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Date:   Wed, 22 Feb 2017 13:55:08 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 3/6] f2fs: remove batched discard in
 f2fs_trim_fs

On 02/22, Chao Yu wrote:
> On 2017/1/13 6:44, Jaegeuk Kim wrote:
> > We don't need to do multiple checkpoints, since we don't actually wait for
> > completion of discard commands during checkpoint.
> > Instead, we still need to avoid very big discard commands, since that large
> > discard can interfere block allocation.
> 
> I hope we can keep this functionality and related sysfs interface, the main
> concern is that: for 16T size fragmented partition, at most there will be
> ~sizeof(u32)/2 4K blocks which could be invalid and needed to be discarded.
> After removal of in-batch discard, we have to handle billions of discard message
> with gc_mutex & cp_rwsem held, which will freeze f2fs for very long time.

The baseline of this patch is that we have a kernel thread to issue discard
commands asynchronously. So, I don't expect such the huge latency anymore.

Thanks,

> 
> So I suggest we just set trim_sections to 512 segments (covers one GB) by
> default. How do you think?
> 
> Thanks,
> 
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> >  Documentation/ABI/testing/sysfs-fs-f2fs |  6 ------
> >  fs/f2fs/f2fs.h                          |  9 +-------
> >  fs/f2fs/segment.c                       | 38 +++++++++++----------------------
> >  fs/f2fs/super.c                         |  2 --
> >  4 files changed, 14 insertions(+), 41 deletions(-)
> > 
> > diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
> > index a809f6005f14..df6b3f6e164a 100644
> > --- a/Documentation/ABI/testing/sysfs-fs-f2fs
> > +++ b/Documentation/ABI/testing/sysfs-fs-f2fs
> > @@ -75,12 +75,6 @@ Contact:	"Jaegeuk Kim" <jaegeuk.kim@...sung.com>
> >  Description:
> >  		 Controls the memory footprint used by f2fs.
> >  
> > -What:		/sys/fs/f2fs/<disk>/trim_sections
> > -Date:		February 2015
> > -Contact:	"Jaegeuk Kim" <jaegeuk@...nel.org>
> > -Description:
> > -		 Controls the trimming rate in batch mode.
> > -
> >  What:		/sys/fs/f2fs/<disk>/cp_interval
> >  Date:		October 2015
> >  Contact:	"Jaegeuk Kim" <jaegeuk@...nel.org>
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 548e75d18ec1..a2850bf2a487 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -128,11 +128,7 @@ enum {
> >  	CP_DISCARD,
> >  };
> >  
> > -#define DEF_BATCHED_TRIM_SECTIONS	2
> > -#define BATCHED_TRIM_SEGMENTS(sbi)	\
> > -		(SM_I(sbi)->trim_sections * (sbi)->segs_per_sec)
> > -#define BATCHED_TRIM_BLOCKS(sbi)	\
> > -		(BATCHED_TRIM_SEGMENTS(sbi) << (sbi)->log_blocks_per_seg)
> > +#define MAX_DISCARD_BLOCKS(sbi) (1 << (sbi)->log_blocks_per_seg)
> >  #define DEF_CP_INTERVAL			60	/* 60 secs */
> >  #define DEF_IDLE_INTERVAL		5	/* 5 secs */
> >  
> > @@ -638,9 +634,6 @@ struct f2fs_sm_info {
> >  	int nr_discards;			/* # of discards in the list */
> >  	int max_discards;			/* max. discards to be issued */
> >  
> > -	/* for batched trimming */
> > -	unsigned int trim_sections;		/* # of sections to trim */
> > -
> >  	struct list_head sit_entry_set;	/* sit entry set list */
> >  
> >  	unsigned int ipu_policy;	/* in-place-update policy */
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index 73282108fa33..e6f3c6db7616 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -823,7 +823,8 @@ static void __add_discard_entry(struct f2fs_sb_info *sbi,
> >  	if (!list_empty(head)) {
> >  		last = list_last_entry(head, struct discard_entry, list);
