[v2,2/5] mm/swapfile: use percpu_ref to serialize against concurrent swapoff
diff mbox series

Message ID 20210417094039.51711-3-linmiaohe@huawei.com
State New
Headers show
Series
  • close various race windows for swap
Related show

Commit Message

Miaohe Lin April 17, 2021, 9:40 a.m. UTC
Use percpu_ref to serialize against concurrent swapoff. Also 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 |  3 +--
 mm/swapfile.c        | 43 +++++++++++++++++--------------------------
 2 files changed, 18 insertions(+), 28 deletions(-)

Comments

Huang, Ying April 19, 2021, 2:54 a.m. UTC | #1
Miaohe Lin <linmiaohe@huawei.com> writes:

> Use percpu_ref to serialize against concurrent swapoff. Also 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 |  3 +--
>  mm/swapfile.c        | 43 +++++++++++++++++--------------------------
>  2 files changed, 18 insertions(+), 28 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 8be36eb58b7a..993693b38109 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 */
>  };
> @@ -514,7 +513,7 @@ 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 66515a3a2824..90e197bc2eeb 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1279,18 +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()
> @@ -1318,21 +1312,24 @@ 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 we will not reference uninitialized fields
> +	 * of swap_info_struct.
> +	 */

/*
 * Guarantee the si->users are checked before accessing other fields of
 * swap_info_struct.
*/

> +	smp_rmb();

Usually, smp_rmb() need to be paired with smp_wmb().  Some comments are
needed for that.  Here smb_rmb() is paired with the spin_unlock() after
setup_swap_info() in enable_swap_info().

>  	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;
>  }
>  
> @@ -2475,7 +2472,7 @@ 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;
>  
> @@ -2507,7 +2504,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
>  	spin_unlock(&swap_lock);
>  	/*
>  	 * Guarantee swap_map, cluster_info, etc. fields are valid
> -	 * between get/put_swap_device() if SWP_VALID bit is set
> +	 * between get/put_swap_device().
>  	 */

The comments need to be revised.  Something likes below?

/* Finished initialized swap device, now it's safe to reference it */

Best Regards,
Huang, Ying

>  	percpu_ref_resurrect(&p->users);
>  	spin_lock(&swap_lock);
> @@ -2625,12 +2622,6 @@ 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);
> -
>  	percpu_ref_kill(&p->users);
>  	/*
>  	 * We need synchronize_rcu() here to protect the accessing
Miaohe Lin April 19, 2021, 6:57 a.m. UTC | #2
On 2021/4/19 10:54, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> Use percpu_ref to serialize against concurrent swapoff. Also 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 |  3 +--
>>  mm/swapfile.c        | 43 +++++++++++++++++--------------------------
>>  2 files changed, 18 insertions(+), 28 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 8be36eb58b7a..993693b38109 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 */
>>  };
>> @@ -514,7 +513,7 @@ 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 66515a3a2824..90e197bc2eeb 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1279,18 +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()
>> @@ -1318,21 +1312,24 @@ 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 we will not reference uninitialized fields
>> +	 * of swap_info_struct.
>> +	 */
> 
> /*
>  * Guarantee the si->users are checked before accessing other fields of
>  * swap_info_struct.
> */
> 
>> +	smp_rmb();
> 
> Usually, smp_rmb() need to be paired with smp_wmb().  Some comments are
> needed for that.  Here smb_rmb() is paired with the spin_unlock() after
> setup_swap_info() in enable_swap_info().
> 
>>  	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;
>>  }
>>  
>> @@ -2475,7 +2472,7 @@ 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;
>>  
>> @@ -2507,7 +2504,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
>>  	spin_unlock(&swap_lock);
>>  	/*
>>  	 * Guarantee swap_map, cluster_info, etc. fields are valid
>> -	 * between get/put_swap_device() if SWP_VALID bit is set
>> +	 * between get/put_swap_device().
>>  	 */
> 
> The comments need to be revised.  Something likes below?
> 
> /* Finished initialized swap device, now it's safe to reference it */
> 

All look good for me. Will do. Many thanks!

> Best Regards,
> Huang, Ying
> 
>>  	percpu_ref_resurrect(&p->users);
>>  	spin_lock(&swap_lock);
>> @@ -2625,12 +2622,6 @@ 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);
>> -
>>  	percpu_ref_kill(&p->users);
>>  	/*
>>  	 * We need synchronize_rcu() here to protect the accessing
> 
> .
>

Patch
diff mbox series

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 8be36eb58b7a..993693b38109 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 */
 };
@@ -514,7 +513,7 @@  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 66515a3a2824..90e197bc2eeb 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1279,18 +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()
@@ -1318,21 +1312,24 @@  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 we will not reference uninitialized fields
+	 * of swap_info_struct.
+	 */
+	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;
 }
 
@@ -2475,7 +2472,7 @@  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;
 
@@ -2507,7 +2504,7 @@  static void enable_swap_info(struct swap_info_struct *p, int prio,
 	spin_unlock(&swap_lock);
 	/*
 	 * Guarantee swap_map, cluster_info, etc. fields are valid
-	 * between get/put_swap_device() if SWP_VALID bit is set
+	 * between get/put_swap_device().
 	 */
 	percpu_ref_resurrect(&p->users);
 	spin_lock(&swap_lock);
@@ -2625,12 +2622,6 @@  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);
-
 	percpu_ref_kill(&p->users);
 	/*
 	 * We need synchronize_rcu() here to protect the accessing