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

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

On 07/03/2024 08:54, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@arm.com> writes:
> 
>> On 07/03/2024 07:34, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>
>>>> On 2024/3/7 13:56, Huang, Ying wrote:
>>>>> Miaohe Lin <linmiaohe@huawei.com> 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@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.
>>>>>>>
>>>>>>> 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.

The only problem with this is that Andrew has already put my v2 into mm-*stable* :-|

So (1) from that list isn't possible. I could do a patch for (2) and (3), but to
be honest, I think you would do a better job of writing it up than I would - any
chance you could post the patch?




> 
> --
> Best Regards,
> Huang, Ying