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

[Jaegeuk Kim]

On 04/10, Chao Yu wrote:
> 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

Testing this.