[PATCH v2] mm: swap: Fix race between free_swap_and_cache() and swapoff()

[Ryan Roberts <ryan.roberts@arm.com>]

There was previously a theoretical window where swapoff() could run and
teardown a swap_info_struct while a call to free_swap_and_cache() was
running in another thread. This could cause, amongst other bad
possibilities, swap_page_trans_huge_swapped() (called by
free_swap_and_cache()) to access the freed memory for swap_map.

This is a theoretical problem and I haven't been able to provoke it from
a test case. But there has been agreement based on code review that this
is possible (see link below).

Fix it by using get_swap_device()/put_swap_device(), which will stall
swapoff(). There was an extra check in _swap_info_get() to confirm that
the swap entry was not free. This isn't present in get_swap_device()
because it doesn't make sense in general due to the race between getting
the reference and swapoff. So I've added an equivalent check directly in
free_swap_and_cache().

Details of how to provoke one possible issue (thanks to David
Hildenbrand for deriving this):

--8<-----

__swap_entry_free() might be the last user and result in
"count == SWAP_HAS_CACHE".

swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0.

So the question is: could someone reclaim the folio and turn
si->inuse_pages==0, before we completed swap_page_trans_huge_swapped().

Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are
still references by swap entries.

Process 1 still references subpage 0 via swap entry.
Process 2 still references subpage 1 via swap entry.

Process 1 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE
[then, preempted in the hypervisor etc.]

Process 2 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE

Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls
__try_to_reclaim_swap().

__try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->
put_swap_folio()->free_swap_slot()->swapcache_free_entries()->
swap_entry_free()->swap_range_free()->
...
WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);

What stops swapoff to succeed after process 2 reclaimed the swap cache
but before process1 finished its call to swap_page_trans_huge_swapped()?

--8<-----

Fixes: 7c00bafee87c ("mm/swap: free swap slots in batch")
Closes: https://lore.kernel.org/linux-mm/65a66eb9-41f8-4790-8db2-0c70ea15979f@redhat.com/
Cc: stable@vger.kernel.org
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---

Hi Andrew,

Please replace v1 of this patch in mm-unstable with this version.

Changes since v1:

 - Added comments for get_swap_device() as suggested by David
 - Moved check that swap entry is not free from get_swap_device() to
   free_swap_and_cache() since there are some paths that legitimately call with
   a free offset.

I haven't addressed the recommendation by Huang Ying [1] to also revert commit
23b230ba8ac3 ("mm/swap: print bad swap offset entry in get_swap_device"). It
should be done separately to this, and and we need to conclude discussion
first.

[1] https://lore.kernel.org/all/875xy0842q.fsf@yhuang6-desk2.ccr.corp.intel.com/

Thanks,
Ryan

 mm/swapfile.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2b3a2d85e350..1155a6304119 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1232,6 +1232,11 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
  * with get_swap_device() and put_swap_device(), unless the swap
  * functions call get/put_swap_device() by themselves.
  *
+ * Note that when only holding the PTL, swapoff might succeed immediately
+ * after freeing a swap entry. Therefore, immediately after
+ * __swap_entry_free(), the swap info might become stale and should not
+ * be touched without a prior get_swap_device().
+ *
  * Check whether swap entry is valid in the swap device.  If so,
  * return pointer to swap_info_struct, and keep the swap entry valid
  * via preventing the swap device from being swapoff, until
@@ -1609,13 +1614,19 @@ int free_swap_and_cache(swp_entry_t entry)
 	if (non_swap_entry(entry))
 		return 1;

-	p = _swap_info_get(entry);
+	p = get_swap_device(entry);
 	if (p) {
+		if (WARN_ON(data_race(!p->swap_map[swp_offset(entry)]))) {
+			put_swap_device(p);
+			return 0;
+		}
+
 		count = __swap_entry_free(p, entry);
 		if (count == SWAP_HAS_CACHE &&
 		    !swap_page_trans_huge_swapped(p, entry))
 			__try_to_reclaim_swap(p, swp_offset(entry),
 					      TTRS_UNMAPPED | TTRS_FULL);
+		put_swap_device(p);
 	}
 	return p != NULL;
 }
--
2.25.1