linux-ext4 - Re: [PATCH] ext2: don't mount filesystems with compat features we know only ext4 supports

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Message-ID: <20150930163635.GK10390@birch.djwong.org>
Date:	Wed, 30 Sep 2015 09:36:35 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Jan Kara <jack@...e.com>, linux-ext4 <linux-ext4@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] ext2: don't mount filesystems with compat features we
 know only ext4 supports

On Wed, Sep 30, 2015 at 10:41:26AM +0200, Jan Kara wrote:
> On Mon 28-09-15 15:36:26, Darrick J. Wong wrote:
> > The ext2 mount code never checks the compat features against the ones
> > it knows about.  This is correct behavior since compat features are
> > supposed to be rw-compatible with old drivers; however, for certain
> > configurations (journalled rootfs) we probably want the ext4 driver
> > to load, not ext2.
> > 
> > Reported-by: Dave Chinner <david@...morbit.com>
> > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> 
> Isn't this a bit too harsh? I agree we should at least warn (and that's
> probably regardless of EXT4 config option) but just refusing mount looks
> too much to me...

I admit that refusing the mount might be a bit much; the goal here was
merely to make it so that if the FS has a journal and ext4 was turned on,
hopefully ext2 rejects the mount and ext4 will probe it next.

<shrug>

--D

> 
> 								Honza
> 
> > ---
> >  fs/ext2/ext2.h  |    6 +++++-
> >  fs/ext2/super.c |   13 ++++++++++---
> >  2 files changed, 15 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/ext2/ext2.h b/fs/ext2/ext2.h
> > index 8d15feb..ce508b1 100644
> > --- a/fs/ext2/ext2.h
> > +++ b/fs/ext2/ext2.h
> > @@ -547,7 +547,11 @@ struct ext2_super_block {
> >  #define EXT2_FEATURE_INCOMPAT_META_BG		0x0010
> >  #define EXT2_FEATURE_INCOMPAT_ANY		0xffffffff
> >  
> > -#define EXT2_FEATURE_COMPAT_SUPP	EXT2_FEATURE_COMPAT_EXT_ATTR
> > +#define EXT2_FEATURE_COMPAT_SUPP	(EXT2_FEATURE_COMPAT_DIR_PREALLOC| \
> > +					 EXT2_FEATURE_COMPAT_IMAGIC_INODES| \
> > +					 EXT2_FEATURE_COMPAT_EXT_ATTR| \
> > +					 EXT2_FEATURE_COMPAT_RESIZE_INO| \
> > +					 EXT2_FEATURE_COMPAT_DIR_INDEX)
> >  #define EXT2_FEATURE_INCOMPAT_SUPP	(EXT2_FEATURE_INCOMPAT_FILETYPE| \
> >  					 EXT2_FEATURE_INCOMPAT_META_BG)
> >  #define EXT2_FEATURE_RO_COMPAT_SUPP	(EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER| \
> > diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> > index 900e19c..4efb018 100644
> > --- a/fs/ext2/super.c
> > +++ b/fs/ext2/super.c
> > @@ -896,6 +896,16 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
> >  	 * previously didn't change the revision level when setting the flags,
> >  	 * so there is a chance incompat flags are set on a rev 0 filesystem.
> >  	 */
> > +#ifdef CONFIG_EXT4_FS
> > +	/* Journalled FS should mount with ext4 if it's available */
> > +	features = EXT2_HAS_COMPAT_FEATURE(sb, ~EXT2_FEATURE_COMPAT_SUPP);
> > +	if (features) {
> > +		ext2_msg(sb, KERN_ERR,	"error: won't mount because of "
> > +		       "unsupported optional features (%x); try ext4",
> > +			le32_to_cpu(features));
> > +		goto failed_mount;
> > +	}
> > +#endif
> >  	features = EXT2_HAS_INCOMPAT_FEATURE(sb, ~EXT2_FEATURE_INCOMPAT_SUPP);
> >  	if (features) {
> >  		ext2_msg(sb, KERN_ERR,	"error: couldn't mount because of "
> > 
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
> --
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