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

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

On 4/3/23 12:10 PM, Matthew Wilcox wrote:
> On Sun, Apr 02, 2023 at 06:19:20AM +0800, Rongwei Wang wrote:
>> Without this modification, a core will wait (mostly)
>> 'swap_info_struct->lock' when completing
>> 'del_from_avail_list(p)'. Immediately, other cores
>> soon calling 'add_to_avail_list()' to add the same
>> object again when acquiring the lock that released
>> by former. It's not the desired result but exists
>> indeed. This case can be described as below:
> This feels like a very verbose way of saying
>
> "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."
It looks better than mine. Sorry for my confusing description, it will 
be fixed in the next version.
>
>> +++ b/mm/swapfile.c
>> @@ -2610,8 +2610,12 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>>   		spin_unlock(&swap_lock);
>>   		goto out_dput;
>>   	}
>> -	del_from_avail_list(p);
>> +	/*
>> +	 * Here lock is used to protect deleting and SWP_WRITEOK clearing
>> +	 * can be seen concurrently.
>> +	 */
> This comment isn't necessary.  But I would add a lockdep assert inside
> __del_from_avail_list() that p->lock is held.

Thanks. Actually, I have this line in previous test version, but delete 
for saving one line of code.

I will update here as you said.


Thanks for your time.

>
>>   	spin_lock(&p->lock);
>> +	del_from_avail_list(p);
>>   	if (p->prio < 0) {
>>   		struct swap_info_struct *si = p;
>>   		int nid;
>> -- 
>> 2.27.0
>>
>>