linux-kernel - Re: erofs: Question on unused fields in on-disk structs

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Message-ID: <20190819204504.GB10075@hsiangkao-HP-ZHAN-66-Pro-G1>
Date:   Tue, 20 Aug 2019 04:45:08 +0800
From:   Gao Xiang <hsiangkao@....com>
To:     Richard Weinberger <richard@....at>
Cc:     linux-erofs@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: erofs: Question on unused fields in on-disk structs

Hi Richard,

On Mon, Aug 19, 2019 at 07:10:33PM +0200, Richard Weinberger wrote:
> Hi!
> 
> struct erofs_super_block has "checksum" and "features" fields,
> but they are not used in the source.
> What is the plan for these?

Yes, both will be used laterly (features is used for compatible
features, we already have some incompatible features in 5.3).

> 
> Same for i_checksum in erofs_inode_v1 and erofs_inode_v2.

checksum field apart from super_block has been reserved again
for linux-next. checksum in the super_block still exists and
will be used sooner.

The reason I discussed with Chao is
since EROFS is a read-only filesystem, we will develop block-based
checksum and integrity check to EROFS in the future version. It's
more effectively than adding such fields to all metadata (some metadata
is too large, we cannot calculate a checksum for the whole metadata
and compare at runtime, but we can do block-based metadata chksum
since EROFS is a read-only fs).

> 
> At least the "features" field in the super block is something I'd
> expect to be used.
> ...such that you can have new filesystem features in future.

Of course, "features" is for compatible features, "requirements" is
for incompatible features. Both will be used in the future.

Thanks,
Gao Xiang

> 
> Thanks,
> //richard