diff mbox series

[v3,1/4] mm/swapfile: use percpu_ref to serialize against concurrent swapoff

Message ID 20210420133048.6773-2-linmiaohe@huawei.com (mailing list archive)
State New
Headers show
Series close various race windows for swap | expand

Commit Message

Miaohe Lin April 20, 2021, 1:30 p.m. UTC
Using current get/put_swap_device() to guard against concurrent swapoff
for some swap ops, e.g. swap_readpage(), looks terrible because they
might take really long time. This patch adds the percpu_ref support to
serialize against concurrent swapoff. Also we remove the SWP_VALID flag
because it's used together with RCU solution.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 include/linux/swap.h |  5 +--
 mm/swapfile.c        | 79 +++++++++++++++++++++++++++-----------------
 2 files changed, 52 insertions(+), 32 deletions(-)

Comments

Huang, Ying April 21, 2021, 1:05 a.m. UTC | #1
Miaohe Lin <linmiaohe@huawei.com> writes:

> Using current get/put_swap_device() to guard against concurrent swapoff
> for some swap ops, e.g. swap_readpage(), looks terrible because they
> might take really long time. This patch adds the percpu_ref support to
> serialize against concurrent swapoff. Also we remove the SWP_VALID flag
> because it's used together with RCU solution.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  include/linux/swap.h |  5 +--
>  mm/swapfile.c        | 79 +++++++++++++++++++++++++++-----------------
>  2 files changed, 52 insertions(+), 32 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 144727041e78..c9e7fea10b83 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -177,7 +177,6 @@ enum {
>  	SWP_PAGE_DISCARD = (1 << 10),	/* freed swap page-cluster discards */
>  	SWP_STABLE_WRITES = (1 << 11),	/* no overwrite PG_writeback pages */
>  	SWP_SYNCHRONOUS_IO = (1 << 12),	/* synchronous IO is efficient */
> -	SWP_VALID	= (1 << 13),	/* swap is valid to be operated on? */
>  					/* add others here before... */
>  	SWP_SCANNING	= (1 << 14),	/* refcount in scan_swap_map */
>  };
> @@ -240,6 +239,7 @@ struct swap_cluster_list {
>   * The in-memory structure used to track swap areas.
>   */
>  struct swap_info_struct {
> +	struct percpu_ref users;	/* indicate and keep swap device valid. */
>  	unsigned long	flags;		/* SWP_USED etc: see above */
>  	signed short	prio;		/* swap priority of this type */
>  	struct plist_node list;		/* entry in swap_active_head */
> @@ -260,6 +260,7 @@ struct swap_info_struct {
>  	struct block_device *bdev;	/* swap device or bdev of swap file */
>  	struct file *swap_file;		/* seldom referenced */
>  	unsigned int old_block_size;	/* seldom referenced */
> +	struct completion comp;		/* seldom referenced */
>  #ifdef CONFIG_FRONTSWAP
>  	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
>  	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
> @@ -511,7 +512,7 @@ sector_t swap_page_sector(struct page *page);
>  
>  static inline void put_swap_device(struct swap_info_struct *si)
>  {
> -	rcu_read_unlock();
> +	percpu_ref_put(&si->users);
>  }
>  
>  #else /* CONFIG_SWAP */
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 149e77454e3c..a640fc84be5b 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -39,6 +39,7 @@
>  #include <linux/export.h>
>  #include <linux/swap_slots.h>
>  #include <linux/sort.h>
> +#include <linux/completion.h>
