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Subject: Re: [f2fs-dev] [PATCH v2] f2fs: introduce blk_alloc_mode mount option
To:     Daeho Jeong <daeho43@gmail.com>, linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com
Cc:     Daeho Jeong <daehojeong@google.com>
References: <20210809185449.2565768-1-daeho43@gmail.com>
From:   Chao Yu <chao@kernel.org>
Message-ID: <425daf77-8020-26ce-dc9f-019d9a881b78@kernel.org>
Date:   Wed, 11 Aug 2021 09:58:45 +0800
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On 2021/8/10 2:54, Daeho Jeong wrote:
> From: Daeho Jeong <daehojeong@google.com>
> 
> Added a mount option to control block allocation mode for filesystem
> developer to simulate filesystem fragmentation and after-GC situation
> for experimental reasons to understand the filesystem behaviors well

At a glance, I think this functionality can be used to initialize filesystem
status as fragmented one, which is commonly used in the first step of aging
test, in order to make filesystem fragmented, aging program needs to fill
file or block sequentially and remove them at fixed interval, that takes a
lot of time. IMO, it will be great to support this by filesystem naturally
to save time of aging filessytem, rather just for experimental reasons to
understand filesystem behavior.

Maybe we can add a new mode for filesystem allocation mode except adaptive
and lfs mode, maybe "fragment" mode... :P

