Re: [PATCH] f2fs: fix to trigger a checkpoint in the end of foreground garbage collection

[Chao Yu]

On 2023/4/5 23:55, Jaegeuk Kim wrote:
> On 04/05, Chao Yu wrote:
> > On 2023/4/5 5:39, Jaegeuk Kim wrote:
> > > Can we do like this?
> > >
> > >   From 9a58f0e59364241aa31b555cfe793d278e39b0dc Mon Sep 17 00:00:00 2001
> > > From: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > > Date: Tue, 4 Apr 2023 14:36:00 -0700
> > > Subject: [PATCH] f2fs: do checkpoint when there's not enough free sections
> > >
> > > We didn't do checkpoint in FG_GC case, which may cause losing to reclaim prefree
> > > sctions in time.
> > >
> > > Fixes: 6f8d4455060d ("f2fs: avoid fi->i_gc_rwsem[WRITE] lock in f2fs_gc")
> > > Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
> > > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > > ---
> > >    fs/f2fs/gc.c | 24 +++++++++++-------------
> > >    1 file changed, 11 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > > index 56c53dbe05c9..f1d0dd9c5a6c 100644
> > > --- a/fs/f2fs/gc.c
> > > +++ b/fs/f2fs/gc.c
> > > @@ -1806,6 +1806,7 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > >    	};
> > >    	unsigned int skipped_round = 0, round = 0;
> > >    	unsigned int upper_secs;
> > > +	bool stop_gc = false;
> > >    	trace_f2fs_gc_begin(sbi->sb, gc_type, gc_control->no_bg_gc,
> > >    				gc_control->nr_free_secs,
> > > @@ -1876,19 +1877,15 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > >    				(gc_type == FG_GC) ? sec_freed : 0, 0)) {
> > >    		if (gc_type == FG_GC && sec_freed < gc_control->nr_free_secs)
> > >    			goto go_gc_more;
> > > -		goto stop;
> > > -	}
> > > -
> > > -	/* FG_GC stops GC by skip_count */
> > > -	if (gc_type == FG_GC) {
> > > +		stop_gc = true;
> >
> > I guess below condition is for emergency recycle of prefree segments during
> > foreground GC, in order to avoid exhausting free sections due to to many
> > metadata allocation during CP.
> >
> > 	if (free_sections(sbi) <= upper_secs + NR_GC_CHECKPOINT_SECS &&
> > 				prefree_segments(sbi)) {
> >
> > But for common case, free_sections() is close to reserved_segments(), and
> > upper_secs + NR_GC_CHECKPOINT_SECS value may be far smaller than free_sections(),
> > so checkpoint may not be trggered as expected, IIUC.
> >
> > So it's fine to just trigger CP in the end of foreground garbage collection?
>
> My major concern is to avoid unnecessary checkpointing given multiple FG_GC
> requests were pending in parallel. And, I don't want to add so many combination
> which gives so many corner cases, and feel f2fs_gc() needs to call checkpoint
> automatically in the worst case scenario only.

Alright.

> By the way, do we just need to call checkpoint here including FG_GC as well?

I didn't get it, do you mean?

- f2fs_balance_fs()
 - f2fs_gc() creates prefree segments but not call checkpoint to reclaim

- f2fs_balance_fs()
 - f2fs_gc()
  - detect prefree segments created by last f2fs_balance_fs, then call
f2fs_write_checkpoint to reclaim

Or could you please provide a draft patch? :-P

Thanks,

> 1832
> 1833         if (gc_type == BG_GC && has_not_enough_free_secs(sbi, 0, 0)) {
> 1834                 /*
> 1835                  * For example, if there are many prefree_segments below given
> 1836                  * threshold, we can make them free by checkpoint. Then, we
> 1837                  * secure free segments which doesn't need fggc any more.
> 1838                  */
> 1839                 if (prefree_segments(sbi)) {
> 1840                         ret = f2fs_write_checkpoint(sbi, &cpc);
> 1841                         if (ret)
> 1842                                 goto stop;
> 1843                 }
> 1844                 if (has_not_enough_free_secs(sbi, 0, 0))
> 1845                         gc_type = FG_GC;
> 1846         }
>
> > One other concern is for those path as below:
> > - disable_checkpoint
> > - ioc_gc
> > - ioc_gc_range
> > - ioc_resize
> > ...
>
> I think the upper caller should decide to call checkpoint, if they want to
> reclaim the prefree likewise f2fs_disable_checkpoint.
>
> > We've passed gc_type as FG_GC, but the demand here is to migrate block in time,
> > rather than dirtying blocks, and callers don't expect checkpoint in f2fs_gc(),
> > instead the callers will do the checkpoit as it needs.
> >
> > That means it's better to decouple FG_GC and write_checkpoint behavior, so I
> > added another parameter .reclaim_space to just let f2fs_balance_fs() to trigger
> > checkpoit in the end of f2fs_gc().
>
> > Thanks,
> >
> > > +	} else if (gc_type == FG_GC) {
> > > +		/* FG_GC stops GC by skip_count */
> > >    		if (sbi->skipped_gc_rwsem)
> > >    			skipped_round++;
> > >    		round++;
> > >    		if (skipped_round > MAX_SKIP_GC_COUNT &&
> > > -				skipped_round * 2 >= round) {
> > > -			ret = f2fs_write_checkpoint(sbi, &cpc);
> > > -			goto stop;
> > > -		}
> > > +				skipped_round * 2 >= round)
> > > +			stop_gc = true;
> > >    	}
> > >    	__get_secs_required(sbi, NULL, &upper_secs, NULL);
> > > @@ -1901,12 +1898,13 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > >    				prefree_segments(sbi)) {
> > >    		ret = f2fs_write_checkpoint(sbi, &cpc);
> > >    		if (ret)
> > > -			goto stop;
> > > +			stop_gc = true;
> > >    	}
> > >    go_gc_more:
> > > -	segno = NULL_SEGNO;
> > > -	goto gc_more;
> > > -
> > > +	if (!stop_gc) {
> > > +		segno = NULL_SEGNO;
> > > +		goto gc_more;
> > > +	}
> > >    stop:
> > >    	SIT_I(sbi)->last_victim[ALLOC_NEXT] = 0;
> > >    	SIT_I(sbi)->last_victim[FLUSH_DEVICE] = gc_control->victim_segno;