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Subject[PATCH] mm/swap: fix race on swap_info reuse between swapoff and swapon
Date
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.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
---
mm/swapfile.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 612a7c9..89071c3
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1922,7 +1922,6 @@ 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);
@@ -1948,6 +1947,16 @@ 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 p->lock after we cleared its SWP_WRITEOK
+ */
+ spin_lock(&swap_lock);
+ p->flags = 0;
+ spin_unlock(&swap_lock);
+
err = 0;
atomic_inc(&proc_poll_event);
wake_up_interruptible(&proc_poll_wait);
--
1.7.10.4



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