> under the severe condition. This supports "normal", "seg_random" and
> "blk_random:<num>" options.
> 
> "normal" (default): f2fs allocates blocks in the normal way.
> "seg_random": f2fs allocates a new segment in ramdom position.
>                With this, we can simulate the after-GC condition.
> "blk_random:<num>": We can make f2fs allocate only 1..<num> blocks
>                      in a row and forcibly change the segment randomly.
>                      With this, the newly allocated blocks will be scatter
>                      throughout the whole partition and we can simulate
>                      filesystem fragmentation condition.
> 
> Signed-off-by: Daeho Jeong <daehojeong@google.com>
> 
> ---
> v2: changed the <num> initialization way.
> ---
>   Documentation/filesystems/f2fs.rst | 16 ++++++++++
>   fs/f2fs/f2fs.h                     | 20 +++++++++++++
>   fs/f2fs/gc.c                       |  5 +++-
>   fs/f2fs/segment.c                  | 12 ++++++++
>   fs/f2fs/super.c                    | 47 ++++++++++++++++++++++++++++++
>   5 files changed, 99 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
> index ff9e7cc97c65..a0ca963fda20 100644
> --- a/Documentation/filesystems/f2fs.rst
> +++ b/Documentation/filesystems/f2fs.rst
> @@ -312,6 +312,22 @@ inlinecrypt		 When possible, encrypt/decrypt the contents of encrypted
>   			 Documentation/block/inline-encryption.rst.
>   atgc			 Enable age-threshold garbage collection, it provides high
>   			 effectiveness and efficiency on background GC.
> +blk_alloc_mode=%s	 Control block allocation mode. This is a developer option
> +			 for experiments to simulate filesystem fragmentation and
> +			 after-GC situation. The developers use this mode to understand
> +			 filesystem fragmentation and after-GC condition well, and
> +			 eventually get the insight to handle them better.
> +			 This supports "normal", "seg_random" and "blk_random:<num>" modes.
> +			 In "normal" mode (default), f2fs allocates blocks in the normal way.
> +			 In "seg_random", f2fs allocates a new segment in ramdom position.
> +			 With this, we can simulate the after-GC condition.
> +			 In "blk_random:<num>", we can make f2fs allocate only 1..<num>
> +			 blocks in a row and forcibly change the segment randomly.
> +			 You can set the <num> within 1 .. 512 number.
> +			 With this, the newly allocated blocks will be scatter throughout
> +			 the whole partition and we can simulate filesystem fragmentation
> +			 condition. Please, use this option for your experiments and we
> +			 strongly recommand a filesystem format after using this option.
>   ======================== ============================================================
>   
>   Debugfs Entries
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index fccee18ab776..aed09e8c0fce 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -155,6 +155,9 @@ struct f2fs_mount_info {
>   	int compress_mode;			/* compression mode */
>   	unsigned char extensions[COMPRESS_EXT_NUM][F2FS_EXTENSION_LEN];	/* extensions */
>   	unsigned char noextensions[COMPRESS_EXT_NUM][F2FS_EXTENSION_LEN]; /* extensions */
> +
> +	int blk_alloc_mode;		/* block allocation mode */
> +	int blk_alloc_random_max;	/* the maximum chunk size for block random allocation mode */
>   };
>   
>   #define F2FS_FEATURE_ENCRYPT		0x0001
> @@ -1740,6 +1743,8 @@ struct f2fs_sb_info {
>   
>   	unsigned long seq_file_ra_mul;		/* multiplier for ra_pages of seq. files in fadvise */
>   
> +	int blk_alloc_remained;			/* remained block count for this block allocation period */
> +
>   #ifdef CONFIG_F2FS_FS_COMPRESSION
>   	struct kmem_cache *page_array_slab;	/* page array entry */
>   	unsigned int page_array_slab_size;	/* default page array slab size */
> @@ -3619,6 +3624,21 @@ unsigned int f2fs_usable_segs_in_sec(struct f2fs_sb_info *sbi,
>   unsigned int f2fs_usable_blks_in_seg(struct f2fs_sb_info *sbi,
>   			unsigned int segno);
>   
> +#define MIN_BLK_ALLOC_RANDOM	1
> +#define MAX_BLK_ALLOC_RANDOM	512
> +
> +enum {
> +	BLK_ALLOC_MODE_NORMAL,		/* normal block allocation mode */
> +	BLK_ALLOC_MODE_SEG_RANDOM,	/* make segment allocation random */
> +	BLK_ALLOC_MODE_BLK_RANDOM,	/* make block allocation random */
> +};
> +
> +static inline bool f2fs_need_seg_random(struct f2fs_sb_info *sbi)
> +{
> +	return F2FS_OPTION(sbi).blk_alloc_mode == BLK_ALLOC_MODE_SEG_RANDOM ||
> +		F2FS_OPTION(sbi).blk_alloc_mode == BLK_ALLOC_MODE_BLK_RANDOM;
> +}
> +
>   /*
>    * checkpoint.c
>    */
> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> index 9dce44619069..571b50322e6e 100644
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -14,6 +14,7 @@
>   #include <linux/delay.h>
>   #include <linux/freezer.h>
>   #include <linux/sched/signal.h>
> +#include <linux/random.h>
>   
>   #include "f2fs.h"
>   #include "node.h"
> @@ -256,7 +257,9 @@ static void select_policy(struct f2fs_sb_info *sbi, int gc_type,
>   		p->max_search = sbi->max_victim_search;
>   
>   	/* let's select beginning hot/small space first in no_heap mode*/
> -	if (test_opt(sbi, NOHEAP) &&
> +	if (f2fs_need_seg_random(sbi))
> +		p->offset = prandom_u32() % (MAIN_SECS(sbi) * sbi->segs_per_sec);
> +	else if (test_opt(sbi, NOHEAP) &&
>   		(type == CURSEG_HOT_DATA || IS_NODESEG(type)))
>   		p->offset = 0;
>   	else
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index f9b7fb785e1d..6dff2d36ad6b 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -15,6 +15,7 @@
>   #include <linux/timer.h>
>   #include <linux/freezer.h>
>   #include <linux/sched/signal.h>
> +#include <linux/random.h>
>   
>   #include "f2fs.h"
>   #include "segment.h"
> @@ -2587,6 +2588,8 @@ static unsigned int __get_next_segno(struct f2fs_sb_info *sbi, int type)
>   	unsigned short seg_type = curseg->seg_type;
>   
>   	sanity_check_seg_type(sbi, seg_type);
> +	if (f2fs_need_seg_random(sbi))
> +		return prandom_u32() % (MAIN_SECS(sbi) * sbi->segs_per_sec);
>   
>   	/* if segs_per_sec is large than 1, we need to keep original policy. */
>   	if (__is_large_section(sbi))
> @@ -3150,6 +3153,15 @@ int f2fs_trim_fs(struct f2fs_sb_info *sbi, struct fstrim_range *range)
>   static bool __has_curseg_space(struct f2fs_sb_info *sbi,
>   					struct curseg_info *curseg)
>   {
> +	/* To allocate block chunks in different sizes, use random number */
> +	if (F2FS_OPTION(sbi).blk_alloc_mode == BLK_ALLOC_MODE_BLK_RANDOM) {
> +		if (--sbi->blk_alloc_remained < 0) {
> +			sbi->blk_alloc_remained = prandom_u32() %
> +				F2FS_OPTION(sbi).blk_alloc_random_max;
> +			return false;
> +		}
> +	}

Why not adjusting __refresh_next_blkoff(), so we can control granularity of
allocation interval, e.g.

	if (seg->alloc_type == SSR)
		seg->next_blkoff =
			__next_free_blkoff(sbi, seg->segno,
						seg->next_blkoff + 1);
	else if (seg->alloc_type == LFS)
		seg->next_blkoff++;
	else
		seg->next_blkoff += sbi->fragment_granularity;

