Re: [PATCH] f2fs: avoid race between zero_range and background GC

[Chao Yu]

On 2018/7/29 10:02, Jaegeuk Kim wrote:
> On 07/27, Chao Yu wrote:
>> On 2018/7/27 18:29, Jaegeuk Kim wrote:
>>> On 07/26, Chao Yu wrote:
>>>> Thread A				Background GC
>>>> - f2fs_zero_range
>>>>  - truncate_pagecache_range
>>>> 					- gc_data_segment
>>>> 					 - get_read_data_page
>>>> 					  - move_data_page
>>>> 					   - set_page_dirty
>>>> 					   - set_cold_data
>>>>  - f2fs_do_zero_range
>>>>   - dn->data_blkaddr = NEW_ADDR;
>>>>   - f2fs_set_data_blkaddr
>>>>
>>>> Actually, we don't need to set dirty & checked flag on the page, since
>>>> all valid data in the page should be zeroed by zero_range().
>>>
>>> But, it doesn't matter too much, right?
>>
>> No, if the dirtied page is writebacked after f2fs_do_zero_range(), result of
>> zero_range() should be wrong, as zeroed page contains valid user data.
> 
> How about truncating page caches after block address change or doing it twice
> before and after?

Thread A				Background GC
- f2fs_zero_range
 - truncate_pagecache_range
					- gc_data_segment
					 - get_read_data_page
					  - move_data_page
					   - set_page_dirty
					   - set_cold_data
 - f2fs_do_zero_range
  - dn->data_blkaddr = NEW_ADDR;
  - f2fs_set_data_blkaddr
					bdi-flusher
					- __write_data_page
					 - f2fs_update_data_blkaddr
					 : data_blkaddr has been updated here.
 - truncate_pagecache_range
 : data & dnode has been writebacked before page cache truncation?

How about this case?

Thanks,

> 
>>
>>>
>>>> Use i_gc_rwsem[WRITE] to avoid such race condition.
>>>
>>> Hope to avoid abusing i_gc_rwsem[] tho.
>>
>> Agreed, let's try avoiding until we have to use it.
>>
>> Thanks,
>>
>>>
>>>>
>>>> Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
>>>> ---
>>>>  fs/f2fs/file.c | 2 ++
>>>>  1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>>> index 267ec3794e1e..7bd2412a8c37 100644
>>>> --- a/fs/f2fs/file.c
>>>> +++ b/fs/f2fs/file.c
>>>> @@ -1309,6 +1309,7 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
>>>>  	if (ret)
>>>>  		return ret;
>>>>  
>>>> +	down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>>>>  	down_write(&F2FS_I(inode)->i_mmap_sem);
>>>>  	ret = filemap_write_and_wait_range(mapping, offset, offset + len - 1);
>>>>  	if (ret)
>>>> @@ -1389,6 +1390,7 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
>>>>  	}
>>>>  out_sem:
>>>>  	up_write(&F2FS_I(inode)->i_mmap_sem);
>>>> +	up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>>>>  
>>>>  	return ret;
>>>>  }
>>>> -- 
>>>> 2.18.0.rc1