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

[Chao Yu]

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.

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;
> > >   }