linux-kernel - Re: [PATCH v2 2/3] mm/swapfile: fix possible data races of inuse

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Date:   Mon, 20 Jun 2022 15:54:55 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <akpm@...ux-foundation.org>, <david@...hat.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] mm/swapfile: fix possible data races of inuse_pages

Miaohe Lin <linmiaohe@...wei.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@...wei.com>
> Reviewed-by: David Hildenbrand <david@...hat.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?

Best Regards,
Huang, Ying