linux-kernel - Re: [PATCH v6 03/24] erofs: add super block operations

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Date:   Mon, 2 Sep 2019 23:24:52 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Gao Xiang <hsiangkao@....com>, Jan Kara <jack@...e.cz>,
        Chao Yu <yuchao0@...wei.com>,
        Dave Chinner <david@...morbit.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Miao Xie <miaoxie@...wei.com>, <devel@...verdev.osuosl.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Theodore Ts'o <tytso@....edu>, Pavel Machek <pavel@...x.de>,
        David Sterba <dsterba@...e.cz>,
        Li Guifu <bluce.liguifu@...wei.com>,
        Fang Wei <fangwei1@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-erofs@...ts.ozlabs.org>
Subject: Re: [PATCH v6 03/24] erofs: add super block operations

Hi Christoph,

On Mon, Sep 02, 2019 at 08:19:10AM -0700, Christoph Hellwig wrote:
> On Mon, Sep 02, 2019 at 10:43:04PM +0800, Gao Xiang wrote:
> > Hi Christoph,
> > > > ...
> > > >  24         __le32 features;        /* (aka. feature_compat) */
> > > > ...
> > > >  38         __le32 requirements;    /* (aka. feature_incompat) */
> > > > ...
> > > >  41 };
> > > 
> > > This is only cosmetic, why not stick to feature_compat and
> > > feature_incompat?
> > 
> > Okay, will fix. (however, in my mind, I'm some confused why
> > "features" could be incompatible...)
> 
> The feature is incompatible if it requires changes to the driver.
> An easy to understand historic example is that ext3 originally did not
> have the file types in the directory entry.  Adding them means old
> file system drivers can not read a file system with this new feature,
> so an incompat flag has to be added.

Got it.

> 
> > > > > > +	memcpy(&sb->s_uuid, layout->uuid, sizeof(layout->uuid));
> > > > > > +	memcpy(sbi->volume_name, layout->volume_name,
> > > > > > +	       sizeof(layout->volume_name));
> > > > > 
> > > > > s_uuid should preferably be a uuid_t (assuming it is a real BE uuid,
> > > > > if it is le it should be a guid_t).
> > > > 
> > > > For this case, I have no idea how to deal with...
> > > > I have little knowledge about this uuid stuff, so I just copied
> > > > from f2fs... (Could be no urgent of this field...)
> > > 
> > > Who fills out this field in the on-disk format and how?
> > 
> > mkfs.erofs, but this field leaves 0 for now. Is that reasonable?
> > (using libuuid can generate it easily...)
> 
> If the filed is always zero for now please don't fill it out.  If you
> decide it is worth adding the uuid eventually please add a compat
> feature flag that you have a valid uuid and only fill out the field
> if the file system actualy has a valid uuid.

Okay. Will do that then (as a note here).

Thanks,
Gao Xiang