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 <linmia...@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.
+ */
+ 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
--
2.19.1