>  
>  #include <asm/tlbflush.h>
>  #include <linux/swapops.h>
> @@ -511,6 +512,14 @@ static void swap_discard_work(struct work_struct *work)
>  	spin_unlock(&si->lock);
>  }
>  
> +static void swap_users_ref_free(struct percpu_ref *ref)
> +{
> +	struct swap_info_struct *si;
> +
> +	si = container_of(ref, struct swap_info_struct, users);
> +	complete(&si->comp);
> +}
> +
>  static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
>  {
>  	struct swap_cluster_info *ci = si->cluster_info;
> @@ -1270,18 +1279,12 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
>   * via preventing the swap device from being swapoff, until
>   * put_swap_device() is called.  Otherwise return NULL.
>   *
> - * The entirety of the RCU read critical section must come before the
> - * return from or after the call to synchronize_rcu() in
> - * enable_swap_info() or swapoff().  So if "si->flags & SWP_VALID" is
> - * true, the si->map, si->cluster_info, etc. must be valid in the
> - * critical section.
> - *
>   * Notice that swapoff or swapoff+swapon can still happen before the
> - * rcu_read_lock() in get_swap_device() or after the rcu_read_unlock()
> - * in put_swap_device() if there isn't any other way to prevent
> - * swapoff, such as page lock, page table lock, etc.  The caller must
> - * be prepared for that.  For example, the following situation is
> - * possible.
> + * percpu_ref_tryget_live() in get_swap_device() or after the
> + * percpu_ref_put() in put_swap_device() if there isn't any other way
> + * to prevent swapoff, such as page lock, page table lock, etc.  The
> + * caller must be prepared for that.  For example, the following
> + * situation is possible.
>   *
>   *   CPU1				CPU2
>   *   do_swap_page()
> @@ -1309,21 +1312,27 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
>  	si = swp_swap_info(entry);
>  	if (!si)
>  		goto bad_nofile;
> -
> -	rcu_read_lock();
> -	if (data_race(!(si->flags & SWP_VALID)))
> -		goto unlock_out;
> +	if (!percpu_ref_tryget_live(&si->users))
> +		goto out;
> +	/*
> +	 * Guarantee the si->users are checked before accessing other
> +	 * fields of swap_info_struct.
> +	 *
> +	 * Paired with the spin_unlock() after setup_swap_info() in
> +	 * enable_swap_info().
> +	 */
> +	smp_rmb();
>  	offset = swp_offset(entry);
>  	if (offset >= si->max)
> -		goto unlock_out;
> +		goto put_out;
>  
>  	return si;
>  bad_nofile:
>  	pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
>  out:
>  	return NULL;
> -unlock_out:
> -	rcu_read_unlock();
> +put_out:
> +	percpu_ref_put(&si->users);
>  	return NULL;
>  }
>  
> @@ -2466,7 +2475,7 @@ static void setup_swap_info(struct swap_info_struct *p, int prio,
>  
>  static void _enable_swap_info(struct swap_info_struct *p)
>  {
> -	p->flags |= SWP_WRITEOK | SWP_VALID;
> +	p->flags |= SWP_WRITEOK;
>  	atomic_long_add(p->pages, &nr_swap_pages);
>  	total_swap_pages += p->pages;
>  
> @@ -2497,10 +2506,9 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
>  	spin_unlock(&p->lock);
>  	spin_unlock(&swap_lock);
>  	/*
> -	 * Guarantee swap_map, cluster_info, etc. fields are valid
> -	 * between get/put_swap_device() if SWP_VALID bit is set
> +	 * Finished initialized swap device, now it's safe to reference it.

s/initialized/initializing/

Otherwise looks good to me!  Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

