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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     <akpm@linux-foundation.org>, <david@redhat.com>,
        <songmuchun@bytedance.com>, <quic_qiancai@quicinc.com>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 1/2] mm/swapfile: fix possible data races of inuse_pages
References: <20220625093346.48894-1-linmiaohe@huawei.com>
        <20220625093346.48894-2-linmiaohe@huawei.com>
Date:   Mon, 27 Jun 2022 09:29:50 +0800
In-Reply-To: <20220625093346.48894-2-linmiaohe@huawei.com> (Miaohe Lin's
        message of "Sat, 25 Jun 2022 17:33:45 +0800")
Message-ID: <874k0699m9.fsf@yhuang6-desk2.ccr.corp.intel.com>
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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. READ_ONCE and WRITE_ONCE is used to fix such data
> races. Note these data races 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>

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

Thanks!

Best Regards,
Huang, Ying

> ---
>  mm/swapfile.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index edc3420d30e7..5c8681a3f1d9 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -695,7 +695,7 @@ static void swap_range_alloc(struct swap_info_struct *si, unsigned long offset,
>  		si->lowest_bit += nr_entries;
>  	if (end == si->highest_bit)
>  		WRITE_ONCE(si->highest_bit, si->highest_bit - nr_entries);
> -	si->inuse_pages += nr_entries;
> +	WRITE_ONCE(si->inuse_pages, si->inuse_pages + nr_entries);
>  	if (si->inuse_pages == si->pages) {
>  		si->lowest_bit = si->max;
>  		si->highest_bit = 0;
> @@ -732,7 +732,7 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
>  			add_to_avail_list(si);
>  	}
>  	atomic_long_add(nr_entries, &nr_swap_pages);
> -	si->inuse_pages -= nr_entries;
> +	WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
>  	if (si->flags & SWP_BLKDEV)
>  		swap_slot_free_notify =
>  			si->bdev->bd_disk->fops->swap_slot_free_notify;
> @@ -2641,7 +2641,7 @@ 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\\");
> @@ -3260,7 +3260,7 @@ 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;