Re: [PATCH v4 11/12] zsmalloc: page migration support

[Chulmin Kim]

Hello, Minchan!

On 2016ë 04ì 27ì 16:48, 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 for own purpose. For that, firstly, we can get first
> object offset of the page via runtime calculation instead of
> page->index so we can use page->index as link for page chaining.
> In case of huge object, it stores handle rather than page chaining.
> To identify huge object, we uses PG_owner_priv_1 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 if it's first
> isolation on subpages of zspage. Thus, further isolation on other
> subpages cannot isolate zspage from class list.
>
> * 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 so go crash.
>
> If it locks all of objects successfully, it copies content from old page
> create new one, finally, create new page chain with new page.
> If it's last isolated page 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.
>
> Lastly, this patch introduces asynchronous zspage free. The reason
> we 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
> with preemption disabled. If it got page_lock of all of pages
> successfully, it can free zspage in the context. Otherwise, it queues
> the free request and free zspage via workqueue in process context.
>
> Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>   include/uapi/linux/magic.h |   1 +
>   mm/zsmalloc.c              | 552 +++++++++++++++++++++++++++++++++++++++------
>   2 files changed, 487 insertions(+), 66 deletions(-)
>
> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> index e1fbe72c39c0..93b1affe4801 100644
> --- a/include/uapi/linux/magic.h
> +++ b/include/uapi/linux/magic.h
> @@ -79,5 +79,6 @@
>   #define NSFS_MAGIC		0x6e736673
>   #define BPF_FS_MAGIC		0xcafe4a11
>   #define BALLOON_KVM_MAGIC	0x13661366
> +#define ZSMALLOC_MAGIC		0x58295829
>
>   #endif /* __LINUX_MAGIC_H__ */
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 8d82e44c4644..042793015ecf 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -17,15 +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: 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
>    */
>
>   #include <linux/module.h>
> @@ -47,6 +46,10 @@
>   #include <linux/debugfs.h>
>   #include <linux/zsmalloc.h>
>   #include <linux/zpool.h>
> +#include <linux/mount.h>
> +#include <linux/migrate.h>
> +
> +#define ZSPAGE_MAGIC	0x58
>
>   /*
>    * This must be power of 2 and greater than of equal to sizeof(link_free).
> @@ -128,8 +131,33 @@
>    *  ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
>    *  (reason above)
>    */
> +
> +/*
> + * A zspage's class index and fullness group
> + * are encoded in its (first)page->mapping
> + */
> +#define FULLNESS_BITS	2
> +#define CLASS_BITS	8
> +#define ISOLATED_BITS	3
> +#define MAGIC_VAL_BITS	8
> +
> +
>   #define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> CLASS_BITS)
>
> +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 */
> +	rwlock_t lock;
> +};
> +
>   /*
>    * We do not maintain any list for completely empty or full pages
>    */
> @@ -161,6 +189,8 @@ struct zs_size_stat {
>   static struct dentry *zs_stat_root;
>   #endif
>
> +static struct vfsmount *zsmalloc_mnt;
> +
>   /*
>    * number of size_classes
>    */
> @@ -243,24 +273,10 @@ struct zs_pool {
>   #ifdef CONFIG_ZSMALLOC_STAT
>   	struct dentry *stat_dentry;
>   #endif
> -};
> -
> -/*
> - * A zspage's class index and fullness group
> - * are encoded in its (first)page->mapping
> - */
> -#define FULLNESS_BITS	2
> -#define CLASS_BITS	8
> -
> -struct zspage {
> -	struct {
> -		unsigned int fullness:FULLNESS_BITS;
> -		unsigned int class:CLASS_BITS;
> -	};
> -	unsigned int inuse;
> -	unsigned int freeobj;
> -	struct page *first_page;
> -	struct list_head list; /* fullness list */
> +	struct inode *inode;
> +	spinlock_t free_lock;
> +	struct work_struct free_work;
> +	struct list_head free_zspage;
>   };
>
>   struct mapping_area {
> @@ -312,8 +328,11 @@ static struct zspage *cache_alloc_zspage(struct zs_pool *pool, gfp_t flags)
>   	struct zspage *zspage;
>
>   	zspage = kmem_cache_alloc(pool->zspage_cachep, flags & ~__GFP_HIGHMEM);
> -	if (zspage)
> +	if (zspage) {
>   		memset(zspage, 0, sizeof(struct zspage));
> +		zspage->magic = ZSPAGE_MAGIC;
> +		rwlock_init(&zspage->lock);

+              INIT_LIST_HEAD(&zspage->list);

If there is no special intention here,
I think we need the list initialization.

