*
>>> + * Caller should hold page_lock of all pages in the zspage
>>> + * In here, we cannot use zspage meta data.
>>> + */
>>> +static void async_free_zspage(struct work_struct *work)
>>> +{
>>> +	int i;
>>> +	struct size_class *class;
>>> +	unsigned int class_idx;
>>> +	enum fullness_group fullness;
>>> +	struct zspage *zspage, *tmp;
>>> +	LIST_HEAD(free_pages);
>>> +	struct zs_pool *pool = container_of(work, struct zs_pool,
>>> +					free_work);
>>> +
>>> +	for (i = 0; i < zs_size_classes; i++) {
>>> +		class = pool->size_class[i];
>>> +		if (class->index != i)
>>> +			continue;
>>> +
>>> +		spin_lock(&class->lock);
>>> +		list_splice_init(&class->fullness_list[ZS_EMPTY], &free_pages);
>>> +		spin_unlock(&class->lock);
>>> +	}
>>> +
>>> +
>>> +	list_for_each_entry_safe(zspage, tmp, &free_pages, list) {
>>> +		list_del(&zspage->list);
>>> +		lock_zspage(zspage);
>>> +
>>> +		get_zspage_mapping(zspage, &class_idx, &fullness);
>>> +		VM_BUG_ON(fullness != ZS_EMPTY);
>>> +		class = pool->size_class[class_idx];
>>> +		spin_lock(&class->lock);
>>> +		__free_zspage(pool, pool->size_class[class_idx], zspage);
>>> +		spin_unlock(&class->lock);
>>> +	}
>>> +};
>>> +
>>> +static void kick_deferred_free(struct zs_pool *pool)
>>> +{
>>> +	schedule_work(&pool->free_work);
>>> +}
>>> +
>>> +static void init_deferred_free(struct zs_pool *pool)
>>> +{
>>> +	INIT_WORK(&pool->free_work, async_free_zspage);
>>> +}
>>> +
>>> +static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage)
>>> +{
>>> +	struct page *page = get_first_page(zspage);
>>> +
>>> +	do {
>>> +		WARN_ON(!trylock_page(page));
>>> +		__SetPageMovable(page, pool->inode->i_mapping);
>>> +		unlock_page(page);
>>> +	} while ((page = get_next_page(page)) != NULL);
>>> +}
>>> +#endif
>>> +
>>> /*
>>>  *
>>>  * Based on the number of unused allocated objects calculate
>>> @@ -1745,10 +2287,10 @@ static void __zs_compact(struct zs_pool *pool, struct size_class *class)
>>> 			break;
>>>
>>> 		cc.index = 0;
>>> -		cc.s_page = src_zspage->first_page;
>>> +		cc.s_page = get_first_page(src_zspage);
>>>
>>> 		while ((dst_zspage = isolate_zspage(class, false))) {
>>> -			cc.d_page = dst_zspage->first_page;
>>> +			cc.d_page = get_first_page(dst_zspage);
>>> 			/*
>>> 			 * If there is no more space in dst_page, resched
>>> 			 * and see if anyone had allocated another zspage.
