linux-kernel - Re: [PATCH] f2fs: introduce on-disk layout version checking functionality

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Date:	Tue, 24 May 2016 18:05:23 -0700
From:	Viacheslav Dubeyko <slava@...eyko.com>
To:	Jaegeuk Kim <jaegeuk@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net,
	Vyacheslav.Dubeyko@...t.com, Cyril.Guyot@...t.com,
	Adam.Manzanares@...t.com, Damien.LeMoal@...t.com
Subject: Re: [PATCH] f2fs: introduce on-disk layout version checking
 functionality

Hi Jaegeuk,

On Mon, 2016-05-23 at 14:13 -0700, Jaegeuk Kim wrote:
> Hi Slava,
> 
> On Thu, May 19, 2016 at 10:46:06AM -0700, Viacheslav Dubeyko wrote:
> ...
> >  
> > +#ifdef CONFIG_F2FS_16TB_VOLUME_SUPPORT
> > +#define F2FS_MAX_SUPP_MAJOR_VERSION		(2)
> > +#define F2FS_MIN_16TB_VOLUME_SUPPORT_VERSION	(2)
> > +#else
> > +#define F2FS_MAX_SUPP_MAJOR_VERSION		(1)
> > +#endif
> > +
> ...
> >  
> > +static int f2fs_check_version_and_features(struct super_block *sb,
> > +					   struct f2fs_super_block *raw_super)
> > +{
> > +	u16 major_ver = le16_to_cpu(raw_super->major_ver);
> > +	u32 feature = le32_to_cpu(raw_super->feature);
> > +
> > +	if (major_ver > F2FS_MAX_SUPP_MAJOR_VERSION) {
> 
> This means, for example, f2fs driver in v4.8 will deny to mount a partition
> formatted by mkfs.f2fs v3.x, which doesn't make sense, IIUC.
> 

I didn't catch the point. Maybe, I've missed something but, as far as I
can judge, f2fs driver v.4.8 will mount as old version of on-disk layout
as the new one. But right now it doesn't make sense to discuss this
topic because we haven't consent about ideology of this patch.

> As Christoph mentioned, how about checking the feature only like this?
> 
> 1. if the feature is ON,
>  - go 64 bits   , when compiled w/  F2FS_MIN_16TB_VOLUME_SUPPORT
>  - fail to mount, when compiled w/o F2FS_MIN_16TB_VOLUME_SUPPORT
> 
> 2. if the feature is OFF,
>  - fail to mount, when compiled w/  F2FS_MIN_16TB_VOLUME_SUPPORT
>  - go 32 bits   , when compiled w/o F2FS_MIN_16TB_VOLUME_SUPPORT
> 
> Thoughts?
> 

So, my logic is simple. We are trying to modify the on-disk layout. As a
result, we need to check the on-disk layout version, from my viewpoint.
And this modification is not "feature" itself but simple bug fix. And I
believe that "major_ver", "minor_ver" in F2FS superblock is the on-disk
layout version.

What do you think? Do you still believe that it should be a feature
flag?

Thanks,
Vyacheslav Dubeyko.