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

[Miaohe Lin]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
>>>> ---
>>>>  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.

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!
>>
>>> .
>>>
>>
>>
> .
>