"Huang, Ying" <ying.huang@xxxxxxxxx> writes:
> From: Huang Ying <ying.huang@xxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>
> 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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