Re: [f2fs-dev] [PATCH] f2fs: Fix indefinite loop in f2fs_gc()

[Chao Yu <yuchao0@huawei.com>]

Hi Sahitya,

On 2019/7/31 11:41, Sahitya Tummala wrote:
> Hi Chao,
> 
> On Tue, Jul 30, 2019 at 08:35:46PM +0800, Chao Yu wrote:
>> Hi Sahitya,
>>
>> On 2019/7/30 12:36, Sahitya Tummala wrote:
>>> Hi Chao,
>>>
>>> On Tue, Jul 30, 2019 at 12:00:45AM +0800, Chao Yu wrote:
>>>> Hi Sahitya,
>>>>
>>>> On 2019-7-29 13:20, Sahitya Tummala wrote:
>>>>> Policy - foreground GC, LFS mode and greedy GC mode.
>>>>>
>>>>> Under this policy, f2fs_gc() loops forever to GC as it doesn't have
>>>>> enough free segements to proceed and thus it keeps calling gc_more
>>>>> for the same victim segment.  This can happen if the selected victim
>>>>> segment could not be GC'd due to failed blkaddr validity check i.e.
>>>>> is_alive() returns false for the blocks set in current validity map.
>>>>>
>>>>> Fix this by not resetting the sbi->cur_victim_sec to NULL_SEGNO, when
>>>>> the segment selected could not be GC'd. This helps to select another
>>>>> segment for GC and thus helps to proceed forward with GC.
>>>>>
>>>>> Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
>>>>> ---
>>>>>  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 8974672..7bbcc4a 100644
>>>>> --- a/fs/f2fs/gc.c
>>>>> +++ b/fs/f2fs/gc.c
>>>>> @@ -1303,7 +1303,7 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync,
>>>>>  		round++;
>>>>>  	}
>>>>>  
>>>>> -	if (gc_type == FG_GC)
>>>>> +	if (gc_type == FG_GC && seg_freed)
>>>>>  		sbi->cur_victim_sec = NULL_SEGNO;
>>>>
>>>> In some cases, we may remain last victim in sbi->cur_victim_sec, and jump out of
>>>> GC cycle, then SSR can skip the last victim due to sec_usage_check()...
>>>>
>>>
>>> I see. I have a few questions on how to fix this issue. Please share your
>>> comments.
>>>
>>> 1. Do you think the scenario described is valid? It happens rarely, not very
>>
>> IIRC, we suffered endless gc loop due to there is valid block belong to an
>> opened atomic write file. (because we will skip directly once we hit atomic file)
>>
>> For your case, I'm not sure that would happen, did you look into is_alive(), why
>> will it fail? block address not match? If so, it looks like summary info and
>> dnode block and nat entry are inconsistent.
> 
> Yes, from the ramdumps, I could see that block address is not matching and
> hence, is_alive() could fail in the issue scenario. Have you observed any such
> cases before? What could be the reason for this mismatch?

Alright, I didn't suffer such case before...

I don't know, too few clues to find the root cause. I guess maybe:
- random data caused by emmc/ufs firmware bugs
- bit-flip or memory overflow
- f2fs bugs

So, for the solution, I suggest to detect such inconsistency, and tag in
somewhere to just get rid of selecting the corrupted section.

BTW, do you try fsck on that image? what's the result?

Thanks,

> 
> Thanks,
> 
>>
>>> easy to reproduce.  From the dumps, I see that only block is set as valid in
>>> the sentry->cur_valid_map for which I see that summary block check is_alive()
>>> could return false. As only one block is set as valid, chances are there it
>>> can be always selected as the victim by get_victim_by_default() under FG_GC.
>>>
>>> 2. What are the possible scenarios where summary block check is_alive() could
>>> fail for a segment?
>>
>> I guess, maybe after check_valid_map(), the block is been truncated before
>> is_alive(). If so the victim should be prefree directly instead of being
>> selected again...
>>
>>>
>>> 3. How does GC handle such segments?
>>
>> I think that's not a normal case, or I'm missing something.
>>
>> Thanks,
>>
>>>
>>> Thanks,
>>>
>>>> Thanks,
>>>>
>>>>>  
>>>>>  	if (sync)
>>>>>
>>>
> 


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