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Subject: Re: [f2fs-dev] [PATCH v4] f2fs: introduce fragment allocation 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: <20210902172404.3517626-1-daeho43@gmail.com>
From:   Chao Yu <chao@kernel.org>
Message-ID: <9ab17089-accc-c3a3-a5dc-007fc4eeaa20@kernel.org>
Date:   Fri, 3 Sep 2021 07:57:38 +0800
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On 2021/9/3 1:24, Daeho Jeong wrote:
> @@ -2630,6 +2631,8 @@ 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_rand_seg(sbi))
> +		return prandom_u32() % (MAIN_SECS(sbi) * sbi->segs_per_sec);

	if (f2fs_need_seq_seg(sbi))
		return 0;

static inline bool f2fs_need_seq_seg(struct f2fs_sb_info *sbi)
{
	return F2FS_OPTION(sbi).fs_mode == FS_MODE_FRAGMENT_FIXED_BLK;
}

> @@ -2707,12 +2715,29 @@ static int __next_free_blkoff(struct f2fs_sb_info *sbi,
>   static void __refresh_next_blkoff(struct f2fs_sb_info *sbi,
>   				struct curseg_info *seg)
>   {
> -	if (seg->alloc_type == SSR)
> +	if (seg->alloc_type == SSR) {
>   		seg->next_blkoff =
>   			__next_free_blkoff(sbi, seg->segno,
>   						seg->next_blkoff + 1);
> -	else
> +	} else {
>   		seg->next_blkoff++;
> +		if (F2FS_OPTION(sbi).fs_mode == FS_MODE_FRAGMENT_FIXED_BLK) {
> +			if (--seg->fragment_remained_chunk <= 0) {
> +				seg->fragment_remained_chunk =
> +				   sbi->fragment_chunk_size;
> +				seg->next_blkoff +=
> +				   sbi->fragment_hole_size;

One more concern... we'd better to save fragment_remained_hole as well
as fragment_remained_chunk,  otherwise, if fragment_chunk_size +
fragment_hole_size > 512, fragment hole will be truncated to 512 -
fragment_chunk_size due to we won't create hole with enough size as
seg->next_blkoff has crossed end of current segment.

Thanks,