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
> percpu_ref_kill(&p->users)
> synchronize_rcu()
> wait_for_completion
I don't think the above 3 lines are relevant for the race.
> ..
> 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 | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..936ba5595297 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
> static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
> struct shmem_inode_info *info, pgoff_t index)
> {
> + struct swap_info_struct *si;
> struct vm_area_struct pvma;
> struct page *page;
> struct vm_fault vmf = {
> .vma = &pvma,
> };
>
> + /* Prevent swapoff from happening to us. */
> + si = get_swap_device(swap);
Better to put get/put_swap_device() in shmem_swapin_page(), that make it
possible for us to remove get/put_swap_device() in lookup_swap_cache().
Best Regards,
Huang, Ying
> + if (unlikely(!si))
> + return NULL;
> shmem_pseudo_vma_init(&pvma, info, index);
> page = swap_cluster_readahead(swap, gfp, &vmf);
> shmem_pseudo_vma_destroy(&pvma);
> + put_swap_device(si);
>
> return page;
> }
