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Message-ID: <3f5c26c0-f44c-4639-a636-8c01c22405f6@kernel.org>
Date: Mon, 31 Mar 2025 19:36:08 +0800
From: Chao Yu <chao@...nel.org>
To: "yohan.joung" <yohan.joung@...com>, linux-f2fs-devel@...ts.sourceforge.net
Cc: chao@...nel.org, jaegeuk@...nel.org, jyh429@...il.com,
linux-kernel@...r.kernel.org, pilhyun.kim@...com
Subject: 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@...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,
>>>
>>> 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
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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