Thanks,

> +
>   	return curseg->next_blkoff < f2fs_usable_blks_in_seg(sbi,
>   							curseg->segno);
>   }
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index 9ead6d2e703b..ef14f3e8b29a 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -155,6 +155,7 @@ enum {
>   	Opt_atgc,
>   	Opt_gc_merge,
>   	Opt_nogc_merge,
> +	Opt_blk_alloc_mode,
>   	Opt_err,
>   };
>   
> @@ -231,6 +232,7 @@ static match_table_t f2fs_tokens = {
>   	{Opt_atgc, "atgc"},
>   	{Opt_gc_merge, "gc_merge"},
>   	{Opt_nogc_merge, "nogc_merge"},
> +	{Opt_blk_alloc_mode, "blk_alloc_mode=%s"},
>   	{Opt_err, NULL},
>   };
>   
> @@ -1173,6 +1175,40 @@ static int parse_options(struct super_block *sb, char *options, bool is_remount)
>   		case Opt_nogc_merge:
>   			clear_opt(sbi, GC_MERGE);
>   			break;
> +		case Opt_blk_alloc_mode:
> +			name = match_strdup(&args[0]);
> +			if (!name)
> +				return -ENOMEM;
> +			if (!strcmp(name, "normal")) {
> +				F2FS_OPTION(sbi).blk_alloc_mode =
> +					BLK_ALLOC_MODE_NORMAL;
> +			} else if (!strcmp(name, "seg_random")) {
> +				F2FS_OPTION(sbi).blk_alloc_mode =
> +					BLK_ALLOC_MODE_SEG_RANDOM;
> +			} else if (!strncmp(name, "blk_random:", 11)) {
> +				const char *num = name + 11;
> +				long size;
> +
> +				ret = kstrtol(num, 10, &size);
> +				if (ret) {
> +					kfree(name);
> +					return ret;
> +				}
> +				if (size < MIN_BLK_ALLOC_RANDOM)
> +					size = MIN_BLK_ALLOC_RANDOM;
> +				else if (size > MAX_BLK_ALLOC_RANDOM)
> +					size = MAX_BLK_ALLOC_RANDOM;
> +
> +				F2FS_OPTION(sbi).blk_alloc_mode =
> +					BLK_ALLOC_MODE_BLK_RANDOM;
> +				F2FS_OPTION(sbi).blk_alloc_random_max =	size;
> +				sbi->blk_alloc_remained = size;
> +			} else {
> +				kfree(name);
> +				return -EINVAL;
> +			}
> +			kfree(name);
> +			break;
>   		default:
>   			f2fs_err(sbi, "Unrecognized mount option \"%s\" or missing value",
>   				 p);
> @@ -1919,6 +1955,14 @@ static int f2fs_show_options(struct seq_file *seq, struct dentry *root)
>   	else if (F2FS_OPTION(sbi).fsync_mode == FSYNC_MODE_NOBARRIER)
>   		seq_printf(seq, ",fsync_mode=%s", "nobarrier");
>   
> +	if (F2FS_OPTION(sbi).blk_alloc_mode == BLK_ALLOC_MODE_NORMAL)
> +		seq_printf(seq, ",blk_alloc_mode=%s", "normal");
> +	else if (F2FS_OPTION(sbi).blk_alloc_mode == BLK_ALLOC_MODE_SEG_RANDOM)
> +		seq_printf(seq, ",blk_alloc_mode=%s", "seg_random");
> +	else if (F2FS_OPTION(sbi).blk_alloc_mode == BLK_ALLOC_MODE_BLK_RANDOM)
> +		seq_printf(seq, ",blk_alloc_mode=%s:%d", "blk_random",
> +				F2FS_OPTION(sbi).blk_alloc_random_max);
> +
>   #ifdef CONFIG_F2FS_FS_COMPRESSION
>   	f2fs_show_compress_options(seq, sbi->sb);
>   #endif
> @@ -1947,6 +1991,9 @@ static void default_options(struct f2fs_sb_info *sbi)
>   	F2FS_OPTION(sbi).compress_ext_cnt = 0;
>   	F2FS_OPTION(sbi).compress_mode = COMPR_MODE_FS;
>   	F2FS_OPTION(sbi).bggc_mode = BGGC_MODE_ON;
> +	F2FS_OPTION(sbi).blk_alloc_mode = BLK_ALLOC_MODE_NORMAL;
> +	F2FS_OPTION(sbi).blk_alloc_random_max = MAX_BLK_ALLOC_RANDOM;
> +	sbi->blk_alloc_remained = MAX_BLK_ALLOC_RANDOM;
>   
>   	sbi->sb->s_flags &= ~SB_INLINECRYPT;
>   
>