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

[Ryan Roberts]

On 06/03/2024 08:51, Miaohe Lin wrote:
> On 2024/3/6 10:52, Huang, Ying wrote:
>> Ryan Roberts <ryan.roberts@xxxxxxx> 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.

I don't beleive it has the PTL when called by shmem.

> 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?

For Huang Ying's example, I agree this can't happen because try_to_unuse() will
be waiting for the PTL (see the reply I just sent).

> 
> 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?

Perhaps you can educate me here; I thought that synchronize_rcu() will only wait
for RCU critical sections to complete. The PTL is a spin lock, so why would
synchronize_rcu() wait for the PTL to become unlocked?


>                                            /* free si->swap_map[] */
> 
> Thanks.
> 
>