Re: [PATCH 5/5] mm/swap_state: fix swap_cluster_readahead() race with swapoff

[Huang, Ying]

Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:

> swap_cluster_readahead() could race with swapoff and might dereference
> si->swap_file after it's released by swapoff. Close this race window by
> using get/put_swap_device() pair.

I think we should fix the callers instead to reduce the overhead.  Now,
do_swap_page() has been fixed.  We need to fix shmem_swapin().

Best Regards,
Huang, Ying

> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
>  mm/swap_state.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 3bf0d0c297bc..eba6b0cf6cf9 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -626,12 +626,17 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>  	unsigned long offset = entry_offset;
>  	unsigned long start_offset, end_offset;
>  	unsigned long mask;
> -	struct swap_info_struct *si = swp_swap_info(entry);
> +	struct swap_info_struct *si;
>  	struct blk_plug plug;
>  	bool do_poll = true, page_allocated;
>  	struct vm_area_struct *vma = vmf->vma;
>  	unsigned long addr = vmf->address;
>  
> +	si = get_swap_device(entry);
> +	/* In case we raced with swapoff. */
> +	if (!si)
> +		return NULL;
> +
>  	mask = swapin_nr_pages(offset) - 1;
>  	if (!mask)
>  		goto skip;
> @@ -673,7 +678,9 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>  
>  	lru_add_drain();	/* Push any new pages onto the LRU now */
>  skip:
> -	return read_swap_cache_async(entry, gfp_mask, vma, addr, do_poll);
> +	page = read_swap_cache_async(entry, gfp_mask, vma, addr, do_poll);
> +	put_swap_device(si);
> +	return page;
>  }
>  
>  int init_swap_address_space(unsigned int type, unsigned long nr_pages)