linux-kernel - Re: [PATCH v2 2/7] erofs: some marcos are much more readable as a function

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Date:   Fri, 30 Aug 2019 23:52:23 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Joe Perches <joe@...ches.com>, Chao Yu <yuchao0@...wei.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <devel@...verdev.osuosl.org>, LKML <linux-kernel@...r.kernel.org>,
        <linux-erofs@...ts.ozlabs.org>, Chao Yu <chao@...nel.org>,
        Miao Xie <miaoxie@...wei.com>, <weidu.du@...wei.com>,
        Fang Wei <fangwei1@...wei.com>
Subject: Re: [PATCH v2 2/7] erofs: some marcos are much more readable as a
 function

Hi Christoph,

On Fri, Aug 30, 2019 at 08:45:51AM -0700, Christoph Hellwig wrote:
> On Thu, Aug 29, 2019 at 08:16:27PM -0700, Joe Perches wrote:
> > > -		sizeof(__u32) * ((__count) - 1); })
> > > +static inline unsigned int erofs_xattr_ibody_size(__le16 d_icount)
> > > +{
> > > +	unsigned int icount = le16_to_cpu(d_icount);
> > > +
> > > +	if (!icount)
> > > +		return 0;
> > > +
> > > +	return sizeof(struct erofs_xattr_ibody_header) +
> > > +		sizeof(__u32) * (icount - 1);
> > 
> > Maybe use struct_size()?
> 
> Declaring a variable that is only used for struct_size is rather ugly.
> But while we are nitpicking: you don't need to byteswap to check for 0,
> so the local variable could be avoided.
> 
> Also what is that magic -1 for?  Normally we use that for the
> deprecated style where a variable size array is declared using
> varname[1], but that doesn't seem to be the case for erofs.

I have to explain more about this (sorry about my awkward English)
here i_xattr_icount is to represent the size of xattr field of erofs, as follows:
 0 - no xattr at all (no erofs_xattr_ibody_header)
  _______
 | inode |
 |_______|

 1 - a erofs_xattr_ibody_header (12 byte) + 4-byte (shared + inline) xattrs
 2 - a erofs_xattr_ibody_header (12 byte) + 8-byte (shared + inline) xattrs
 ....
 (that is the magic -1 means...)

In order to keep the number continuously, actually the content could be
 an array of shared_xattr_id and
 an inline xattr combination (struct erofs_xattr_entry + name + value)

Thanks,
Gao Xiang