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

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Message-ID: <20190830155617.GB69026@architecture4>
Date:   Fri, 30 Aug 2019 23:56:17 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     <devel@...verdev.osuosl.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Miao Xie <miaoxie@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>, <weidu.du@...wei.com>,
        Joe Perches <joe@...ches.com>, <linux-erofs@...ts.ozlabs.org>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v2 2/7] erofs: some marcos are much more readable as a
 function

On Fri, Aug 30, 2019 at 11:52:23PM +0800, Gao Xiang wrote:
> 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)

...Add a word, large xattrs should use shared xattr (which save xattrs
in another area) rather than inline xattr, shared xattr stores xattr_id
just after erofs_xattr_ibody_header and before inline xattrs...

Thanks,
Gao Xiang

> 
> Thanks,
> Gao Xiang
>