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

[Chao Yu]

On 2018/7/30 9:38, Jaegeuk Kim wrote:
> On 07/28, Jaegeuk Kim wrote:
>> On 07/29, Chao Yu wrote:
>>> On 2018/7/29 10:59, Jaegeuk Kim wrote:
>>>> On 07/29, Chao Yu wrote:
>>>>> 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?
>>>>
>>>> So, truncating pages under dnode lock can address it?
>>>
>>> Normally, our lock dependency is
>>>
>>> ->writepage()
>>> lock data page -> lock dnode page
>>>
>>> here
>>> lock dnode page -> truncate_pagecache_range::lock data page
>>>
>>> Will easily cause deadlock?
>>
>> Yeah. Can we add an inode flag to bypass GC in this case, then?
> 
> Hmm, BTW, how about using i_gc_rwsem[WRITE] in a very narrow scope?

Oh, I can see that you submitted a patch to change lock dependency to:

f2fs_lock_op() -> down_write(i_gc_rwsem[WRITE])

At a glance, I haven't see any place can cause deadlock now.

> 
> for (index = pg_start; index < pg_end;) {
> 	f2fs_lock_op();
> 	down_write(i_gc_rwsem[WRITE]);
> 	truncate_page_cache_range(index, index + 4k);
> 	f2fs_do_zero_range(&dn, index, end);
> 	up_write(i_gc_rwsem[WRITE]);
> 	f2fs_unlock_op();
> 	f2fs_balance_fs();
> }

Let me update the patch as you suggested.

Thanks,

> 
>>
>>>
>>> Thanks,
>>>
>>>>
>>>>>
>>>>> 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
>>
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