>  	 */
> -	synchronize_rcu();
> +	percpu_ref_resurrect(&p->users);
>  	spin_lock(&swap_lock);
>  	spin_lock(&p->lock);
>  	_enable_swap_info(p);
> @@ -2616,16 +2624,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  
>  	reenable_swap_slots_cache_unlock();
>  
> -	spin_lock(&swap_lock);
> -	spin_lock(&p->lock);
> -	p->flags &= ~SWP_VALID;		/* mark swap device as invalid */
> -	spin_unlock(&p->lock);
> -	spin_unlock(&swap_lock);
>  	/*
> -	 * wait for swap operations protected by get/put_swap_device()
> -	 * to complete
> +	 * Wait for swap operations protected by get/put_swap_device()
> +	 * to complete.
> +	 *
> +	 * We need synchronize_rcu() here to protect the accessing to
> +	 * the swap cache data structure.
>  	 */
> +	percpu_ref_kill(&p->users);
>  	synchronize_rcu();
> +	wait_for_completion(&p->comp);
>  
>  	flush_work(&p->discard_work);
>  
> @@ -2857,6 +2865,12 @@ static struct swap_info_struct *alloc_swap_info(void)
>  	if (!p)
>  		return ERR_PTR(-ENOMEM);
>  
> +	if (percpu_ref_init(&p->users, swap_users_ref_free,
> +			    PERCPU_REF_INIT_DEAD, GFP_KERNEL)) {
> +		kvfree(p);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
>  	spin_lock(&swap_lock);
>  	for (type = 0; type < nr_swapfiles; type++) {
>  		if (!(swap_info[type]->flags & SWP_USED))
> @@ -2864,6 +2878,7 @@ static struct swap_info_struct *alloc_swap_info(void)
>  	}
>  	if (type >= MAX_SWAPFILES) {
>  		spin_unlock(&swap_lock);
> +		percpu_ref_exit(&p->users);
>  		kvfree(p);
>  		return ERR_PTR(-EPERM);
>  	}
> @@ -2891,9 +2906,13 @@ static struct swap_info_struct *alloc_swap_info(void)
>  		plist_node_init(&p->avail_lists[i], 0);
>  	p->flags = SWP_USED;
>  	spin_unlock(&swap_lock);
> -	kvfree(defer);
> +	if (defer) {
> +		percpu_ref_exit(&defer->users);
> +		kvfree(defer);
> +	}
>  	spin_lock_init(&p->lock);
>  	spin_lock_init(&p->cont_lock);
> +	init_completion(&p->comp);
>  
>  	return p;
>  }
Miaohe Lin April 21, 2021, 1:59 a.m. UTC | #2
On 2021/4/21 9:05, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> Using current get/put_swap_device() to guard against concurrent swapoff
>> for some swap ops, e.g. swap_readpage(), looks terrible because they
>> might take really long time. This patch adds the percpu_ref support to
>> serialize against concurrent swapoff. Also we remove the SWP_VALID flag
>> because it's used together with RCU solution.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  include/linux/swap.h |  5 +--
>>  mm/swapfile.c        | 79 +++++++++++++++++++++++++++-----------------
>>  2 files changed, 52 insertions(+), 32 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 144727041e78..c9e7fea10b83 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -177,7 +177,6 @@ enum {
>>  	SWP_PAGE_DISCARD = (1 << 10),	/* freed swap page-cluster discards */
>>  	SWP_STABLE_WRITES = (1 << 11),	/* no overwrite PG_writeback pages */
>>  	SWP_SYNCHRONOUS_IO = (1 << 12),	/* synchronous IO is efficient */
>> -	SWP_VALID	= (1 << 13),	/* swap is valid to be operated on? */
>>  					/* add others here before... */
>>  	SWP_SCANNING	= (1 << 14),	/* refcount in scan_swap_map */
>>  };
>> @@ -240,6 +239,7 @@ struct swap_cluster_list {
>>   * The in-memory structure used to track swap areas.
>>   */
>>  struct swap_info_struct {
>> +	struct percpu_ref users;	/* indicate and keep swap device valid. */
>>  	unsigned long	flags;		/* SWP_USED etc: see above */
>>  	signed short	prio;		/* swap priority of this type */
>>  	struct plist_node list;		/* entry in swap_active_head */
>> @@ -260,6 +260,7 @@ struct swap_info_struct {
>>  	struct block_device *bdev;	/* swap device or bdev of swap file */
>>  	struct file *swap_file;		/* seldom referenced */
>>  	unsigned int old_block_size;	/* seldom referenced */
>> +	struct completion comp;		/* seldom referenced */
>>  #ifdef CONFIG_FRONTSWAP
>>  	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
>>  	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
