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SubjectRe: [PATCH -mm] mm, swap: Fix race between swapoff and some swap operations
On Thu, Dec 07, 2017 at 04:29:37PM -0800, Andrew Morton wrote:
> On Thu, 7 Dec 2017 09:14:26 +0800 "Huang, Ying" <ying.huang@intel.com> wrote:
>
> > When the swapin is performed, after getting the swap entry information
> > from the page table, the PTL (page table lock) will be released, then
> > system will go to 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 swap off 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.
>
> swapoff is so rare that it's hard to get motivated about any fix which
> adds overhead to the regular codepaths.

That was my concern, too when I see this patch.

>
> Is there something we can do to ensure that all the overhead of this
> fix is placed into the swapoff side? stop_machine() may be a bit
> brutal, but a surprising amount of code uses it. Any other ideas?

How about this?

I think It's same approach with old where we uses si->lock everywhere
instead of more fine-grained cluster lock.

The reason I repeated to reset p->max to zero in the loop is to avoid
using lockdep annotation(maybe, spin_lock_nested(something) to prevent
false positive.

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 42fe5653814a..9ce007a42bbc 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2644,6 +2644,19 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
swap_file = p->swap_file;
old_block_size = p->old_block_size;
p->swap_file = NULL;
+
+ if (p->flags & SWP_SOLIDSTATE) {
+ unsigned long ci, nr_cluster;
+
+ nr_cluster = DIV_ROUND_UP(p->max, SWAPFILE_CLUSTER);
+ for (ci = 0; ci < nr_cluster; ci++) {
+ struct swap_cluster_info *sci;
+
+ sci = lock_cluster(p, ci * SWAPFILE_CLUSTER);
+ p->max = 0;
+ unlock_cluster(sci);
+ }
+ }
p->max = 0;
swap_map = p->swap_map;
p->swap_map = NULL;
@@ -3369,10 +3382,10 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
goto bad_file;
p = swap_info[type];
offset = swp_offset(entry);
- if (unlikely(offset >= p->max))
- goto out;

ci = lock_cluster_or_swap_info(p, offset);
+ if (unlikely(offset >= p->max))
+ goto unlock_out;

count = p->swap_map[offset];

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