Re: [PATCH v2] mm/swap: fix swap_info_struct race between swapoff and get_swap_pages()

[Rongwei Wang <rongwei.wang@linux.alibaba.com>]

On 2023/4/6 22:57, Aaron Lu wrote:
> On Thu, Apr 06, 2023 at 10:04:16PM +0800, Aaron Lu wrote:
>> On Tue, Apr 04, 2023 at 11:47:16PM +0800, Rongwei Wang wrote:
>>> The si->lock must be held when deleting the si from
>>> the available list.  Otherwise, another thread can
>>> re-add the si to the available list, which can lead
>>> to memory corruption. The only place we have found
>>> where this happens is in the swapoff path. This case
>>> can be described as below:
>>>
>>> core 0                       core 1
>>> swapoff
>>>
>>> del_from_avail_list(si)      waiting
>>>
>>> try lock si->lock            acquire swap_avail_lock
>>>                               and re-add si into
>>>                               swap_avail_head
>>                                 confused here.
>>
>> If del_from_avail_list(si) finished in swaoff path, then this si should
>> not exist in any of the per-node avail list and core 1 should not be
>> able to re-add it.
> I think a possible sequence could be like this:
>
> cpuX                             cpuY
> swapoff                          put_swap_folio()
>
> del_from_avail_list(si)
>                                   taken si->lock
> spin_lock(&si->lock);
>
> 				 swap_range_free()
> 				 was_full && SWP_WRITEOK -> re-add!
> 				 drop si->lock
>
> taken si->lock
> proceed removing si
>
> End result: si left on avail_list after being swapped off.
>
> The problem is, in add_to_avail_list(), it has no idea this si is being
> swapped off and taking si->lock then del_from_avail_list() could avoid
> this problem, so I think this patch did the right thing but the
> changelog about how this happened needs updating and after that:

Hi Aaron

That's my fault. Actually, I don't refers specifically to 
swap_range_free() path in commit, mainly because cpuY can stand all 
threads which is waiting swap_avail_lock, then to call 
add_to_avail_list(), not only swap_range_free(), e.g. maybe swapon().

>
> Reviewed-by: Aaron Lu <aaron.lu@intel.com>

Thanks for your time.

-wrw

>
> Thanks,
> Aaron
>