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

[Miaohe Lin]

On 2021/4/13 9:36, Huang, Ying wrote:
> 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().
> 

Will do. Many thanks.

> 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)
> .
>