Re: [PATCH v4 4/4] mm/shmem: fix shmem_swapin() race with swapoff

[Huang, Ying]

Miaohe Lin <linmiaohe@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  mm/shmem.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..be388d0cf8b5 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 (unlikely(!si)) {
> +		error = EINVAL;
> +		goto failed;
> +	}
>  	/* Look it up and read it in.. */
>  	page = lookup_swap_cache(swap, NULL, 0);
>  	if (!page) {
> @@ -1720,6 +1727,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  			goto failed;
>  		}
>  	}
> +	put_swap_device(si);

I think it's better to put_swap_device() just before returning from the
function.  It's not a big issue to slow down swapoff() a little.  And
this will make the logic easier to be understood.

Best Regards,
Huang, Ying

>  
>  	/* We have to do this with page locked to prevent races */
>  	lock_page(page);
> @@ -1775,6 +1783,9 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  		put_page(page);
>  	}
>  
> +	if (si)
> +		put_swap_device(si);
> +
>  	return error;
>  }