Re: [PATCH v2 5/5] 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->flags &= ~SWP_VALID;
>                                                   ..
>                                                   synchronize_rcu();
>                                                   ..

You have removed these code in the previous patches of the series.  And
they are not relevant in this patch.

>                                                   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: 1da177e4c3f4 ("Linux-2.6.12-rc2")

No.  This isn't the commit that introduces the race condition.  Please
recheck your git blame result.

Best Regards,
Huang, Ying

> 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);
> +	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;
>  }