>> @@ -511,7 +512,7 @@ sector_t swap_page_sector(struct page *page);
>>  
>>  static inline void put_swap_device(struct swap_info_struct *si)
>>  {
>> -	rcu_read_unlock();
>> +	percpu_ref_put(&si->users);
>>  }
>>  
>>  #else /* CONFIG_SWAP */
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 149e77454e3c..a640fc84be5b 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -39,6 +39,7 @@
>>  #include <linux/export.h>
>>  #include <linux/swap_slots.h>
>>  #include <linux/sort.h>
>> +#include <linux/completion.h>
>>  
>>  #include <asm/tlbflush.h>
>>  #include <linux/swapops.h>
>> @@ -511,6 +512,14 @@ static void swap_discard_work(struct work_struct *work)
>>  	spin_unlock(&si->lock);
>>  }
>>  
>> +static void swap_users_ref_free(struct percpu_ref *ref)
>> +{
>> +	struct swap_info_struct *si;
>> +
>> +	si = container_of(ref, struct swap_info_struct, users);
>> +	complete(&si->comp);
>> +}
>> +
>>  static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
>>  {
>>  	struct swap_cluster_info *ci = si->cluster_info;
>> @@ -1270,18 +1279,12 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
>>   * via preventing the swap device from being swapoff, until
>>   * put_swap_device() is called.  Otherwise return NULL.
>>   *
>> - * The entirety of the RCU read critical section must come before the
>> - * return from or after the call to synchronize_rcu() in
>> - * enable_swap_info() or swapoff().  So if "si->flags & SWP_VALID" is
>> - * true, the si->map, si->cluster_info, etc. must be valid in the
>> - * critical section.
>> - *
>>   * Notice that swapoff or swapoff+swapon can still happen before the
>> - * rcu_read_lock() in get_swap_device() or after the rcu_read_unlock()
>> - * in put_swap_device() if there isn't any other way to prevent
>> - * swapoff, such as page lock, page table lock, etc.  The caller must
>> - * be prepared for that.  For example, the following situation is
>> - * possible.
>> + * percpu_ref_tryget_live() in get_swap_device() or after the
>> + * percpu_ref_put() in put_swap_device() if there isn't any other way
>> + * to prevent swapoff, such as page lock, page table lock, etc.  The
>> + * caller must be prepared for that.  For example, the following
>> + * situation is possible.
>>   *
>>   *   CPU1				CPU2
>>   *   do_swap_page()
>> @@ -1309,21 +1312,27 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
>>  	si = swp_swap_info(entry);
>>  	if (!si)
>>  		goto bad_nofile;
>> -
>> -	rcu_read_lock();
>> -	if (data_race(!(si->flags & SWP_VALID)))
>> -		goto unlock_out;
>> +	if (!percpu_ref_tryget_live(&si->users))
>> +		goto out;
>> +	/*
>> +	 * Guarantee the si->users are checked before accessing other
>> +	 * fields of swap_info_struct.
>> +	 *
>> +	 * Paired with the spin_unlock() after setup_swap_info() in
>> +	 * enable_swap_info().
>> +	 */
>> +	smp_rmb();
>>  	offset = swp_offset(entry);
>>  	if (offset >= si->max)
>> -		goto unlock_out;
>> +		goto put_out;
>>  
>>  	return si;
>>  bad_nofile:
>>  	pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
>>  out:
>>  	return NULL;
>> -unlock_out:
>> -	rcu_read_unlock();
>> +put_out:
>> +	percpu_ref_put(&si->users);
>>  	return NULL;
>>  }
>>  
>> @@ -2466,7 +2475,7 @@ static void setup_swap_info(struct swap_info_struct *p, int prio,
>>  
>>  static void _enable_swap_info(struct swap_info_struct *p)
>>  {
>> -	p->flags |= SWP_WRITEOK | SWP_VALID;
>> +	p->flags |= SWP_WRITEOK;
>>  	atomic_long_add(p->pages, &nr_swap_pages);
>>  	total_swap_pages += p->pages;
>>  
>> @@ -2497,10 +2506,9 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
>>  	spin_unlock(&p->lock);
>>  	spin_unlock(&swap_lock);
>>  	/*
>> -	 * Guarantee swap_map, cluster_info, etc. fields are valid
>> -	 * between get/put_swap_device() if SWP_VALID bit is set
>> +	 * Finished initialized swap device, now it's safe to reference it.
> 
> s/initialized/initializing/
> 
> Otherwise looks good to me!  Thanks!
> 
> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
> 

