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

[Huang, Ying]

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")

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;
>>>  }
>> .
>>