Re: [f2fs-dev] [PATCH 1/2] f2fs: do not select same victim right again

[Chao Yu]

On 2019/9/18 11:12, Jaegeuk Kim wrote:
> On 09/18, Chao Yu wrote:
>> On 2019/9/18 4:55, Jaegeuk Kim wrote:
>>> On 09/17, Chao Yu wrote:
>>>> On 2019/9/16 23:37, Jaegeuk Kim wrote:
>>>>> On 09/16, Chao Yu wrote:
>>>>>> On 2019/9/9 20:04, Jaegeuk Kim wrote:
>>>>>>> On 09/09, Chao Yu wrote:
>>>>>>>> On 2019/9/9 16:06, Jaegeuk Kim wrote:
>>>>>>>>> On 09/09, Chao Yu wrote:
>>>>>>>>>> On 2019/9/9 9:25, Jaegeuk Kim wrote:
>>>>>>>>>>> GC must avoid select the same victim again.
>>>>>>>>>>
>>>>>>>>>> Blocks in previous victim will occupy addition free segment, I doubt after this
>>>>>>>>>> change, FGGC may encounter out-of-free space issue more frequently.
>>>>>>>>>
>>>>>>>>> Hmm, actually this change seems wrong by sec_usage_check().
>>>>>>>>> We may be able to avoid this only in the suspicious loop?
>>>>>>>>>
>>>>>>>>> ---
>>>>>>>>>  fs/f2fs/gc.c | 2 +-
>>>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>>>
>>>>>>>>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
>>>>>>>>> index e88f98ddf396..5877bd729689 100644
>>>>>>>>> --- a/fs/f2fs/gc.c
>>>>>>>>> +++ b/fs/f2fs/gc.c
>>>>>>>>> @@ -1326,7 +1326,7 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync,
>>>>>>>>>  		round++;
>>>>>>>>>  	}
>>>>>>>>>  
>>>>>>>>> -	if (gc_type == FG_GC)
>>>>>>>>> +	if (gc_type == FG_GC && seg_freed)
>>>>>>>>
>>>>>>>> That's original solution Sahitya provided to avoid infinite loop of GC, but I
>>>>>>>> suggest to find the root cause first, then we added .invalid_segmap for that
>>>>>>>> purpose.
>>>>>>>
>>>>>>> I've checked the Sahitya's patch. So, it seems the problem can happen due to
>>>>>>> is_alive or atomic_file.
>>>>>>
>>>>>> For some conditions, this doesn't help, for example, two sections contain the
>>>>>> same fewest valid blocks, it will cause to loop selecting them if it fails to
>>>>>> migrate blocks.
>>>>>>
>>>>>> How about keeping it as it is to find potential bug.
>>>>>
>>>>> I think it'd be fine to merge this. Could you check the above scenario in more
>>>>> detail?
>>>>
>>>> I haven't saw this in real scenario yet.
>>>>
>>>> What I mean is if there is a bug (maybe in is_alive()) failing us to GC on one
>>>> section, when that bug happens in two candidates, there could be the same
>>>> condition that GC will run into loop (select A, fail to migrate; select B, fail
>>>> to migrate, select A...).
>>>>
>>>> But I guess the benefit of this change is, if FGGC fails to migrate block due to
>>>> i_gc_rwsem race, selecting another section and later retrying previous one may
>>>> avoid lock race, right?
>>>
>>> In any case, I think this can avoid potenial GC loop. At least to me, it'd be
>>> quite risky, if we remain this just for debugging purpose only.
>>
>> Yup,
>>
>> One more concern is would this cur_victim_sec remain after FGGC? then BGGC/SSR
>> will always skip the section cur_victim_sec points to.
> 
> Then, we can get another loop before using it by BGGC/SSR.

I guess I didn't catch your point, do you mean, if we reset it in the end of
FGGC, we may encounter the loop during BGGC/SSR?

I meant:

f2fs_gc()
...

+	if (gc_type == FG_GC)
+		sbi->cur_victim_sec = NULL_SEGNO;

	mutex_unlock(&sbi->gc_mutex);

	put_gc_inode(&gc_list);
...

Thanks,

> 
>>
>> So could we reset cur_victim_sec in the end of FGGC?
>>
>> Thanks,
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>>>  		sbi->cur_victim_sec = NULL_SEGNO;
>>>>>>>>>  
>>>>>>>>>  	if (sync)
>>>>>>>>>
>>>>>>> .
>>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>