Re: [f2fs-dev] [External Mail] Re: [External Mail] Re: [PATCH] f2fs: prevent the current section from being selected as a victim during garbage collection

[Chao Yu <chao@kernel.org>]

On 2025/3/28 11:40, yohan.joung wrote:
>> From: Chao Yu <chao@kernel.org>
>> Sent: Thursday, March 27, 2025 10:48 PM
>> To: 정요한(JOUNG YOHAN) Mobile AE <yohan.joung@sk.com>; Yohan Joung
>> <jyh429@gmail.com>; jaegeuk@kernel.org; daeho43@gmail.com
>> Cc: chao@kernel.org; linux-f2fs-devel@lists.sourceforge.net; linux-
>> kernel@vger.kernel.org; 김필현(KIM PILHYUN) Mobile AE <pilhyun.kim@sk.com>
>> Subject: [External Mail] Re: [External Mail] Re: [External Mail] Re:
>> [PATCH] f2fs: prevent the current section from being selected as a victim
>> during garbage collection
>>
>> On 2025/3/27 16:00, yohan.joung@sk.com wrote:
>>>> From: Chao Yu <chao@kernel.org>
>>>> Sent: Thursday, March 27, 2025 4:30 PM
>>>> To: 정요한(JOUNG YOHAN) Mobile AE <yohan.joung@sk.com>; Yohan Joung
>>>> <jyh429@gmail.com>; jaegeuk@kernel.org; daeho43@gmail.com
>>>> Cc: chao@kernel.org; linux-f2fs-devel@lists.sourceforge.net; linux-
>>>> kernel@vger.kernel.org; 김필현(KIM PILHYUN) Mobile AE
>>>> <pilhyun.kim@sk.com>
>>>> Subject: [External Mail] Re: [External Mail] Re: [PATCH] f2fs:
>>>> prevent the current section from being selected as a victim during
>>>> garbage collection
>>>>
>>>> On 3/27/25 14:43, yohan.joung@sk.com wrote:
>>>>>> From: Chao Yu <chao@kernel.org>
>>>>>> Sent: Thursday, March 27, 2025 3:02 PM
>>>>>> To: Yohan Joung <jyh429@gmail.com>; jaegeuk@kernel.org;
>>>>>> daeho43@gmail.com
>>>>>> Cc: chao@kernel.org; linux-f2fs-devel@lists.sourceforge.net; linux-
>>>>>> kernel@vger.kernel.org; 정요한(JOUNG YOHAN) Mobile AE
>>>>>> <yohan.joung@sk.com>
>>>>>> Subject: [External Mail] Re: [PATCH] f2fs: prevent the current
>>>>>> section from being selected as a victim during garbage collection
>>>>>>
>>>>>> On 3/26/25 22:14, Yohan Joung wrote:
>>>>>>> When selecting a victim using next_victim_seg in a large section,
>>>>>>> the selected section might already have been cleared and
>>>>>>> designated as the new current section, making it actively in use.
>>>>>>> This behavior causes inconsistency between the SIT and SSA.
>>>>>>
>>>>>> Hi, does this fix your issue?
>>>>>
>>>>> This is an issue that arises when dividing a large section into
>>>>> segments for garbage collection.
>>>>> caused by the background GC (garbage collection) thread in large
>>>>> section
>>>>> f2fs_gc(victim_section) ->
>>>>> f2fs_clear_prefree_segments(victim_section)->
>>>>> cursec(victim_section) -> f2fs_gc(victim_section by next_victim_seg)
>>>>
>>>> I didn't get it, why f2fs_get_victim() will return section which is
>>>> used by curseg? It should be avoided by checking w/ sec_usage_check().
>>>>
>>>> Or we missed to check gcing section which next_victim_seg points to
>>>> during get_new_segment()?
>>>>
>>>> Can this happen?
>>>>
>>>> e.g.
>>>> - bggc selects sec #0
>>>> - next_victim_seg: seg #0
>>>> - migrate seg #0 and stop
>>>> - next_victim_seg: seg #1
>>>> - checkpoint, set sec #0 free if sec #0 has no valid blocks
>>>> - allocate seg #0 in sec #0 for curseg
>>>> - curseg moves to seg #1 after allocation
>>>> - bggc tries to migrate seg #1
>>>>
>>>> Thanks,
>>> That's correct
>>> In f2fs_get_victim, we use next_victim_seg to directly jump to
>>> got_result, thereby bypassing sec_usage_check What do you think about
>>> this change?
>>>
>>> @@ -850,15 +850,20 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi,
>> unsigned int *result,
>>>                           p.min_segno = sbi->next_victim_seg[BG_GC];
>>>                           *result = p.min_segno;
>>>                           sbi->next_victim_seg[BG_GC] = NULL_SEGNO;
>>> -                       goto got_result;
>>>                   }
>>>                   if (gc_type == FG_GC &&
>>>                                   sbi->next_victim_seg[FG_GC] != NULL_SEGNO) {
>>>                           p.min_segno = sbi->next_victim_seg[FG_GC];
>>>                           *result = p.min_segno;
>>>                           sbi->next_victim_seg[FG_GC] = NULL_SEGNO;
>>> -                       goto got_result;
>>>                   }
>>> +
>>> +               secno = GET_SEC_FROM_SEG(sbi, segno);
>>> +
>>> +               if (sec_usage_check(sbi, secno))
>>> +                       goto next;
>>> +
>>> +               goto got_result;
>>>           }
>>
>> But still allocator can assign this segment after sec_usage_check() in
>> race condition, right?
> Since the BG GC using next_victim  takes place after the SIT update in do_checkpoint,
> it seems unlikely that a race condition with sec_usage_check will occur.

