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

[Daeho Jeong]

On Fri, Aug 21, 2026 at 1:29 AM Yeongjin Gil <youngjin.gil@xxxxxxxxxxx> wrote:
>
> > 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.
> >
> I wonder if the free-section adjustment could still be relevant
> to the SSR/LFS allocation decision during resize.
>
> Could you please also check whether free_sections may affect
> allocation through the following paths?
>
>   free_segment_range()
>     -> f2fs_allocate_segment_for_resize()
>       -> f2fs_need_SSR()
>       -> new_curseg()
>         -> get_new_segment()
>
> Also, when a current segment becomes full during block migration:
>
>   free_segment_range()
>     -> f2fs_gc_range()
>       -> do_garbage_collect()
>         -> f2fs_allocate_data_block()
>           -> need_new_seg()
>             -> f2fs_need_SSR()
>             -> new_curseg()
>               -> get_new_segment()
>
> If free_sections still include free sections in the range being
> removed, f2fs_need_SSR() may choose LFS allocation instead of SSR.
> Since get_new_segment() searches only within the temporarily reduced
> MAIN_SECS range, it may fail to find a free section.
>
> Could you please confirm whether this case also needs to be handled?

Thank you for pointing this out — this is a very sharp and valid observation!

It makes sense to me. Chao, WDYT?

>
> 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;
> > > >>>   }
> > > >>
> > >
> >
> >
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