A potential asynchronous frontswap_register_ops could happen while swapoff
or swapon is happening, there is a need to protect init frontswap type and
prevent NULL point reference to si->frontswap_map.
This patch utilize swapon_mutex to prevent this scenario, see comments of
frontswap_register_ops for detail.
This patch is just for a rare scenario, aim to correct of code.
Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
---
include/linux/swapfile.h | 1 +
mm/frontswap.c | 7 ++++---
mm/swapfile.c | 20 +++++++++++++++-----
3 files changed, 20 insertions(+), 8 deletions(-)
diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
index e282624..1fd5494 100644
--- a/include/linux/swapfile.h
+++ b/include/linux/swapfile.h
@@ -6,6 +6,7 @@
* want to expose them to the dozens of source files that include swap.h
*/
extern spinlock_t swap_lock;
+extern struct mutex swapon_mutex;
extern struct swap_list_t swap_list;
extern struct swap_info_struct *swap_info[];
extern int try_to_unuse(unsigned int, bool, unsigned long);
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 1b24bdc..a0c68db 100644
--- a/mm/frontswap.c
+++ b/mm/frontswap.c
@@ -88,10 +88,9 @@ static inline void inc_frontswap_invalidates(void) { }
* bitmap) to create tmem_pools and set the respective poolids. All of that is
* guarded by us using atomic bit operations on the 'need_init' bitmap.
*
- * This would not guards us against the user deciding to call swapoff right as
+ * swapon_mutex guards us against the user deciding to call swapoff right as
* we are calling the backend to initialize (so swapon is in action).
- * Fortunatly for us, the swapon_mutex has been taked by the callee so we are
- * OK. The other scenario where calls to frontswap_store (called via
+ * The other scenario where calls to frontswap_store (called via
* swap_writepage) is racing with frontswap_invalidate_area (called via
* swapoff) is again guarded by the swap subsystem.
*
@@ -120,6 +119,7 @@ struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
struct frontswap_ops *old = frontswap_ops;
int i;
+ mutex_lock(&swapon_mutex);
for (i = 0; i < MAX_SWAPFILES; i++) {
if (test_and_clear_bit(i, need_init)) {
struct swap_info_struct *sis = swap_info[i];
@@ -129,6 +129,7 @@ struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
ops->init(i);
}
}
+ mutex_unlock(&swapon_mutex);
/*
* We MUST have frontswap_ops set _after_ the frontswap_init's
* have been called. Otherwise __frontswap_store might fail. Hence
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 413c213..5c8a086 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -112,7 +112,7 @@ struct swap_list_t swap_list = {-1, -1};
struct swap_info_struct *swap_info[MAX_SWAPFILES];
-static DEFINE_MUTEX(swapon_mutex);
+DEFINE_MUTEX(swapon_mutex);
static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
/* Activity counter to indicate that a swapon or swapoff has occurred */
@@ -1954,11 +1954,9 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
if (p->flags & SWP_CONTINUED)
free_swap_count_continuations(p);
- mutex_lock(&swapon_mutex);
spin_lock(&swap_lock);
spin_lock(&p->lock);
drain_mmlist();
-
/* wait for anyone still in scan_swap_map */
p->highest_bit = 0; /* cuts scans short */
while (p->flags >= SWP_SCANNING) {
@@ -1968,7 +1966,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
spin_lock(&swap_lock);
spin_lock(&p->lock);
}
+ spin_unlock(&p->lock);
+ spin_unlock(&swap_lock);
+ /*
+ * Now nobody use or allocate swap_entry from this swapfile,
+ * there is no need to hold swap_lock or p->lock.
+ * Use swapon_mutex to mutex against swap_start() and a potential
+ * asynchronous frontswap_register_ops, which will init this swap type
+ */
+ mutex_lock(&swapon_mutex);
swap_file = p->swap_file;
old_block_size = p->old_block_size;
p->swap_file = NULL;
@@ -1978,11 +1985,10 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
cluster_info = p->cluster_info;
p->cluster_info = NULL;
frontswap_map = frontswap_map_get(p);
- spin_unlock(&p->lock);
- spin_unlock(&swap_lock);
frontswap_invalidate_area(type);
frontswap_map_set(p, NULL);
mutex_unlock(&swapon_mutex);
+
free_percpu(p->percpu_cluster);
p->percpu_cluster = NULL;
vfree(swap_map);
@@ -2577,6 +2583,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
}
}
+ /*
+ * Use swapon_mutex to mutex against swap_start() and a potential
+ * asynchronous frontswap_register_ops, which will init this swap type
+ */
mutex_lock(&swapon_mutex);
prio = -1;
if (swap_flags & SWAP_FLAG_PREFER)
--
1.7.10.4
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