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

[Chao Yu]

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.

Thanks,

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