Many thanks for your quick review and respond again. All these comments look good. Will fix it
in V4. Thank you very much !

>>  	 */
>> -	synchronize_rcu();
>> +	percpu_ref_resurrect(&p->users);
>>  	spin_lock(&swap_lock);
>>  	spin_lock(&p->lock);
>>  	_enable_swap_info(p);
>> @@ -2616,16 +2624,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>>  
>>  	reenable_swap_slots_cache_unlock();
>>  
>> -	spin_lock(&swap_lock);
>> -	spin_lock(&p->lock);
>> -	p->flags &= ~SWP_VALID;		/* mark swap device as invalid */
>> -	spin_unlock(&p->lock);
>> -	spin_unlock(&swap_lock);
>>  	/*
>> -	 * wait for swap operations protected by get/put_swap_device()
>> -	 * to complete
>> +	 * Wait for swap operations protected by get/put_swap_device()
>> +	 * to complete.
>> +	 *
>> +	 * We need synchronize_rcu() here to protect the accessing to
>> +	 * the swap cache data structure.
>>  	 */
>> +	percpu_ref_kill(&p->users);
>>  	synchronize_rcu();
>> +	wait_for_completion(&p->comp);
>>  
>>  	flush_work(&p->discard_work);
>>  
>> @@ -2857,6 +2865,12 @@ static struct swap_info_struct *alloc_swap_info(void)
>>  	if (!p)
>>  		return ERR_PTR(-ENOMEM);
>>  
>> +	if (percpu_ref_init(&p->users, swap_users_ref_free,
>> +			    PERCPU_REF_INIT_DEAD, GFP_KERNEL)) {
>> +		kvfree(p);
>> +		return ERR_PTR(-ENOMEM);
>> +	}
>> +
>>  	spin_lock(&swap_lock);
>>  	for (type = 0; type < nr_swapfiles; type++) {
>>  		if (!(swap_info[type]->flags & SWP_USED))
>> @@ -2864,6 +2878,7 @@ static struct swap_info_struct *alloc_swap_info(void)
>>  	}
>>  	if (type >= MAX_SWAPFILES) {
>>  		spin_unlock(&swap_lock);
>> +		percpu_ref_exit(&p->users);
>>  		kvfree(p);
>>  		return ERR_PTR(-EPERM);
>>  	}
>> @@ -2891,9 +2906,13 @@ static struct swap_info_struct *alloc_swap_info(void)
>>  		plist_node_init(&p->avail_lists[i], 0);
>>  	p->flags = SWP_USED;
>>  	spin_unlock(&swap_lock);
>> -	kvfree(defer);
>> +	if (defer) {
>> +		percpu_ref_exit(&defer->users);
>> +		kvfree(defer);
>> +	}
>>  	spin_lock_init(&p->lock);
>>  	spin_lock_init(&p->cont_lock);
>> +	init_completion(&p->comp);
>>  
>>  	return p;
>>  }
> .
>
diff mbox series

