Re: [PATCH v1] mm: swap: Fix race between free_swap_and_cache() and swapoff()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2024/3/6 10:52, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@arm.com> writes:
> 
>> 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 valid. This wasn't present in get_swap_device() so
>> I've added it. I couldn't find any existing get_swap_device() call sites
>> where this extra check would cause any false alarms.
>>
>> Details of how to provoke one possible issue (thanks to David Hilenbrand
>> 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<-----
> 
> I think that this can be simplified.  Even for a 4K folio, this could
> happen.
> 
> CPU0                                     CPU1
> ----                                     ----
> 
> zap_pte_range
>   free_swap_and_cache
>   __swap_entry_free
>   /* swap count become 0 */
>                                          swapoff
>                                            try_to_unuse
>                                              filemap_get_folio
>                                              folio_free_swap
>                                              /* remove swap cache */
>                                            /* free si->swap_map[] */
> 
>   swap_page_trans_huge_swapped <-- access freed si->swap_map !!!

Sorry for jumping the discussion here. IMHO, free_swap_and_cache is called with pte lock held.
So synchronize_rcu (called by swapoff) will wait zap_pte_range to release the pte lock. So this
theoretical problem can't happen. Or am I miss something?

CPU0                                     CPU1
----                                     ----

zap_pte_range
  pte_offset_map_lock -- spin_lock is held.
  free_swap_and_cache
   __swap_entry_free
   /* swap count become 0 */
                                         swapoff
                                           try_to_unuse
                                             filemap_get_folio
                                             folio_free_swap
                                             /* remove swap cache */
					    percpu_ref_kill(&p->users);
   swap_page_trans_huge_swapped
  pte_unmap_unlock -- spin_lock is released.
					    synchronize_rcu();  --> Will wait pte_unmap_unlock to be called?
                                           /* free si->swap_map[] */

Thanks.