Re: [External Mail] 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 15:25, yohan.joung wrote:
>> 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,
> 
> 						  do_checkpoint sec0 free
> 						  If sec0 is not freed, then segno1 within sec0 cannot be allocated
> - gc_thread
>    - f2fs_gc
>     - f2fs_get_victim
>      - sec_usage_check  --- segno #1 is not used in any cursegs (but sec0 is already used)
> 							- f2fs_allocate_data_block
> 							- new_curseg
> 							- get_new_segment find segno #1
> 					
>     - do_garbage_collect
> 
> I appreciate your patch, it is under testing.
> but I'm wondering if there's a risk of a race condition in this situation

Oh, yes, I may missed that get_new_segment can return a free segment in
partial used section.

So what do you think of this?
- check CURSEG() in do_garbage_collect() and get_victim()
- reset next_victim_seg[] in get_new_segment() and __set_test_and_free()
during checkpoint.

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);
>>>>>>>
>>>>>
>>>
>