Patch

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 144727041e78..c9e7fea10b83 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -177,7 +177,6 @@  enum {
 	SWP_PAGE_DISCARD = (1 << 10),	/* freed swap page-cluster discards */
 	SWP_STABLE_WRITES = (1 << 11),	/* no overwrite PG_writeback pages */
 	SWP_SYNCHRONOUS_IO = (1 << 12),	/* synchronous IO is efficient */
-	SWP_VALID	= (1 << 13),	/* swap is valid to be operated on? */
 					/* add others here before... */
 	SWP_SCANNING	= (1 << 14),	/* refcount in scan_swap_map */
 };
@@ -240,6 +239,7 @@  struct swap_cluster_list {
  * The in-memory structure used to track swap areas.
  */
 struct swap_info_struct {
+	struct percpu_ref users;	/* indicate and keep swap device valid. */
 	unsigned long	flags;		/* SWP_USED etc: see above */
 	signed short	prio;		/* swap priority of this type */
 	struct plist_node list;		/* entry in swap_active_head */
@@ -260,6 +260,7 @@  struct swap_info_struct {
 	struct block_device *bdev;	/* swap device or bdev of swap file */
 	struct file *swap_file;		/* seldom referenced */
 	unsigned int old_block_size;	/* seldom referenced */
+	struct completion comp;		/* seldom referenced */
 #ifdef CONFIG_FRONTSWAP
 	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
 	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
@@ -511,7 +512,7 @@  sector_t swap_page_sector(struct page *page);
 
 static inline void put_swap_device(struct swap_info_struct *si)
 {
-	rcu_read_unlock();
+	percpu_ref_put(&si->users);
 }
 
 #else /* CONFIG_SWAP */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 149e77454e3c..a640fc84be5b 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -39,6 +39,7 @@ 
 #include <linux/export.h>
 #include <linux/swap_slots.h>
 #include <linux/sort.h>
+#include <linux/completion.h>
 
 #include <asm/tlbflush.h>
 #include <linux/swapops.h>
@@ -511,6 +512,14 @@  static void swap_discard_work(struct work_struct *work)
 	spin_unlock(&si->lock);
 }
 
+static void swap_users_ref_free(struct percpu_ref *ref)
+{
+	struct swap_info_struct *si;
+
+	si = container_of(ref, struct swap_info_struct, users);
+	complete(&si->comp);
+}
+
 static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
 {
 	struct swap_cluster_info *ci = si->cluster_info;
@@ -1270,18 +1279,12 @@  static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
  * via preventing the swap device from being swapoff, until
  * put_swap_device() is called.  Otherwise return NULL.
  *
- * The entirety of the RCU read critical section must come before the
- * return from or after the call to synchronize_rcu() in
- * enable_swap_info() or swapoff().  So if "si->flags & SWP_VALID" is
- * true, the si->map, si->cluster_info, etc. must be valid in the
- * critical section.
- *
  * Notice that swapoff or swapoff+swapon can still happen before the
- * rcu_read_lock() in get_swap_device() or after the rcu_read_unlock()
- * in put_swap_device() if there isn't any other way to prevent
- * swapoff, such as page lock, page table lock, etc.  The caller must
- * be prepared for that.  For example, the following situation is
- * possible.
+ * percpu_ref_tryget_live() in get_swap_device() or after the
+ * percpu_ref_put() in put_swap_device() if there isn't any other way
+ * to prevent swapoff, such as page lock, page table lock, etc.  The
+ * caller must be prepared for that.  For example, the following
+ * situation is possible.
  *
  *   CPU1				CPU2
  *   do_swap_page()
@@ -1309,21 +1312,27 @@  struct swap_info_struct *get_swap_device(swp_entry_t entry)
 	si = swp_swap_info(entry);
 	if (!si)
 		goto bad_nofile;
-
-	rcu_read_lock();
-	if (data_race(!(si->flags & SWP_VALID)))
-		goto unlock_out;
+	if (!percpu_ref_tryget_live(&si->users))
+		goto out;
+	/*
+	 * Guarantee the si->users are checked before accessing other
+	 * fields of swap_info_struct.
+	 *
+	 * Paired with the spin_unlock() after setup_swap_info() in
+	 * enable_swap_info().
+	 */
+	smp_rmb();
 	offset = swp_offset(entry);
 	if (offset >= si->max)
-		goto unlock_out;
+		goto put_out;
 
 	return si;
 bad_nofile:
 	pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
 out:
 	return NULL;
-unlock_out:
-	rcu_read_unlock();
+put_out:
+	percpu_ref_put(&si->users);
 	return NULL;
 }
 
