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Date:   Mon, 10 Apr 2023 16:21:46 -0700
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Chao Yu <chao@kernel.org>
Cc:     linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] f2fs: fix to trigger a checkpoint in the end of
 foreground garbage collection
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References: <20230324071028.336982-1-chao@kernel.org>
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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.

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);
-- 
2.40.0.577.gac1e443424-goog


> 
> 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;