>>> @@ -1765,11 +2307,7 @@ static void __zs_compact(struct zs_pool *pool, struct size_class *class)
>>>
>>> 		putback_zspage(class, dst_zspage);
>>> 		if (putback_zspage(class, src_zspage) == ZS_EMPTY) {
>>> -			zs_stat_dec(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
>>> -					class->size, class->pages_per_zspage));
>>> -			atomic_long_sub(class->pages_per_zspage,
>>> -					&pool->pages_allocated);
>>> -			free_zspage(pool, src_zspage);
>>> +			free_zspage(pool, class, src_zspage);
>>> 			pool->stats.pages_compacted += class->pages_per_zspage;
>>> 		}
>>> 		spin_unlock(&class->lock);
>>> @@ -1885,6 +2423,7 @@ struct zs_pool *zs_create_pool(const char *name)
>>> 	if (!pool)
>>> 		return NULL;
>>>
>>> +	init_deferred_free(pool);
>>> 	pool->size_class = kcalloc(zs_size_classes, sizeof(struct size_class *),
>>> 			GFP_KERNEL);
>>> 	if (!pool->size_class) {
>>> @@ -1939,12 +2478,10 @@ struct zs_pool *zs_create_pool(const char *name)
>>> 		class->pages_per_zspage = pages_per_zspage;
>>> 		class->objs_per_zspage = class->pages_per_zspage *
>>> 						PAGE_SIZE / class->size;
>>> -		if (pages_per_zspage == 1 && class->objs_per_zspage == 1)
>>> -			class->huge = true;
>>> 		spin_lock_init(&class->lock);
>>> 		pool->size_class[i] = class;
>>> -		for (fullness = ZS_ALMOST_FULL; fullness <= ZS_ALMOST_EMPTY;
>>> -								fullness++)
>>> +		for (fullness = ZS_EMPTY; fullness < NR_ZS_FULLNESS;
>>> +							fullness++)
>>> 			INIT_LIST_HEAD(&class->fullness_list[fullness]);
>>>
>>> 		prev_class = class;
>>> @@ -1953,6 +2490,9 @@ struct zs_pool *zs_create_pool(const char *name)
>>> 	/* debug only, don't abort if it fails */
>>> 	zs_pool_stat_create(pool, name);
>>>
>>> +	if (zs_register_migration(pool))
>>> +		goto err;
>>> +
>>> 	/*
>>> 	 * Not critical, we still can use the pool
>>> 	 * and user can trigger compaction manually.
>>> @@ -1972,6 +2512,7 @@ void zs_destroy_pool(struct zs_pool *pool)
>>> 	int i;
>>>
>>> 	zs_unregister_shrinker(pool);
>>> +	zs_unregister_migration(pool);
>>> 	zs_pool_stat_destroy(pool);
>>>
>>> 	for (i = 0; i < zs_size_classes; i++) {
>>> @@ -1984,7 +2525,7 @@ void zs_destroy_pool(struct zs_pool *pool)
>>> 		if (class->index != i)
>>> 			continue;
>>>
>>> -		for (fg = ZS_ALMOST_FULL; fg <= ZS_ALMOST_EMPTY; fg++) {
>>> +		for (fg = ZS_EMPTY; fg < NR_ZS_FULLNESS; fg++) {
>>> 			if (!list_empty(&class->fullness_list[fg])) {
>>> 				pr_info("Freeing non-empty class with size %db, fullness group %d\n",
>>> 					class->size, fg);
>>> @@ -2002,7 +2543,13 @@ EXPORT_SYMBOL_GPL(zs_destroy_pool);
>>>
>>> static int __init zs_init(void)
>>> {
>>> -	int ret = zs_register_cpu_notifier();
>>> +	int ret;
>>> +
>>> +	ret = zsmalloc_mount();
>>> +	if (ret)
>>> +		goto out;
>>> +
>>> +	ret = zs_register_cpu_notifier();
>>>
>>> 	if (ret)
>>> 		goto notifier_fail;
>>> @@ -2019,7 +2566,8 @@ static int __init zs_init(void)
>>>
>>> notifier_fail:
>>> 	zs_unregister_cpu_notifier();
>>> -
>>> +	zsmalloc_unmount();
>>> +out:
>>> 	return ret;
>>> }
>>>
>>> @@ -2028,6 +2576,7 @@ static void __exit zs_exit(void)
>>> #ifdef CONFIG_ZPOOL
>>> 	zpool_unregister_driver(&zs_zpool_driver);
>>> #endif
>>> +	zsmalloc_unmount();
>>> 	zs_unregister_cpu_notifier();
>>>
>>> 	zs_stat_exit();
>>>
>>
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