There are some functions checking "list_empty(&zspage->list)".
and they might be executed before the list initialization in rare cases.
(AFAIK, the list initialization is being done by insert_zspage(),etc.)
I guess, checking the uninitialized list is not intended at all.

Thanks!
Chulmin


> +	}
>   	return zspage;
>   };
>
> @@ -419,11 +438,27 @@ 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 void inc_zspage_isolation(struct zspage *zspage)
> +{
> +	zspage->isolated++;
> +}
> +
> +static void dec_zspage_isolation(struct zspage *zspage)
> +{
> +	zspage->isolated--;
> +}
> +
> +static int get_zspage_isolation(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;
> @@ -439,20 +474,12 @@ 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)
> @@ -469,6 +496,8 @@ 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;
>   }
> @@ -738,6 +767,7 @@ static void remove_zspage(struct size_class *class,
>   		return;
>
>   	VM_BUG_ON(list_empty(&class->fullness_list[fullness]));
> +	VM_BUG_ON(get_zspage_isolation(zspage));
>
>   	list_del_init(&zspage->list);
>   	zs_stat_dec(class, fullness == ZS_ALMOST_EMPTY ?
> @@ -764,8 +794,10 @@ 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 (!get_zspage_isolation(zspage)) {
> +		remove_zspage(class, zspage, currfg);
> +		insert_zspage(class, zspage, newfg);
> +	}
>   	set_zspage_mapping(zspage, class_idx, newfg);
>
>   out:
> @@ -808,19 +840,47 @@ 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 (PageOwnerPriv1(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, bound;
> +	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);
> +	}
> +
> +	bound = PAGE_SIZE * page_idx;
> +	pos = (((class->objs_per_zspage * class->size) *
> +		page_idx / class->pages_per_zspage) / class->size
> +	      ) * class->size;
> +
> +	ofs = (pos + class->size) % PAGE_SIZE;
> +out:
> +	return ofs;
>   }
>
>   /**
> @@ -867,6 +927,11 @@ static unsigned long obj_to_head(struct size_class *class, struct page *page,
>   		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);
> @@ -884,22 +949,60 @@ 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);
> +	ClearPageOwnerPriv1(page);
>   	set_page_private(page, 0);
> -	page->index = 0;
> +	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 zspage *zspage)
>   {
>   	struct page *page, *next;
>
>   	VM_BUG_ON(get_zspage_inuse(zspage));
>
> -	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);
> @@ -907,20 +1010,34 @@ static void free_zspage(struct zs_pool *pool, struct zspage *zspage)
>   	cache_free_zspage(pool, zspage);
>   }
>
> +static void free_zspage(struct zs_pool *pool, struct zspage *zspage)
> +{
> +	VM_BUG_ON(get_zspage_inuse(zspage));
> +
> +	if (!trylock_zspage(zspage)) {
> +		spin_lock(&pool->free_lock);
> +		VM_BUG_ON(!list_empty(&zspage->list));
> +		list_add(&zspage->list, &pool->free_zspage);
> +		spin_unlock(&pool->free_lock);
> +		schedule_work(&pool->free_work);
> +		return;
> +	}
> +
> +	__free_zspage(pool, zspage);
> +}
> +
>   /* Initialize a newly allocated zspage */
>   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 +1069,17 @@ 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
> @@ -970,20 +1088,43 @@ 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 (class->huge)
> +				SetPageOwnerPriv1(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;
>   	}
>   }
>
> +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 (class->huge)
> +		newpage->index = oldpage->index;
> +	__SetPageMovable(newpage, page_mapping(oldpage));
> +}
> +