@@ -2466,7 +2475,7 @@  static void setup_swap_info(struct swap_info_struct *p, int prio,
 
 static void _enable_swap_info(struct swap_info_struct *p)
 {
-	p->flags |= SWP_WRITEOK | SWP_VALID;
+	p->flags |= SWP_WRITEOK;
 	atomic_long_add(p->pages, &nr_swap_pages);
 	total_swap_pages += p->pages;
 
@@ -2497,10 +2506,9 @@  static void enable_swap_info(struct swap_info_struct *p, int prio,
 	spin_unlock(&p->lock);
 	spin_unlock(&swap_lock);
 	/*
-	 * Guarantee swap_map, cluster_info, etc. fields are valid
-	 * between get/put_swap_device() if SWP_VALID bit is set
+	 * Finished initialized swap device, now it's safe to reference it.
 	 */
-	synchronize_rcu();
+	percpu_ref_resurrect(&p->users);
 	spin_lock(&swap_lock);
 	spin_lock(&p->lock);
 	_enable_swap_info(p);
@@ -2616,16 +2624,16 @@  SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 
 	reenable_swap_slots_cache_unlock();
 
-	spin_lock(&swap_lock);
-	spin_lock(&p->lock);
-	p->flags &= ~SWP_VALID;		/* mark swap device as invalid */
-	spin_unlock(&p->lock);
-	spin_unlock(&swap_lock);
 	/*
-	 * wait for swap operations protected by get/put_swap_device()
-	 * to complete
+	 * Wait for swap operations protected by get/put_swap_device()
+	 * to complete.
+	 *
+	 * We need synchronize_rcu() here to protect the accessing to
+	 * the swap cache data structure.
 	 */
+	percpu_ref_kill(&p->users);
 	synchronize_rcu();
+	wait_for_completion(&p->comp);
 
 	flush_work(&p->discard_work);
 
@@ -2857,6 +2865,12 @@  static struct swap_info_struct *alloc_swap_info(void)
 	if (!p)
 		return ERR_PTR(-ENOMEM);
 
+	if (percpu_ref_init(&p->users, swap_users_ref_free,
+			    PERCPU_REF_INIT_DEAD, GFP_KERNEL)) {
+		kvfree(p);
+		return ERR_PTR(-ENOMEM);
+	}
+
 	spin_lock(&swap_lock);
 	for (type = 0; type < nr_swapfiles; type++) {
 		if (!(swap_info[type]->flags & SWP_USED))
@@ -2864,6 +2878,7 @@  static struct swap_info_struct *alloc_swap_info(void)
 	}
 	if (type >= MAX_SWAPFILES) {
 		spin_unlock(&swap_lock);
+		percpu_ref_exit(&p->users);
 		kvfree(p);
 		return ERR_PTR(-EPERM);
 	}
@@ -2891,9 +2906,13 @@  static struct swap_info_struct *alloc_swap_info(void)
 		plist_node_init(&p->avail_lists[i], 0);
 	p->flags = SWP_USED;
 	spin_unlock(&swap_lock);
-	kvfree(defer);
+	if (defer) {
+		percpu_ref_exit(&defer->users);
+		kvfree(defer);
+	}
 	spin_lock_init(&p->lock);
 	spin_lock_init(&p->cont_lock);
+	init_completion(&p->comp);
 
 	return p;
 }