> From: Chao Yu <c...@kernel.org>
> Sent: Thursday, March 27, 2025 10:48 PM
> To: 정요한(JOUNG YOHAN) Mobile AE <yohan.jo...@sk.com>; Yohan Joung
> <jyh...@gmail.com>; jaeg...@kernel.org; daeh...@gmail.com
> Cc: c...@kernel.org; linux-f2fs-devel@lists.sourceforge.net; linux-
> ker...@vger.kernel.org; 김필현(KIM PILHYUN) Mobile AE <pilhyun....@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.jo...@sk.com wrote:
> >> From: Chao Yu <c...@kernel.org>
> >> Sent: Thursday, March 27, 2025 4:30 PM
> >> To: 정요한(JOUNG YOHAN) Mobile AE <yohan.jo...@sk.com>; Yohan Joung
> >> <jyh...@gmail.com>; jaeg...@kernel.org; daeh...@gmail.com
> >> Cc: c...@kernel.org; linux-f2fs-devel@lists.sourceforge.net; linux-
> >> ker...@vger.kernel.org; 김필현(KIM PILHYUN) Mobile AE
> >> <pilhyun....@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.jo...@sk.com wrote:
> >>>> From: Chao Yu <c...@kernel.org>
> >>>> Sent: Thursday, March 27, 2025 3:02 PM
> >>>> To: Yohan Joung <jyh...@gmail.com>; jaeg...@kernel.org;
> >>>> daeh...@gmail.com
> >>>> Cc: c...@kernel.org; linux-f2fs-devel@lists.sourceforge.net; linux-
> >>>> ker...@vger.kernel.org; 정요한(JOUNG YOHAN) Mobile AE
> >>>> <yohan.jo...@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.
>
> 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-
> >>>> c...@kernel.org
> >>>>
> >>>> Thanks,
> >>>>
> >>>>>
> >>>>> Signed-off-by: Yohan Joung <yohan.jo...@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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