Re: [PATCH v2 2/3] mm/swapfile: fix possible data races of inuse_pages

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/6/20 20:23, Miaohe Lin wrote:
> On 2022/6/20 17:23, Muchun Song wrote:
>> On Mon, Jun 20, 2022 at 05:04:50PM +0800, Miaohe Lin wrote:
>>> On 2022/6/20 15:54, Huang, Ying wrote:
>>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>>
>>>>> si->inuse_pages could still be accessed concurrently now. The plain reads
>>>>> outside si->lock critical section, i.e. swap_show and si_swapinfo, which
>>>>> results in data races. But these should be ok because they're just used
>>>>> for showing swap info.
>>>>>
>>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>>> Reviewed-by: David Hildenbrand <david@redhat.com>
>>>>> ---
>>>>>  mm/swapfile.c | 4 ++--
>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>>>>> index d2bead7b8b70..3fa26f6971e9 100644
>>>>> --- a/mm/swapfile.c
>>>>> +++ b/mm/swapfile.c
>>>>> @@ -2646,7 +2646,7 @@ static int swap_show(struct seq_file *swap, void *v)
>>>>>  	}
>>>>>  
>>>>>  	bytes = si->pages << (PAGE_SHIFT - 10);
>>>>> -	inuse = si->inuse_pages << (PAGE_SHIFT - 10);
>>>>> +	inuse = READ_ONCE(si->inuse_pages) << (PAGE_SHIFT - 10);
>>>>>  
>>>>>  	file = si->swap_file;
>>>>>  	len = seq_file_path(swap, file, " \t\n\\");
>>>>> @@ -3265,7 +3265,7 @@ void si_swapinfo(struct sysinfo *val)
>>>>>  		struct swap_info_struct *si = swap_info[type];
>>>>>  
>>>>>  		if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
>>>>> -			nr_to_be_unused += si->inuse_pages;
>>>>> +			nr_to_be_unused += READ_ONCE(si->inuse_pages);
>>>>>  	}
>>>>>  	val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused;
>>>>>  	val->totalswap = total_swap_pages + nr_to_be_unused;
>>>>
>>>> READ_ONCE() should be paired with WRITE_ONCE().  So, change the writer
>>>> side too?
>>>
>>> READ_ONCE() is used to fix the complaint of concurrent accessing to si->inuse_pages from KCSAN here.
>>> The similar commit is 218209487c3d ("mm/swapfile: fix data races in try_to_unuse()"). IMHO, it's fine
>>
>> I think the fix 218209487c3d is incomplete. The write side in swap_range_free() should
>> also be fixed. Otherwise, IIUC, it cannot stop KCSAN complaining.
> 
> I tend to agree with you. READ_ONCE() should be paired with WRITE_ONCE() theoretically. But WRITTE_ONCE()
> is ignored while the commit is introduced. Add Qian Cai for helping verify it. It's very kind of @Qian Cai
> if he could tell us whether WRITTE_ONCE() is ignored deliberately.

Update the email address of Qian Cai.

> 
> Thanks all of you. :)
> 
>>
>>> to see a not-uptodate value of si->inuse_pages because it's just used for showing swap info. So
>>> WRITE_ONCE() is not obligatory. Or am I miss something?
>>>
>>>>
>>>> Best Regards,
>>>> Huang, Ying
>>>
>>> Thanks!
>>>
>>>> .
>>>>
>>>
>>>
>> .
>>
>