Re: [PATCH -V3 -mm] mm, swap: Fix race between swapoff and some swap operations

["Huang\, Ying" <ying.huang@intel.com>]

"Huang, Ying" <ying.huang@intel.com> writes:

> From: Huang Ying <ying.huang@intel.com>
>
> When the swapin is performed, after getting the swap entry information
> from the page table, system will swap in the swap entry, without any
> lock held to prevent the swap device from being swapoff.  This may
> cause the race like below,
>
> CPU 1				CPU 2
> -----				-----
> 				do_swap_page
> 				  swapin_readahead
> 				    __read_swap_cache_async
> swapoff				      swapcache_prepare
>   p->swap_map = NULL		        __swap_duplicate
> 					  p->swap_map[?] /* !!! NULL pointer access */
>
> Because swapoff is usually done when system shutdown only, the race
> may not hit many people in practice.  But it is still a race need to
> be fixed.
>
> To fix the race, get_swap_device() is added to check whether the
> specified swap entry is valid in its swap device.  If so, it will keep
> the swap entry valid via preventing the swap device from being
> swapoff, until put_swap_device() is called.
>
> Because swapoff() is very race code path, to make the normal path runs
> as fast as possible, RCU instead of reference count is used to
> implement get/put_swap_device().  From get_swap_device() to
> put_swap_device(), the RCU read lock is held, so synchronize_rcu() in
> swapoff() will wait until put_swap_device() is called.
>
> In addition to swap_map, cluster_info, etc. data structure in the
> struct swap_info_struct, the swap cache radix tree will be freed after
> swapoff, so this patch fixes the race between swap cache looking up
> and swapoff too.
>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Shaohua Li <shli@fb.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: "JA(C)rA'me Glisse" <jglisse@redhat.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Aaron Lu <aaron.lu@intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>
> Changelog:
>
> v3:
>
> - Re-implemented with RCU to reduce the overhead of normal paths
>
> v2:
>
> - Re-implemented with SRCU to reduce the overhead of normal paths.
>
> - Avoid to check whether the swap device has been swapoff in
>   get_swap_device().  Because we can check the origin of the swap
>   entry to make sure the swap device hasn't bee swapoff.

A version implemented via stop_machine() could be gotten via a small
patch as below.  If you still prefer stop_machine(), I can resend a
version implemented with stop_machine().

And, it appears that if we replace smp_wmb() in _enable_swap_info() with
stop_machine() in some way, we can avoid smp_rmb() in get_swap_device().
This can reduce overhead in normal path further.  Can we get same effect
with RCU?  For example, use synchronize_rcu() instead of stop_machine()?

Hi, Paul, can you help me on this?

Best Regards,
Huang, Ying

----------------8<------------------------------
---
 include/linux/swap.h |  2 +-
 mm/swapfile.c        | 12 +++++++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index f7e8f26cf07f..1027169d5a04 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -475,7 +475,7 @@ extern struct swap_info_struct *get_swap_device(swp_entry_t entry);
 
 static inline void put_swap_device(struct swap_info_struct *si)
 {
-	rcu_read_unlock();
+	preempt_enable();
 }
 
 #else /* CONFIG_SWAP */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index ca7b4c5ebe34..feb13ce01045 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -38,6 +38,7 @@
 #include <linux/export.h>
 #include <linux/swap_slots.h>
 #include <linux/sort.h>
+#include <linux/stop_machine.h>
 
 #include <asm/pgtable.h>
 #include <asm/tlbflush.h>
@@ -1125,7 +1126,7 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
 		goto bad_nofile;
 	si = swap_info[type];
 
-	rcu_read_lock();
+	preempt_disable();
 	if (!(si->flags & SWP_VALID))
 		goto unlock_out;
 	/*
@@ -1143,7 +1144,7 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
 out:
 	return NULL;
 unlock_out:
-	rcu_read_unlock();
+	preempt_enable();
 	return NULL;
 }
 
@@ -2581,6 +2582,11 @@ bool has_usable_swap(void)
 	return ret;
 }
 
+static int swapoff_stop(void *arg)
+{
+	return 0;
+}
+
 SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 {
 	struct swap_info_struct *p = NULL;
@@ -2677,7 +2683,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	 * wait for swap operations protected by get/put_swap_device()
 	 * to complete
 	 */
-	synchronize_rcu();
+	stop_machine(swapoff_stop, NULL, cpu_online_mask);
 
 	flush_work(&p->discard_work);
 

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