Re: [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 2025/3/27 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?

IMO, we can clear next_victim_seg[] once section is free in
__set_test_and_free()? something like this:

---
  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);
>>>
>