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

["Huang, Ying" <ying.huang@intel.com>]

Miaohe Lin <linmiaohe@huawei.com> writes:

> On 2021/4/19 15:04, Huang, Ying wrote:
>> Miaohe Lin <linmiaohe@huawei.com> writes:
>> 
>>> On 2021/4/19 10:15, Huang, Ying wrote:
>>>> Miaohe Lin <linmiaohe@huawei.com> 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.

Best Regards,
Huang, Ying

>> Best Regards,
>> Huang, Ying
>> 
>>>> Best Regards,
>>>> Huang, Ying
>>>>
>>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>>> ---
>>>>>  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;
>>>>>  }
>>>> .
>>>>
>> .
>>