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

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

[Weijie Yang]

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

[Andrew Morton]

On Mon, 27 Jan 2014 18:03:04 +0800 Weijie Yang <weijie.yang@samsung.com> wrote:

> 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.

I believe that
http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-on-swap_info-reuse-between-swapoff-and-swapon.patch
makes this patch redundant?

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

[Andrew Morton]

On Mon, 3 Feb 2014 15:23:40 -0800 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Mon, 27 Jan 2014 18:03:04 +0800 Weijie Yang <weijie.yang@samsung.com> wrote:
> 
> > 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.
> 
> I believe that
> http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-on-swap_info-reuse-between-swapoff-and-swapon.patch
> makes this patch redundant?
> 

oop, hang on.  This patch *is* a stealth-updated version of
http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-on-swap_info-reuse-between-swapoff-and-swapon.patch.

Undocumented removals of si->swap_map have been added.  What's going on
there?

I think I'll stick with the original patch for now.  If you see
additional optimisations or changes, let's address that separately?

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

[Hugh Dickins]

On Mon, 3 Feb 2014, Andrew Morton wrote:
> On Mon, 3 Feb 2014 15:23:40 -0800 Andrew Morton <akpm@linux-foundation.org> wrote:
> > On Mon, 27 Jan 2014 18:03:04 +0800 Weijie Yang <weijie.yang@samsung.com> wrote:
> > 
> > > 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.
> > 
> > I believe that
> > http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-on-swap_info-reuse-between-swapoff-and-swapon.patch
> > makes this patch redundant?
> > 
> 
> oop, hang on.  This patch *is* a stealth-updated version of
> http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-on-swap_info-reuse-between-swapoff-and-swapon.patch.
> 
> Undocumented removals of si->swap_map have been added.  What's going on
> there?
> 
> I think I'll stick with the original patch for now.  If you see
> additional optimisations or changes, let's address that separately?

Correct decision, thanks: I explained in an answer when Acking the
previous version why I dislike this version (it would prevent you
from watching the slow progress of swapoff).

Hugh

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