swapoff clear swap_info's SWP_USED flag prematurely and free its resources
after that. A concurrent swapon will reuse this swap_info while its previous
resources are not cleared completely.
These late freed resources are:
- p->percpu_cluster
- swap_cgroup_ctrl[type]
- block_device setting
- inode->i_flags &= ~S_SWAPFILE
This patch clear SWP_USED flag after all its resources freed, so that swapon
can reuse this swap_info by alloc_swap_info() safely.
This patch is just for a rare scenario, aim to correct of code.
Suggested-by: Heesub Shin <heesub.shin@xxxxxxxxxxx>
Suggested-by: Mateusz Guzik <mguzik@xxxxxxxxxx>
Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
---
mm/swapfile.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 0a623a9..4d24158 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1977,7 +1977,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
p->swap_map = NULL;
cluster_info = p->cluster_info;
p->cluster_info = NULL;
- p->flags = 0;
frontswap_map = frontswap_map_get(p);
spin_unlock(&p->lock);
spin_unlock(&swap_lock);
@@ -2003,6 +2002,15 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
mutex_unlock(&inode->i_mutex);
}
filp_close(swap_file, NULL);
+
+ /*
+ * clear SWP_USED flag after all resources freed so that
+ * swapon can reuse this swap_info in alloc_swap_info() safely
+ * it is ok to not hold any lock after we cleared SWP_WRITEOK flag
+ */
+ smp_wmb();
+ p->flags = 0;
+
err = 0;
atomic_inc(&proc_poll_event);
wake_up_interruptible(&proc_poll_wait);
@@ -2050,7 +2058,7 @@ static void *swap_start(struct seq_file *swap, loff_t *pos)
for (type = 0; type < nr_swapfiles; type++) {
smp_rmb(); /* read nr_swapfiles before swap_info[type] */
si = swap_info[type];
- if (!(si->flags & SWP_USED) || !si->swap_map)
+ if (!(si->flags & SWP_WRITEOK))
continue;
if (!--l)
return si;
@@ -2072,7 +2080,7 @@ static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
for (; type < nr_swapfiles; type++) {
smp_rmb(); /* read nr_swapfiles before swap_info[type] */
si = swap_info[type];
- if (!(si->flags & SWP_USED) || !si->swap_map)
+ if (!(si->flags & SWP_WRITEOK))
continue;
++*pos;
return si;
--
1.7.10.4
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