I mean this:

- gc_thread
  - f2fs_gc
   - f2fs_get_victim
    - sec_usage_check --- segno #1 is not used in any cursegs
					- f2fs_allocate_data_block
					 - new_curseg
					  - get_new_segment find segno #1
					
   - do_garbage_collect

Thanks,

>>
>> IMO, we can clear next_victim_seg[] once section is free in
>> __set_test_and_free()? something like this:
> I will test it according to your suggestion.
> If there are no issues, can I submit it again with the patch?
> Thanks
>>
>> ---
>>    fs/f2fs/segment.h | 13 ++++++++++---
>>    1 file changed, 10 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h index
>> 0465dc00b349..826e37999085 100644
>> --- a/fs/f2fs/segment.h
>> +++ b/fs/f2fs/segment.h
>> @@ -473,9 +473,16 @@ static inline void __set_test_and_free(struct
>> f2fs_sb_info *sbi,
>>    			goto skip_free;
>>    		next = find_next_bit(free_i->free_segmap,
>>    				start_segno + SEGS_PER_SEC(sbi), start_segno);
>> -		if (next >= start_segno + usable_segs) {
>> -			if (test_and_clear_bit(secno, free_i->free_secmap))
>> -				free_i->free_sections++;
>> +		if ((next >= start_segno + usable_segs) &&
>> +			test_and_clear_bit(secno, free_i->free_secmap)) {
>> +			free_i->free_sections++;
>> +
>> +			if (GET_SEC_FROM_SEG(sbi->next_victim_seg[BG_GC]) ==
>> +									secno)
>> +				sbi->next_victim_seg[BG_GC] = NULL_SEGNO;
>> +			if (GET_SEC_FROM_SEG(sbi->next_victim_seg[FG_GC]) ==
>> +									secno)
>> +				sbi->next_victim_seg[FG_GC] = NULL_SEGNO;
>>    		}
>>    	}
>>    skip_free:
>> --
>> 2.40.1
>>
>>>>
>>>>>
>>>>> Because the call stack is different, I think that in order to handle
>>>>> everything at once, we need to address it within do_garbage_collect,
>>>>> or otherwise include it on both sides.
>>>>> What do you think?
>>>>>
>>>>> [30146.337471][ T1300] F2FS-fs (dm-54): Inconsistent segment (70961)
>>>>> type [0, 1] in SSA and SIT [30146.346151][ T1300] Call trace:
>>>>> [30146.346152][ T1300]  dump_backtrace+0xe8/0x10c [30146.346157][
>>>>> T1300]  show_stack+0x18/0x28 [30146.346158][ T1300]
>>>>> dump_stack_lvl+0x50/0x6c [30146.346161][ T1300]
>>>>> dump_stack+0x18/0x28 [30146.346162][ T1300]
>>>>> f2fs_stop_checkpoint+0x1c/0x3c [30146.346165][ T1300]
>>>>> do_garbage_collect+0x41c/0x271c [30146.346167][ T1300]
>>>>> f2fs_gc+0x27c/0x828 [30146.346168][ T1300]
>>>>> gc_thread_func+0x290/0x88c [30146.346169][ T1300]
>>>>> kthread+0x11c/0x164 [30146.346172][ T1300]
>>>>> ret_from_fork+0x10/0x20
>>>>>
>>>>> struct curseg_info : 0xffffff803f95e800 {
>>>>> 	segno        : 0x11531 : 70961
>>>>> }
>>>>>
>>>>> struct f2fs_sb_info : 0xffffff8811d12000 {
>>>>> 	next_victim_seg[0] : 0x11531 : 70961 }
>>>>>
>>>>>>
>>>>>> https://lore.kernel.org/linux-f2fs-devel/20250325080646.3291947-2-
>>>>>> chao@kernel.org
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>> Signed-off-by: Yohan Joung <yohan.joung@sk.com>
>>>>>>> ---
>>>>>>>    fs/f2fs/gc.c | 4 ++++
>>>>>>>    1 file changed, 4 insertions(+)
>>>>>>>
>>>>>>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c index
>>>>>>> 2b8f9239bede..4b5d18e395eb 100644
>>>>>>> --- a/fs/f2fs/gc.c
>>>>>>> +++ b/fs/f2fs/gc.c
>>>>>>> @@ -1926,6 +1926,10 @@ int f2fs_gc(struct f2fs_sb_info *sbi,
>>>>>>> struct
>>>>>> f2fs_gc_control *gc_control)
>>>>>>>    		goto stop;
>>>>>>>    	}
>>>>>>>
>>>>>>> +	if (__is_large_section(sbi) &&
>>>>>>> +			IS_CURSEC(sbi, GET_SEC_FROM_SEG(sbi, segno)))
>>>>>>> +		goto stop;
>>>>>>> +
>>>>>>>    	seg_freed = do_garbage_collect(sbi, segno, &gc_list, gc_type,
>>>>>>>    				gc_control->should_migrate_blocks,
>>>>>>>    				gc_control->one_time);
>>>>>
>>>
>