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

> .
>

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