Re: [f2fs-dev] [PATCH] f2fs: accurately adjust free_sections during free_segment_range

[Daeho Jeong]

On Thu, Aug 20, 2026 at 6:19 AM Chao Yu <chao@xxxxxxxxxx> wrote:
>
> On 8/20/26 02:26, Daeho Jeong wrote:
> > On Tue, Aug 18, 2026 at 8:57 PM Chao Yu <chao@xxxxxxxxxx> wrote:
> >>
> >> On 8/19/26 01:14, Daeho Jeong wrote:
> >>> From: Daeho Jeong <daehojeong@xxxxxxxxxx>
> >>>
> >>> In free_segment_range(), MAIN_SECS(sbi) is temporarily reduced by `secs`
> >>> while valid blocks in the truncated range are evacuated by GC.
> >>>
> >>> However, if any sections within the truncated range were already free,
> >>> failing to deduct them from FREE_I(sbi)->free_sections leads to an
> >>> over-estimation of available space in the reduced main area, causing
> >>> inconsistent free section accounting.
> >>
> >> Can you please show me an example for above case? I didn't get it.
> >
> > Here is a concrete example explaining why this adjustment is needed:
>
> Thanks for the detailed explanation.
>
> >
> > Suppose:
> > - Total main sections: MAIN_SECS = 100 (sections 0 .. 99)
> > - Total free sections: free_sections = 30
> > - We want to shrink the filesystem by 10 sections (secs = 10, range 90 .. 99).
> > - Within the truncated range (sections 90 .. 99):
> >    * 6 sections are already free (free_secmap bit is 0)
> >    * 4 sections are in-use with valid blocks that need to be migrated by GC.
> > When free_segment_range() enters:
> > 1. MAIN_SECS is temporarily reduced from 100 to 90 so that new block
> >     allocations are constrained to sections 0 .. 89.
> > 2. The actual number of free sections available in the reduced range
> >     (0 .. 89) is only 24 (30 - 6 = 24).
> > 3. Without this patch:
> >     - free_sections remains 30 while MAIN_SECS is 90.
> >     - During the subsequent GC migrations, free section checks (such as
> >       has_not_enough_free_secs()) will over-estimate available space by 6
>
> But free_segment_range() won't call into has_not_enough_free_secs(), if I'm not
> missing anything.

Oh, right. I missed it's FG_GC.
I think we can drop this patch.

Thanks.

>
> Not sure, maybe you mean other threads will call has_not_enough_free_secs(), are
> you worried about that we may miss chances to call fggc earlier in below cases:
>
> f2fs_balance_fs -> has_enough_free_secs -> f2fs_gc? I guess it will blocked on
> gc_lock.

>
> >       sections in the active 0 .. 89 range.
> >     - If free_segment_range() fails midway (e.g. -EAGAIN), free_sections
> >       accounting becomes inconsistent.
>
> Why free_sections accounting becomes inconsistent if free_segment_range() fails
> midway?
>
> Thanks,
>
> > With this patch:
> > - We count the 6 already-free sections in the truncated range (90 .. 99) and
> >    deduct them from free_sections upon entering (30 - 6 = 24), perfectly
> >    matching the actual free sections in the active range 0 .. 89.
> > - On exit, the deducted amount is restored, keeping free_sections consistent
> >    throughout the entire resize lifecycle.
> >
> > Hope this clarifies the scenario!
> >
> > Thanks,
> >
> >>
> >> BTW, it needs to rebase this patch on dev-test branch.
> >>
> >> Thanks,
> >>
> >>>
> >>> Fix this by calculating the number of already-free sections in the
> >>> truncated range under segmap_lock, deducting them from free_sections upon
> >>> entering free_segment_range(), and restoring them under segmap_lock on exit.
> >>>
> >>> Signed-off-by: Daeho Jeong <daehojeong@xxxxxxxxxx>
> >>> Signed-off-by: Sunmin Jeong <s_min.jeong@xxxxxxxxxxx>
> >>> ---
> >>>   fs/f2fs/gc.c | 15 ++++++++++++++-
> >>>   1 file changed, 14 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> >>> index 787133ee2eb2..f3a6fc6d08ae 100644
> >>> --- a/fs/f2fs/gc.c
> >>> +++ b/fs/f2fs/gc.c
> >>> @@ -2200,8 +2200,9 @@ int f2fs_gc_range(struct f2fs_sb_info *sbi,
> >>>   static int free_segment_range(struct f2fs_sb_info *sbi,
> >>>                                unsigned int secs, bool dry_run)
> >>>   {
> >>> -     unsigned int next_inuse, start, end;
> >>> +     unsigned int secno, next_inuse, start, end, end_secno;
> >>>        struct cp_control cpc = { CP_RESIZE, 0, 0, 0 };
> >>> +     unsigned int freed_secs = 0;
> >>>        int gc_mode, gc_type;
> >>>        int err = 0;
> >>>        int type;
> >>> @@ -2210,6 +2211,7 @@ static int free_segment_range(struct f2fs_sb_info *sbi,
> >>>        MAIN_SECS(sbi) -= secs;
> >>>        start = MAIN_SECS(sbi) * SEGS_PER_SEC(sbi);
> >>>        end = MAIN_SEGS(sbi) - 1;
> >>> +     end_secno = GET_SEC_FROM_SEG(sbi, end);
> >>>
> >>>        mutex_lock(&DIRTY_I(sbi)->seglist_lock);
> >>>        for (gc_mode = 0; gc_mode < MAX_GC_POLICY; gc_mode++)
> >>> @@ -2221,6 +2223,14 @@ static int free_segment_range(struct f2fs_sb_info *sbi,
> >>>                        sbi->next_victim_seg[gc_type] = NULL_SEGNO;
> >>>        mutex_unlock(&DIRTY_I(sbi)->seglist_lock);
> >>>
> >>> +     spin_lock(&FREE_I(sbi)->segmap_lock);
> >>> +     for (secno = MAIN_SECS(sbi); secno <= end_secno; secno++) {
> >>> +             if (!test_bit(secno, FREE_I(sbi)->free_secmap))
> >>> +                     freed_secs++;
> >>> +     }
> >>> +     FREE_I(sbi)->free_sections -= freed_secs;
> >>> +     spin_unlock(&FREE_I(sbi)->segmap_lock);
> >>> +
> >>>        /* Move out cursegs from the target range */
> >>>        for (type = CURSEG_HOT_DATA; type < NR_CURSEG_TYPE; type++) {
> >>>                err = f2fs_allocate_segment_for_resize(sbi, type, start, end);
> >>> @@ -2245,6 +2255,9 @@ static int free_segment_range(struct f2fs_sb_info *sbi,
> >>>                f2fs_bug_on(sbi, 1);
> >>>        }
> >>>   out:
> >>> +     spin_lock(&FREE_I(sbi)->segmap_lock);
> >>> +     FREE_I(sbi)->free_sections += freed_secs;
> >>> +     spin_unlock(&FREE_I(sbi)->segmap_lock);
> >>>        MAIN_SECS(sbi) += secs;
> >>>        return err;
> >>>   }
> >>
>