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;
> }
