linux-kernel - Re: [PATCH 1/2] f2fs: use flexible array for f2fs_checkpoint::sit_nat_version

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Message-ID: <20180104061658.GA51983@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Wed, 3 Jan 2018 22:16:58 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, chao@...nel.org
Subject: Re: [PATCH 1/2] f2fs: use flexible array for
 f2fs_checkpoint::sit_nat_version_bitmap

On 01/04, Chao Yu wrote:
> On 2018/1/4 4:12, Jaegeuk Kim wrote:
> > On 01/03, Chao Yu wrote:
> >> If we need an array with variable size in the end of structure, we
> >> can utilize flexible array feature which is supported in C99, so
> >> let's change sit_nat_version_bitmap[] to flexible array in struct
> >> f2fs_checkpoint for readability.
> >>
> >> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> >> ---
> >>  fs/f2fs/f2fs.h          | 4 ++--
> >>  include/linux/f2fs_fs.h | 2 +-
> >>  2 files changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> >> index 83d1f697388b..ad19d29688ae 100644
> >> --- a/fs/f2fs/f2fs.h
> >> +++ b/fs/f2fs/f2fs.h
> >> @@ -1719,13 +1719,13 @@ static inline void *__bitmap_ptr(struct f2fs_sb_info *sbi, int flag)
> >>  
> >>  	if (__cp_payload(sbi) > 0) {
> >>  		if (flag == NAT_BITMAP)
> >> -			return &ckpt->sit_nat_version_bitmap;
> >> +			return ckpt->sit_nat_version_bitmap;
> >>  		else
> >>  			return (unsigned char *)ckpt + F2FS_BLKSIZE;
> >>  	} else {
> >>  		offset = (flag == NAT_BITMAP) ?
> >>  			le32_to_cpu(ckpt->sit_ver_bitmap_bytesize) : 0;
> >> -		return &ckpt->sit_nat_version_bitmap + offset;
> >> +		return ckpt->sit_nat_version_bitmap + offset;
> >>  	}
> >>  }
> >>  
> >> diff --git a/include/linux/f2fs_fs.h b/include/linux/f2fs_fs.h
> >> index 43e98d30d2df..564f65fc192f 100644
> >> --- a/include/linux/f2fs_fs.h
> >> +++ b/include/linux/f2fs_fs.h
> >> @@ -157,7 +157,7 @@ struct f2fs_checkpoint {
> >>  	unsigned char alloc_type[MAX_ACTIVE_LOGS];
> >>  
> >>  	/* SIT and NAT version bitmap */
> >> -	unsigned char sit_nat_version_bitmap[1];
> >> +	unsigned char sit_nat_version_bitmap[];
> > 
> > I cannot find any benefit to do this. Moreover, it just makes the header
> 
> I intend to change both kernel and tools, sorry, I didn't send out the
> patch on tools. Anyway, I think it could be used to avoid checkpoint
> structure size calculation by "sizeof(struct checkpoint) - 1", with the
> patch, readability can be improved. Right?

I don't think so. It's even defined only in f2fs-tools, and well covered by a
macro. There's no reason to spend time to sync it between f2fs and f2fs-tools.

> 
> Thanks,
> 
> > structure be different from the one in f2fs-tools.
> > 
> >>  } __packed;
> >>  
> >>  /*
> >> -- 
> >> 2.15.0.55.gc2ece9dc4de6
> > 
> > .
> >