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

[Miaohe Lin]

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
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!

> .
>