Re: [PATCH v2 3/5] swap: fix do_swap_page() race with swapoff

["Huang, Ying" <ying.huang@intel.com>]

Miaohe Lin <linmiaohe@huawei.com> writes:

> When I was investigating the swap code, I found the below possible race
> window:
>
> CPU 1                                   	CPU 2
> -----                                   	-----
> do_swap_page

This is OK for swap cache cases.  So

  if (data_race(si->flags & SWP_SYNCHRONOUS_IO))

should be shown here.

>   swap_readpage(skip swap cache case)
>     if (data_race(sis->flags & SWP_FS_OPS)) {
>                                         	swapoff
> 					  	  p->flags = &= ~SWP_VALID;
> 					  	  ..
> 					  	  synchronize_rcu();
> 					  	  ..
> 					  	  p->swap_file = NULL;
>     struct file *swap_file = sis->swap_file;
>     struct address_space *mapping = swap_file->f_mapping;[oops!]
>
> Note that for the pages that are swapped in through swap cache, this isn't
> an issue. Because the page is locked, and the swap entry will be marked
> with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been
> unlocked.
>
> Using current get/put_swap_device() to guard against concurrent swapoff for
> swap_readpage() looks terrible because swap_readpage() may take really long
> time. And this race may not be really pernicious because swapoff is usually
> done when system shutdown only. To reduce the performance overhead on the
> hot-path as much as possible, it appears we can use the percpu_ref to close
> this race window(as suggested by Huang, Ying).

I still suggest to squash PATCH 1-3, at least PATCH 1-2.  That will
change the relevant code together and make it easier to review.

Best Regards,
Huang, Ying

> Fixes: 0bcac06f27d7 ("mm,swap: skip swapcache for swapin of synchronous device")
> Reported-by: kernel test robot <lkp@intel.com> (auto build test ERROR)
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  include/linux/swap.h | 9 +++++++++
>  mm/memory.c          | 9 +++++++++
>  2 files changed, 18 insertions(+)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 993693b38109..523c2411a135 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -528,6 +528,15 @@ static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry)
>  	return NULL;
>  }
>  
> +static inline struct swap_info_struct *get_swap_device(swp_entry_t entry)
> +{
> +	return NULL;
> +}
> +
> +static inline void put_swap_device(struct swap_info_struct *si)
> +{
> +}
> +
>  #define swap_address_space(entry)		(NULL)
>  #define get_nr_swap_pages()			0L
>  #define total_swap_pages			0L
> diff --git a/mm/memory.c b/mm/memory.c
> index 27014c3bde9f..7a2fe12cf641 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3311,6 +3311,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>  	struct page *page = NULL, *swapcache;
> +	struct swap_info_struct *si = NULL;
>  	swp_entry_t entry;
>  	pte_t pte;
>  	int locked;
> @@ -3338,6 +3339,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		goto out;
>  	}
>  
> +	/* Prevent swapoff from happening to us. */
> +	si = get_swap_device(entry);
> +	if (unlikely(!si))
> +		goto out;
>  
>  	delayacct_set_flag(current, DELAYACCT_PF_SWAPIN);
>  	page = lookup_swap_cache(entry, vma, vmf->address);
> @@ -3514,6 +3519,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  unlock:
>  	pte_unmap_unlock(vmf->pte, vmf->ptl);
>  out:
> +	if (si)
> +		put_swap_device(si);
>  	return ret;
>  out_nomap:
>  	pte_unmap_unlock(vmf->pte, vmf->ptl);
> @@ -3525,6 +3532,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		unlock_page(swapcache);
>  		put_page(swapcache);
>  	}
> +	if (si)
> +		put_swap_device(si);
>  	return ret;
>  }