[PATCH 2/8] mm/swap: fix race on swap_info reuse between swapoff and swapon

[Weijie Yang <weijie.yang@samsung.com>]

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@samsung.com>
Suggested-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
---
 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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