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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: <e823b534-f4de-7f59-0c26-ff2c463260d1@huawei.com>
Date:   Wed, 18 Sep 2019 09:43:44 +0800
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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.

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)
>>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>