Re: [Question] Is there a race window between swapoff vs synchronous swap_readpage

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/3/30 9:57, Huang, Ying wrote:
> Hi, Miaohe,
> 
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> Hi all,
>> I am investigating the swap code, and I found the below possible race window:
>>
>> CPU 1							CPU 2
>> -----							-----
>> do_swap_page
>>   skip swapcache case (synchronous swap_readpage)
>>     alloc_page_vma
>> 							swapoff
>> 							  release swap_file, bdev, or ...
>>       swap_readpage
>> 	check sis->flags is ok
>> 	  access swap_file, bdev or ...[oops!]
>> 							    si->flags = 0
>>
>> The swapcache case is ok because swapoff will wait on the page_lock of swapcache page.
>> Is this will really happen or Am I miss something ?
>> Any reply would be really grateful. Thanks! :)
> 
> This appears possible.  Even for swapcache case, we can't guarantee the

Many thanks for reply!

> swap entry gotten from the page table is always valid too.  The

The page table may change at any time. And we may thus do some useless work.
But the pte_same() check could handle these races correctly if these do not
result in oops.

> underlying swap device can be swapped off at the same time.  So we use
> get/put_swap_device() for that.  Maybe we need similar stuff here.

Using get/put_swap_device() to guard against swapoff for swap_readpage() sounds
really bad as swap_readpage() may take really long time. Also such race may not be
really hurtful because swapoff is usually done when system shutdown only.
I can not figure some simple and stable stuff out to fix this. Any suggestions or
could anyone help get rid of such race?

Anyway, thanks again!

> 
> Best Regards,
> Huang, Ying
> .
>