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

[Huang, Ying]

Ryan Roberts <ryan.roberts@xxxxxxx> writes:

> On 07/03/2024 07:34, Huang, Ying wrote:
>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>> 
>>> On 2024/3/7 13:56, Huang, Ying wrote:
>>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>>>
>>>>> On 2024/3/6 17:31, Ryan Roberts wrote:
>>>>>> 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.
>>>>>
>>>>> In the case of shmem, folio_lock is used to guard against the race.
>>>>
>>>> I don't find folio is lock for shmem.  find_lock_entries() will only
>>>> lock the folio if (!xa_is_value()), that is, not swap entry.  Can you
>>>> point out where the folio is locked for shmem?
>>>
>>> You're right, folio is locked if not swap entry. That's my mistake. But it seems above race is still nonexistent.
>>> shmem_unuse() will first be called to read all the shared memory data that resides in the swap device back into
>>> memory when doing swapoff. In that case, all the swapped pages are moved to page cache thus there won't be any
>>> xa_is_value(folio) cases when calling shmem_undo_range(). free_swap_and_cache() even won't be called from
>>> shmem_undo_range() after shmem_unuse(). Or am I miss something?
>> 
>> I think the following situation is possible.  Right?
>> 
>> CPU0                               CPU1
>> ----                               ----
>> shmem_undo_range
>>   shmem_free_swap
>>     xa_cmpxchg_irq
>>     free_swap_and_cache
>>       __swap_entry_free
>>       /* swap count become 0 */
>>                                    swapoff
>>                                      try_to_unuse
>>                                        shmem_unuse /* cannot find swap entry */
>>                                        find_next_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 !!!
>> 
>> shmem_undo_range can run earlier.
>
> Yes that's the shmem problem I've been trying to convey. Perhaps there are other
> (extremely subtle) mechanisms that make this impossible, I don't know.
>
> Either way, given the length of this discussion, and the subtleties in the
> syncrhonization mechanisms that have so far been identified, I think the safest
> thing to do is just apply the patch. Then we have explicit syncrhonization that
> we can trivially reason about.

Yes.  This is tricky and we can improve it.  So I suggest to,

- Revise the patch description to use shmem race as example except
  someone found it's impossible.

- Revise the comments of get_swap_device() about RCU reader side lock
  (including IRQ off, spinlock, etc.) can prevent swapoff via
  synchronize_rcu() in swapoff().

- Revise the comments of synchronize_rcu() in swapoff(), which can
  prevent swapoff in parallel with RCU reader side lock including swap
  cache operations, etc.

--
Best Regards,
Huang, Ying