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

[Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

On 4/1/25 09:51, yohan.joung wrote:
>> From: Chao Yu <chao@kernel.org>
>> Sent: Monday, March 31, 2025 8:36 PM
>> To: 정요한(JOUNG YOHAN) Mobile AE <yohan.joung@sk.com>; linux-f2fs-
>> devel@lists.sourceforge.net
>> Cc: chao@kernel.org; jaegeuk@kernel.org; jyh429@gmail.com; linux-
>> kernel@vger.kernel.org; 김필현(KIM PILHYUN) Mobile AE <pilhyun.kim@sk.com>
>> Subject: [External Mail] 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
>>
>> On 3/31/25 13:13, yohan.joung wrote:
>>>> 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,
>>>
>>> How about using victim_secmap?
>>> gc_thread
>>> 				mutex_lock(&DIRTY_I(sbi)->seglist_lock);
>>> 				__set_test_and_free
>>> 				check cur section next_victim clear
>>> 				mutex_unlock(&dirty_i->seglist_lock);
>>>
>>> mutex_lock(&dirty->seglist_lock);
>>> f2fs_get_victim
>>> mutex_unlock(&dirty_i->seglist_lock);
>>>
>>> static inline void __set_test_and_free(struct f2fs_sb_info *sbi,
>>>                 if (next >= start_segno + usable_segs) {
>>>                         if (test_and_clear_bit(secno, free_i->free_secmap))
>>>                                 free_i->free_sections++;
>>> +
>>> +                       if (test_and_clear_bit(secno, dirty_i->victim_secmap))
>>> +                               sbi->next_victim_seg[BG_GC] =
>>> + NULL_SEGNO;
>>
>> Can this happen?
>>
>> segs_per_sec=2
>>
>> - seg#0 and seg#1 are all dirty
>> - all valid blocks are removed in seg#1
>> - checkpoint -> seg#1 becomes free
>> - gc select this sec and next_victim_seg=seg#0
>> - migrate seg#0, next_victim_seg=seg#1
>> - allocator assigns seg#1 to curseg
>> - gc tries to migrate seg#1

I meant for above case, below change still can not catch it, right? since
next_victim_seg[] was assigned after checkpoint.

+	if (test_and_clear_bit(secno, dirty_i->victim_secmap))
+		sbi->next_victim_seg[BG_GC] = NULL_SEGNO;

Thanks,

>>
>> Thanks,
> The detailed scenario
> segs_per_sec=2
> - seg#0 and seg#1 are all dirty
> - all valid blocks are removed in seg#1
> - gc select this sec and next_victim_seg=seg#0
> - migrate seg#0, next_victim_seg=seg#1
> - checkpoint -> sec(seg#0, seg#1)  becomes free
> - allocator assigns sec(seg#0, seg#1) to curseg
> - gc tries to migrate seg#1
>>
>>>                 }
>>>         }
>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>>>
>>>>>>>> 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/20250325080646.32919
>>>>>>>>>>>> 47
>>>>>>>>>>>> -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);
>>>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>
> 
> 



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