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

[정요한(JOUNG YOHAN) Mobile AE]

> From: Chao Yu <chao@xxxxxxxxxx>
> Sent: Thursday, March 27, 2025 4:30 PM
> To: 정요한(JOUNG YOHAN) Mobile AE <yohan.joung@xxxxxx>; Yohan Joung
> <jyh429@xxxxxxxxx>; jaegeuk@xxxxxxxxxx; daeho43@xxxxxxxxx
> Cc: chao@xxxxxxxxxx; linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; 김필현(KIM PILHYUN) Mobile AE <pilhyun.kim@xxxxxx>
> 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@xxxxxx wrote:
> >> From: Chao Yu <chao@xxxxxxxxxx>
> >> Sent: Thursday, March 27, 2025 3:02 PM
> >> To: Yohan Joung <jyh429@xxxxxxxxx>; jaegeuk@xxxxxxxxxx;
> >> daeho43@xxxxxxxxx
> >> Cc: chao@xxxxxxxxxx; linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx; linux-
> >> kernel@xxxxxxxxxxxxxxx; 정요한(JOUNG YOHAN) Mobile AE
> >> <yohan.joung@xxxxxx>
> >> 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;
        }
> 
> >
> > 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@xxxxxxxxxx
> >>
> >> Thanks,
> >>
> >>>
> >>> Signed-off-by: Yohan Joung <yohan.joung@xxxxxx>
> >>> ---
> >>>  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);
> >