Re: [f2fs-dev] [PATCH] f2fs: fix to skip empty sections in f2fs_get_victim

[Daeho Jeong]

On Thu, Mar 12, 2026 at 2:07 AM Chao Yu <chao@xxxxxxxxxx> wrote:
>
> On 2026/3/12 00:05, Daeho Jeong wrote:
> > On Wed, Mar 11, 2026 at 6:44 AM Chao Yu <chao@xxxxxxxxxx> wrote:
> >>
> >> On 2026/3/11 01:54, Daeho Jeong wrote:
> >>> From: Daeho Jeong <daehojeong@xxxxxxxxxx>
> >>>
> >>> In age-based victim selection (ATGC, AT_SSR, or GC_CB), f2fs_get_victim
> >>> can encounter sections with zero valid blocks. This situation often
> >>> arises when checkpoint is disabled or due to race conditions between
> >>> SIT updates and dirty list management.
> >>>
> >>> In such cases, f2fs_get_section_mtime() returns INVALID_MTIME, which
> >>> subsequently triggers a fatal f2fs_bug_on(sbi, mtime == INVALID_MTIME)
> >>> in add_victim_entry() or get_cb_cost().
> >>>
> >>> This patch adds a check in f2fs_get_victim's selection loop to skip
> >>> sections with no valid blocks. This prevents unnecessary age
> >>> calculations for empty sections and avoids the associated kernel panic.
> >>> This change also allows removing redundant checks in add_victim_entry().
> >>>
> >>> Signed-off-by: Daeho Jeong <daehojeong@xxxxxxxxxx>
> >>> ---
> >>>    fs/f2fs/gc.c | 9 +++------
> >>>    1 file changed, 3 insertions(+), 6 deletions(-)
> >>>
> >>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> >>> index 2e0f67946914..981eac629fe9 100644
> >>> --- a/fs/f2fs/gc.c
> >>> +++ b/fs/f2fs/gc.c
> >>> @@ -521,12 +521,6 @@ static void add_victim_entry(struct f2fs_sb_info *sbi,
> >>>        struct sit_info *sit_i = SIT_I(sbi);
> >>>        unsigned long long mtime = 0;
> >>>
> >>> -     if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED))) {
> >>> -             if (p->gc_mode == GC_AT &&
> >>> -                     get_valid_blocks(sbi, segno, true) == 0)
> >>> -                     return;
> >>> -     }
> >>> -
> >>>        mtime = f2fs_get_section_mtime(sbi, segno);
> >>>        f2fs_bug_on(sbi, mtime == INVALID_MTIME);
> >>>
> >>> @@ -889,6 +883,9 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
> >>>                if (sec_usage_check(sbi, secno))
> >>>                        goto next;
> >>>
> >>> +             if (!get_valid_blocks(sbi, segno, true))
> >>> +                     goto next;
> >>
> >> Well, for f2fs_get_victim(, AT_SSR), once there are no dirty segment, if we
> >> don't count free segment as candidates, then, we can not find any valid victim?
> >
> > Oh, AT_SSR needs to select the free section in this case?
>
> I think so, for extreme case.
>
> > I am confused. Why do we need the below logic?
> > Looks like WA for the AT_SSR case?
> >
> > In f2fs_get_section_mtime()
> > out:
> >          if (unlikely(mtime == INVALID_MTIME))
> >                  mtime -= 1;
> >          return mtime;
>
> There are two conditions, in a section:
>
> a) if there are no valid blocks, it will return INVALID_MTIME.
> b) if there are vaild blocks, it tries to return mtime which is calculated, but
> if unlucky the calculated mtime is equal to INVALID_MTIME, in order to distinguish
> from case a), we will return INVALID_MTIME - 1 instead.

If we find a free segment and pass it to f2fs_get_section_mtime() for
(!__is_large_section(sbi)) case.
What is the expected output of it? (INVALID_MTIME - 1)?
I don't think this is just an unlucky case. Is this expected result?

>
> Thanks,
>
> >
> >
> >>
> >> Thanks,
> >>
> >>> +
> >>>                /* Don't touch checkpointed data */
> >>>                if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED))) {
> >>>                        if (p.alloc_mode == LFS) {
> >>
>