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Date:   Thu, 13 Apr 2023 08:58:53 -0700
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Chao Yu <chao@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix to trigger a checkpoint in the end
 of foreground garbage collection
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On 04/13, Jaegeuk Kim wrote:
> On 04/13, Chao Yu wrote:
> > On 2023/4/11 7:21, Jaegeuk Kim wrote:
> > > 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@kernel.org>
> > > > > > > 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@kernel.org>
> > > > > > > Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> > > > > > > ---
> > > > > > >     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.
> > > 
> > >  From ec5f37bbe33110257c04e0ec97a80b0111465b52 Mon Sep 17 00:00:00 2001
> > > From: Jaegeuk Kim <jaegeuk@kernel.org>
> > > Date: Mon, 10 Apr 2023 14:48:50 -0700
> > > Subject: [PATCH] f2fs: refactor f2fs_gc to call checkpoint in urgent condition
> > > 
> > > The major change is to call checkpoint, if there's not enough space while having
> > > some prefree segments in FG_GC case.
> > 
> > I found generic/269 will hang w/ this patch.
> 
> Yeah, I got 270 as well. I removed it in the tree first, and will check it out
> later.

And, I figured out somehow this patch falls in a loop:
- get_victim(FG_GC) returns a same segno
- do nothing when migrating blocks due to i_gc_rwsem[WRITE]
- !has_not_enough_free_secs() is true, so goto gc_more loop

> 
> > 
> > Thanks,
> > 
> > > 
> > > Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> > > ---
> > >   fs/f2fs/gc.c | 26 ++++++++++++--------------
> > >   1 file changed, 12 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > > index c748cdfb0501..0a823d2e8b9d 100644
> > > --- a/fs/f2fs/gc.c
> > > +++ b/fs/f2fs/gc.c
> > > @@ -1829,7 +1829,10 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > >   		goto stop;
> > >   	}
> > > -	if (gc_type == BG_GC && has_not_enough_free_secs(sbi, 0, 0)) {
> > > +	/* Let's run FG_GC, if we don't have enough space. */
> > > +	if (has_not_enough_free_secs(sbi, 0, 0)) {
> > > +		gc_type = FG_GC;
> > > +
> > >   		/*
> > >   		 * For example, if there are many prefree_segments below given
> > >   		 * threshold, we can make them free by checkpoint. Then, we
> > > @@ -1840,8 +1843,6 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > >   			if (ret)
> > >   				goto stop;
> > >   		}
> > > -		if (has_not_enough_free_secs(sbi, 0, 0))
> > > -			gc_type = FG_GC;
> > >   	}
> > >   	/* f2fs_balance_fs doesn't need to do BG_GC in critical path. */
> > > @@ -1868,19 +1869,14 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > >   	if (seg_freed == f2fs_usable_segs_in_sec(sbi, segno))
> > >   		sec_freed++;
> > > -	if (gc_type == FG_GC)
> > > +	if (gc_type == FG_GC) {
> > >   		sbi->cur_victim_sec = NULL_SEGNO;
> > > -	if (gc_control->init_gc_type == FG_GC ||
> > > -	    !has_not_enough_free_secs(sbi,
> > > -				(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) {
> > > +		if (!has_not_enough_free_secs(sbi, sec_freed, 0)) {
> > > +			if (sec_freed < gc_control->nr_free_secs)
> > > +				goto go_gc_more;
> > > +			goto stop;
> > > +		}
> > >   		if (sbi->skipped_gc_rwsem)
> > >   			skipped_round++;
> > >   		round++;
> > > @@ -1889,6 +1885,8 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > >   			ret = f2fs_write_checkpoint(sbi, &cpc);
> > >   			goto stop;
> > >   		}
> > > +	} else if (!has_not_enough_free_secs(sbi, 0, 0)) {
> > > +		goto stop;
> > >   	}
> > >   	__get_secs_required(sbi, NULL, &upper_secs, NULL);
> 
> 
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