> >  		if (START_BLOCK(sbi, cpc->trim_start) + start ==
> > -						last->blkaddr + last->len) {
> > +				last->blkaddr + last->len &&
> > +				last->len <= MAX_DISCARD_BLOCKS(sbi)) {
> >  			last->len += end - start;
> >  			goto done;
> >  		}
> > @@ -1513,36 +1514,25 @@ int f2fs_trim_fs(struct f2fs_sb_info *sbi, struct fstrim_range *range)
> >  			"Found FS corruption, run fsck to fix.");
> >  		goto out;
> >  	}
> > +	if (sbi->discard_blks == 0)
> > +		goto out;
> >  
> >  	/* start/end segment number in main_area */
> >  	start_segno = (start <= MAIN_BLKADDR(sbi)) ? 0 : GET_SEGNO(sbi, start);
> >  	end_segno = (end >= MAX_BLKADDR(sbi)) ? MAIN_SEGS(sbi) - 1 :
> >  						GET_SEGNO(sbi, end);
> > +	/*
> > +	 * do checkpoint to issue discard commands safely since we now can
> > +	 * use async discard.
> > +	 */
> >  	cpc.reason = CP_DISCARD;
> >  	cpc.trim_minlen = max_t(__u64, 1, F2FS_BYTES_TO_BLK(range->minlen));
> > +	cpc.trim_start = start_segno;
> > +	cpc.trim_end = end_segno;
> >  
> > -	/* do checkpoint to issue discard commands safely */
> > -	for (; start_segno <= end_segno; start_segno = cpc.trim_end + 1) {
> > -		cpc.trim_start = start_segno;
> > -
> > -		if (sbi->discard_blks == 0)
> > -			break;
> > -		else if (sbi->discard_blks < BATCHED_TRIM_BLOCKS(sbi))
> > -			cpc.trim_end = end_segno;
> > -		else
> > -			cpc.trim_end = min_t(unsigned int,
> > -				rounddown(start_segno +
> > -				BATCHED_TRIM_SEGMENTS(sbi),
> > -				sbi->segs_per_sec) - 1, end_segno);
> > -
> > -		mutex_lock(&sbi->gc_mutex);
> > -		err = write_checkpoint(sbi, &cpc);
> > -		mutex_unlock(&sbi->gc_mutex);
> > -		if (err)
> > -			break;
> > -
> > -		schedule();
> > -	}
> > +	mutex_lock(&sbi->gc_mutex);
> > +	err = write_checkpoint(sbi, &cpc);
> > +	mutex_unlock(&sbi->gc_mutex);
> >  out:
> >  	range->len = F2FS_BLK_TO_BYTES(cpc.trimmed);
> >  	return err;
> > @@ -2708,8 +2698,6 @@ int build_segment_manager(struct f2fs_sb_info *sbi)
> >  	sm_info->nr_discards = 0;
> >  	sm_info->max_discards = 0;
> >  
> > -	sm_info->trim_sections = DEF_BATCHED_TRIM_SECTIONS;
> > -
> >  	INIT_LIST_HEAD(&sm_info->sit_entry_set);
> >  
> >  	if (test_opt(sbi, FLUSH_MERGE) && !f2fs_readonly(sbi->sb)) {
> > diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> > index 461c29043aec..37d40d8aa9c4 100644
> > --- a/fs/f2fs/super.c
> > +++ b/fs/f2fs/super.c
> > @@ -284,7 +284,6 @@ F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_no_gc_sleep_time, no_gc_sleep_time);
> >  F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_idle, gc_idle);
> >  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, reclaim_segments, rec_prefree_segments);
> >  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, max_small_discards, max_discards);
> > -F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, batched_trim_sections, trim_sections);
> >  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, ipu_policy, ipu_policy);
> >  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, min_ipu_util, min_ipu_util);
> >  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, min_fsync_blocks, min_fsync_blocks);
> > @@ -309,7 +308,6 @@ static struct attribute *f2fs_attrs[] = {
> >  	ATTR_LIST(gc_idle),
> >  	ATTR_LIST(reclaim_segments),
> >  	ATTR_LIST(max_small_discards),
> > -	ATTR_LIST(batched_trim_sections),
> >  	ATTR_LIST(ipu_policy),
> >  	ATTR_LIST(min_ipu_util),
> >  	ATTR_LIST(min_fsync_blocks),
> > 
> 
> 
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