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Message-ID: <3a2149b5-68b7-413d-8098-27ee5049f146@kernel.org>
Date: Fri, 28 Mar 2025 14:17:08 +0800
From: Chao Yu <chao@...nel.org>
To: "yohan.joung" <yohan.joung@...com>, linux-f2fs-devel@...ts.sourceforge.net
Cc: chao@...nel.org, daeho43@...il.com, jaegeuk@...nel.org, jyh429@...il.com,
linux-kernel@...r.kernel.org, pilhyun.kim@...com
Subject: 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 2025/3/28 11:40, yohan.joung wrote:
>> From: Chao Yu <chao@...nel.org>
>> Sent: Thursday, March 27, 2025 10:48 PM
>> To: 정요한(JOUNG YOHAN) Mobile AE <yohan.joung@...com>; Yohan Joung
>> <jyh429@...il.com>; jaegeuk@...nel.org; daeho43@...il.com
>> Cc: chao@...nel.org; linux-f2fs-devel@...ts.sourceforge.net; linux-
>> kernel@...r.kernel.org; 김필현(KIM PILHYUN) Mobile AE <pilhyun.kim@...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@...com wrote:
>>>> From: Chao Yu <chao@...nel.org>
>>>> Sent: Thursday, March 27, 2025 4:30 PM
>>>> To: 정요한(JOUNG YOHAN) Mobile AE <yohan.joung@...com>; Yohan Joung
>>>> <jyh429@...il.com>; jaegeuk@...nel.org; daeho43@...il.com
>>>> Cc: chao@...nel.org; linux-f2fs-devel@...ts.sourceforge.net; linux-
>>>> kernel@...r.kernel.org; 김필현(KIM PILHYUN) Mobile AE
>>>> <pilhyun.kim@...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@...com wrote:
>>>>>> From: Chao Yu <chao@...nel.org>
>>>>>> Sent: Thursday, March 27, 2025 3:02 PM
>>>>>> To: Yohan Joung <jyh429@...il.com>; jaegeuk@...nel.org;
>>>>>> daeho43@...il.com
>>>>>> Cc: chao@...nel.org; linux-f2fs-devel@...ts.sourceforge.net; linux-
>>>>>> kernel@...r.kernel.org; 정요한(JOUNG YOHAN) Mobile AE
>>>>>> <yohan.joung@...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@...nel.org
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>> Signed-off-by: Yohan Joung <yohan.joung@...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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