Message-id: <572801FA.4090304@samsung.com>
Date:	Tue, 03 May 2016 10:42:18 +0900
From:	Chulmin Kim <cmlaika.kim@...sung.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH v4 11/12] zsmalloc: page migration support

On 2016년 05월 03일 09:43, Minchan Kim wrote:
> Good morning, Chulmin
>
> On Tue, May 03, 2016 at 08:33:16AM +0900, Chulmin Kim wrote:
>> Hello, Minchan!
>>
>> On 2016년 04월 27일 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@...il.com>
>>> Signed-off-by: Minchan Kim <minchan@...nel.org>
>>> ---
>>>   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.
>
> Intention was that I just watned to add unncessary instruction there
> although it was not expensive. :)
>
>>
>> There are some functions checking "list_empty(&zspage->list)".
>> and they might be executed before the list initialization in rare cases.
>
> There are two places now.
>
> 1. zspage_isolate
>
> It's okay because zs_page_isolate checks get_zspage_inuse under
> class->lock while alloc_zspage adds newly created zspage to list
> under class->lock with increasing used object count.
>
> 2. free_zspage
>
> It's okay because every zspage passed free_zspage should
> remove from list and remove_zspage has list_del_init and
> the used object in the zspage should be zero so zs_page_isolate
> cannot pick it up.
>
>> (AFAIK, the list initialization is being done by insert_zspage(),etc.)
>> I guess, checking the uninitialized list is not intended at all.
>
> You have been great to spot something until now so you are saying
> with some clue already and I might miss something. :)
>
> Do you have another scenario to make race problem?
> Otherwise, I want to remain as it is because I want to reveal the
> problem rather than hiding problems with safe-guard. :)

A weak clue, yes.
my team saw the problem cases with zspage->list filled with 0.

I just found a scenario.

In case of huge page,
the list initialization in remove_zspage() and insert_zspage()
will not work as one zs_malloc will make the zspage to ZS_FULL.

I guess this is the cause of the problem I saw.


>
> Thanks for the review, Chulmin!
>>
>> 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;
>>>   }
>>>
>>>
>>
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