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

[Muchun Song <songmuchun@bytedance.com>]

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.

> 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!
> 
> > .
> > 
> 
>