From: Gao Xiang <gaoxiang25@huawei.com>
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: Richard Weinberger <richard.weinberger@gmail.com>,
Gao Xiang <hsiangkao@aol.com>,
Richard Weinberger <richard@nod.at>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
<linux-erofs@lists.ozlabs.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: erofs: Question on unused fields in on-disk structs
Date: Thu, 22 Aug 2019 22:34:32 +0800 [thread overview]
Message-ID: <20190822143432.GB195034@architecture4> (raw)
In-Reply-To: <20190822142142.GB2730@mit.edu>
Hi Ted,
On Thu, Aug 22, 2019 at 10:21:42AM -0400, Theodore Y. Ts'o wrote:
> On Thu, Aug 22, 2019 at 10:33:01AM +0200, Richard Weinberger wrote:
> > > 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.
> >
> > Yes. But you need some why to indicate that the chksum field is now
> > valid and must be used.
> >
> > The features field can be used for that, but you don't use it right now.
> > I recommend to check it for being 0, 0 means then "no features".
> > If somebody creates in future a erofs with more features this code
> > can refuse to mount because it does not support these features.
>
> The whole point of "compat" features is that the kernel can go ahead
> and mount the file system even if there is some new "compat" feature
> which it doesn't understand. So the fact that right now erofs doesn't
> have any "compat" features means it's not surprising, and perfectly
> OK, if it's not referenced by the kernel.
>
> For ext4, we have some more complex feature bitmasks, "compat",
> "ro_compat" (OK to mount read-only if there are features you don't
> understand) and "incompat" (if there are any bits you don't
> understand, fail the mount). But since erofs is a read-only file
> system, things are much simpler.
>
> It might make life easier for other kernel developers if "features"
> was named "compat_features" and "requirements" were named
> "incompat_features", just because of the long-standing use of that in
> ext2, ext3, ext4, ocfs2, etc. But that naming scheme really is a
> legacy of ext2 and its descendents, and there's no real reason why it
> has to be that way on other file systems.
Thanks for your detailed explanation, thanks a lot!
Thanks,
Gao Xiang
>
> Cheers,
>
> - Ted
prev parent reply other threads:[~2019-08-22 14:35 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-19 17:10 erofs: Question on unused fields in on-disk structs Richard Weinberger
2019-08-19 20:45 ` Gao Xiang
2019-08-21 21:37 ` Richard Weinberger
2019-08-21 22:03 ` Gao Xiang
2019-08-22 8:33 ` Richard Weinberger
2019-08-22 9:05 ` Gao Xiang
2019-08-22 9:08 ` Gao Xiang
2019-08-22 14:21 ` Theodore Y. Ts'o
2019-08-22 14:29 ` Richard Weinberger
2019-08-22 14:38 ` Gao Xiang
2019-08-22 14:34 ` Gao Xiang [this message]
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