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

[Miaohe Lin]

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.

> 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;
>>  }
> .
>