Re: [PATCH v2 5/5] mm/shmem: fix shmem_swapin() race with swapoff

[Miaohe Lin]

On 2021/4/19 15:41, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
> 
>> On 2021/4/19 15:04, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>>
>>>> On 2021/4/19 10:15, Huang, Ying wrote:
>>>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>>>>
>>>>>> When I was investigating the swap code, I found the below possible race
>>>>>> window:
>>>>>>
>>>>>> CPU 1                                           CPU 2
>>>>>> -----                                           -----
>>>>>> shmem_swapin
>>>>>>   swap_cluster_readahead
>>>>>>     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>>>>>>                                                 swapoff
>>>>>>                                                   si->flags &= ~SWP_VALID;
>>>>>>                                                   ..
>>>>>>                                                   synchronize_rcu();
>>>>>>                                                   ..
>>>>>
>>>>> You have removed these code in the previous patches of the series.  And
>>>>> they are not relevant in this patch.
>>>>
>>>> Yes, I should change these. Thanks.
>>>>
>>>>>
>>>>>>                                                   si->swap_file = NULL;
>>>>>>     struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>>>>>>
>>>>>> Close this race window by using get/put_swap_device() to guard against
>>>>>> concurrent swapoff.
>>>>>>
>>>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>>>
>>>>> No.  This isn't the commit that introduces the race condition.  Please
>>>>> recheck your git blame result.
>>>>>
>>>>
>>>> I think this is really hard to find exact commit. I used git blame and found
>>>> this race should be existed when this is introduced. Any suggestion ?
>>>> Thanks.
>>>
>>> I think the commit that introduces the race condition is commit
>>> 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or
>>> not")
>>>
>>
>> Thanks.
>> The commit log only describes one race condition. And for that one, this should be correct
>> Fixes tag. But there are still many other race conditions inside swap_cluster_readahead,
>> such as swap_readpage() called from swap_cluster_readahead. This tag could not cover the
>> all race windows.
> 
> No. swap_readpage() in swap_cluster_readahead() is OK.  Because
> __read_swap_cache_async() is called before that, so the swap entry will
> be marked with SWAP_HAS_CACHE, and page will be locked.
> 

Oh... I missed this. Many thanks for your remind.

> Best Regards,
> Huang, Ying
> 
>>> Best Regards,
>>> Huang, Ying
>>>
>>>>> Best Regards,
>>>>> Huang, Ying
>>>>>
>>>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>>>> ---
>>>>>>  mm/shmem.c | 6 ++++++
>>>>>>  1 file changed, 6 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>>>> index 26c76b13ad23..936ba5595297 100644
>>>>>> --- a/mm/shmem.c
>>>>>> +++ b/mm/shmem.c
>>>>>> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
>>>>>>  static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>>>>>>  			struct shmem_inode_info *info, pgoff_t index)
>>>>>>  {
>>>>>> +	struct swap_info_struct *si;
>>>>>>  	struct vm_area_struct pvma;
>>>>>>  	struct page *page;
>>>>>>  	struct vm_fault vmf = {
>>>>>>  		.vma = &pvma,
>>>>>>  	};
>>>>>>  
>>>>>> +	/* Prevent swapoff from happening to us. */
>>>>>> +	si = get_swap_device(swap);
>>>>>> +	if (unlikely(!si))
>>>>>> +		return NULL;
>>>>>>  	shmem_pseudo_vma_init(&pvma, info, index);
>>>>>>  	page = swap_cluster_readahead(swap, gfp, &vmf);
>>>>>>  	shmem_pseudo_vma_destroy(&pvma);
>>>>>> +	put_swap_device(si);
>>>>>>  
>>>>>>  	return page;
>>>>>>  }
>>>>> .
>>>>>
>>> .
>>>
> .
>