linux-kernel - Re: [PATCH v5 4/4] mm/shmem: fix shmem

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Date:   Mon, 26 Apr 2021 08:56:00 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <akpm@...ux-foundation.org>, <dennis@...nel.org>,
        <tim.c.chen@...ux.intel.com>, <hughd@...gle.com>,
        <hannes@...xchg.org>, <mhocko@...e.com>, <iamjoonsoo.kim@....com>,
        <alexs@...nel.org>, <willy@...radead.org>, <minchan@...nel.org>,
        <richard.weiyang@...il.com>, <shy828301@...il.com>,
        <david@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>
Subject: Re: [PATCH v5 4/4] mm/shmem: fix shmem_swapin() race with swapoff

Miaohe Lin <linmiaohe@...wei.com> writes:

> When I was investigating the swap code, I found the below possible race
> window:
>
> CPU 1                                         CPU 2
> -----                                         -----
> shmem_swapin
>   swap_cluster_readahead
>     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>                                               swapoff
>                                                 ..
>                                                 si->swap_file = NULL;
>                                                 ..
>     struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>
> Close this race window by using get/put_swap_device() to guard against
> concurrent swapoff.
>
> Fixes: 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or not")
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>

Thanks!

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

> ---
>  mm/shmem.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..2dafd65b0b42 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1696,6 +1696,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  	struct address_space *mapping = inode->i_mapping;
>  	struct shmem_inode_info *info = SHMEM_I(inode);
>  	struct mm_struct *charge_mm = vma ? vma->vm_mm : current->mm;
> +	struct swap_info_struct *si;
>  	struct page *page;
>  	swp_entry_t swap;
>  	int error;
> @@ -1704,6 +1705,12 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  	swap = radix_to_swp_entry(*pagep);
>  	*pagep = NULL;
>  
> +	/* Prevent swapoff from happening to us. */
> +	si = get_swap_device(swap);
> +	if (!si) {
> +		error = EINVAL;
> +		goto failed;
> +	}
>  	/* Look it up and read it in.. */
>  	page = lookup_swap_cache(swap, NULL, 0);
>  	if (!page) {
> @@ -1765,6 +1772,8 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  	swap_free(swap);
>  
>  	*pagep = page;
> +	if (si)
> +		put_swap_device(si);
>  	return 0;
>  failed:
>  	if (!shmem_confirm_swap(mapping, index, swap))
> @@ -1775,6 +1784,9 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  		put_page(page);
>  	}
>  
> +	if (si)
> +		put_swap_device(si);
> +
>  	return error;
>  }