>   /*
>    * Allocate a zspage for the given size class
>    */
> @@ -1010,7 +1151,7 @@ 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;
> @@ -1286,6 +1427,10 @@ 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 */
> +	read_lock(&zspage->lock);
> +
>   	get_zspage_mapping(zspage, &class_idx, &fg);
>   	class = pool->size_class[class_idx];
>   	off = (class->size * obj_idx) & ~PAGE_MASK;
> @@ -1345,6 +1490,8 @@ 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);
> +
> +	read_unlock(&zspage->lock);
>   	unpin_tag(handle);
>   }
>   EXPORT_SYMBOL_GPL(zs_unmap_object);
> @@ -1421,6 +1568,8 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
>   	zspage = find_get_zspage(class);
>
>   	if (!zspage) {
> +		struct page *page;
> +
>   		spin_unlock(&class->lock);
>   		zspage = alloc_zspage(pool, class);
>   		if (unlikely(!zspage)) {
> @@ -1432,6 +1581,14 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
>   		atomic_long_add(class->pages_per_zspage,
>   					&pool->pages_allocated);
>
> +		/* We completely set up zspage so mark them as movable */
> +		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);
> +
>   		spin_lock(&class->lock);
>   		zs_stat_inc(class, OBJ_ALLOCATED, get_maxobj_per_zspage(
>   				class->size, class->pages_per_zspage));
> @@ -1490,6 +1647,8 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
>   	obj_to_location(obj, &f_page, &f_objidx);
>   	zspage = get_zspage(f_page);
>
> +	read_lock(&zspage->lock);
> +
>   	get_zspage_mapping(zspage, &class_idx, &fullness);
>   	class = pool->size_class[class_idx];
>
> @@ -1501,11 +1660,14 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
>   				class->size, class->pages_per_zspage));
>   		atomic_long_sub(class->pages_per_zspage,
>   				&pool->pages_allocated);
> +		read_unlock(&zspage->lock);
>   		free_zspage(pool, zspage);
> +		spin_unlock(&class->lock);
> +	} else {
> +		read_unlock(&zspage->lock);
> +		spin_unlock(&class->lock);
>   	}
> -	spin_unlock(&class->lock);
>   	unpin_tag(handle);
> -
>   	cache_free_handle(pool, handle);
>   }
>   EXPORT_SYMBOL_GPL(zs_free);
> @@ -1584,8 +1746,9 @@ 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) {
> @@ -1681,6 +1844,7 @@ 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(get_zspage_isolation(zspage));
>   			remove_zspage(class, zspage, fg[i]);
>   			return zspage;
>   		}
> @@ -1701,6 +1865,8 @@ static enum fullness_group putback_zspage(struct size_class *class,
>   {
>   	enum fullness_group fullness;
>
> +	VM_BUG_ON(get_zspage_isolation(zspage));
> +
>   	fullness = get_fullness_group(class, zspage);
>   	insert_zspage(class, zspage, fullness);
>   	set_zspage_mapping(zspage, class->index, fullness);
> @@ -1739,10 +1905,10 @@ 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.
> @@ -1859,6 +2025,218 @@ static int zs_register_shrinker(struct zs_pool *pool)
>   	return register_shrinker(&pool->shrinker);
>   }
>
> +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) && !get_zspage_isolation(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) && !get_zspage_isolation(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 */
> +	write_lock(&zspage->lock);
> +	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))
> +		goto unlock_class;
> +
> +	pos = offset;
> +	s_addr = kmap_atomic(page);
> +	while (pos < PAGE_SIZE) {
> +		head = obj_to_head(class, 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(class, 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 (!get_zspage_isolation(zspage))
