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Subject: Re: [f2fs-dev] [PATCH 1/2] f2fs: do not select same victim right
 again
To:     Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>
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From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <7d7e8e46-0261-ddec-881a-e720ca2badac@huawei.com>
Date:   Thu, 19 Sep 2019 08:53:34 +0800
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On 2019/9/19 0:47, Jaegeuk Kim wrote:
> On 09/18, Chao Yu wrote:
>> 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?
> 
> FGGC failed in a loop and last victim was remained in cur_victim_sec.

It won't run into a loop because we keep below condition?

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

	if (sync)
		goto stop;

I meant add below logic in addition:

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

	mutex_unlock(&sbi->gc_mutex);

Thanks,

> Next FGGC kicked in and did the same thing again. I don't expect BGGC/SSR
> wants to select this victim much, since it will have CB policy.
> 
>>
>> 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)
>>>>>>>>>>>
>>>>>>>>> .
>>>>>>>>>
>>>>>>> .
>>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>