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

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Message-ID: <20190821220251.GA3954@hsiangkao-HP-ZHAN-66-Pro-G1>
Date:   Thu, 22 Aug 2019 06:03:03 +0800
From:   Gao Xiang <hsiangkao@....com>
To:     Richard Weinberger <richard.weinberger@...il.com>
Cc:     Richard Weinberger <richard@....at>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-erofs@...ts.ozlabs.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: erofs: Question on unused fields in on-disk structs

Hi Richard,

On Wed, Aug 21, 2019 at 11:37:30PM +0200, Richard Weinberger wrote:
> Gao Xiang,
> 
> On Mon, Aug 19, 2019 at 10:45 PM Gao Xiang via Linux-erofs
> <linux-erofs@...ts.ozlabs.org> wrote:
> > > 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).
> 
> Good. :-)
> I suggest to check the fields being 0 right now.
> Otherwise you are in danger that they get burned if an mkfs.erofs does not
> initialize the fields.

Sorry... I cannot get the point...

super block chksum could be a compatible feature right? which means
new kernel can support it (maybe we can add a warning if such image
doesn't have a chksum then when mounting) but old kernel doesn't
care it.

Or maybe you mean these reserved fields? I have no idea all other
filesystems check these fields to 0 or not... But I think it should
be used with some other flag is set rather than directly use, right?

Thanks,
Gao Xiang

> 
> -- 
> Thanks,
> //richard