> +		putback_zspage(class, zspage);
> +
> +	reset_page(page);
> +	put_page(page);
> +	page = newpage;
> +
> +	ret = MIGRATEPAGE_SUCCESS;
> +unpin_objects:
> +	for (addr = s_addr + offset; addr < s_addr + pos;
> +						addr += class->size) {
> +		head = obj_to_head(class, 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);
> +	write_unlock(&zspage->lock);
> +
> +	return ret;
> +}
> +
> +void zs_page_putback(struct page *page)
> +{
> +	struct zs_pool *pool;
> +	struct size_class *class;
> +	int class_idx;
> +	enum fullness_group dummy;
> +	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, &dummy);
> +	mapping = page_mapping(page);
> +	pool = mapping->private_data;
> +	class = pool->size_class[class_idx];
> +
> +	spin_lock(&class->lock);
> +	dec_zspage_isolation(zspage);
> +	if (!get_zspage_isolation(zspage))
> +		putback_zspage(class, zspage);
> +	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,
> +};
> +
> +/*
> + * 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)
> +{
> +	struct zspage *zspage, *tmp;
> +	LIST_HEAD(free_pages);
> +	struct zs_pool *pool = container_of(work, struct zs_pool,
> +					free_work);
> +
> +	spin_lock(&pool->free_lock);
> +	list_splice_init(&pool->free_zspage, &free_pages);
> +	spin_unlock(&pool->free_lock);
> +
> +	list_for_each_entry_safe(zspage, tmp, &free_pages, list) {
> +		list_del(&zspage->list);
> +		lock_zspage(zspage);
> +		__free_zspage(pool, zspage);
> +	}
> +};
> +
>   /**
>    * zs_create_pool - Creates an allocation pool to work from.
>    * @flags: allocation flags used to allocate pool metadata
> @@ -1879,6 +2257,10 @@ struct zs_pool *zs_create_pool(const char *name, gfp_t flags)
>   	if (!pool)
>   		return NULL;
>
> +	INIT_WORK(&pool->free_work, async_free_zspage);
> +	INIT_LIST_HEAD(&pool->free_zspage);
> +	spin_lock_init(&pool->free_lock);
> +
>   	pool->size_class = kcalloc(zs_size_classes, sizeof(struct size_class *),
>   			GFP_KERNEL);
>   	if (!pool->size_class) {
> @@ -1944,11 +2326,21 @@ struct zs_pool *zs_create_pool(const char *name, gfp_t flags)
>   		prev_class = class;
>   	}
>
> +	INIT_LIST_HEAD(&pool->free_zspage);
>   	pool->flags = flags;
>
>   	if (zs_pool_stat_create(pool, name))
>   		goto err;
>
> +	pool->inode = alloc_anon_inode(zsmalloc_mnt->mnt_sb);
> +	if (IS_ERR(pool->inode)) {
> +		pool->inode = NULL;
> +		goto err;
> +	}
> +
> +	pool->inode->i_mapping->a_ops = &zsmalloc_aops;
> +	pool->inode->i_mapping->private_data = pool;
> +
>   	/*
>   	 * Not critical, we still can use the pool
>   	 * and user can trigger compaction manually.
> @@ -1967,7 +2359,11 @@ void zs_destroy_pool(struct zs_pool *pool)
>   {
>   	int i;
>
> +	flush_work(&pool->free_work);
> +
>   	zs_unregister_shrinker(pool);
> +	if (pool->inode)
> +		iput(pool->inode);
>   	zs_pool_stat_destroy(pool);
>
>   	for (i = 0; i < zs_size_classes; i++) {
> @@ -1996,10 +2392,33 @@ void zs_destroy_pool(struct zs_pool *pool)
>   }
>   EXPORT_SYMBOL_GPL(zs_destroy_pool);
>
> +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 __init zs_init(void)
>   {
> -	int ret = zs_register_cpu_notifier();
> +	int ret;
>
> +	zsmalloc_mnt = kern_mount(&zsmalloc_fs);
> +	if (IS_ERR(zsmalloc_mnt)) {
> +		ret = PTR_ERR(zsmalloc_mnt);
> +		goto out;
> +	}
> +
> +	ret = zs_register_cpu_notifier();
>   	if (ret)
>   		goto notifier_fail;
>
> @@ -2022,7 +2441,8 @@ static int __init zs_init(void)
>   #endif
>   notifier_fail:
>   	zs_unregister_cpu_notifier();
> -
> +	kern_unmount(zsmalloc_mnt);
> +out:
>   	return ret;
>   }