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

[Miaohe Lin]

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.

> 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;
>>>>  }
>>> .
>>>
> .
>