On Thu, Sep 03, 2015 at 07:16:19PM +0000, Richard Yao wrote: > On Thu, Sep 03, 2015 at 11:36:57AM -0700, Darrick J. Wong wrote: > > On Thu, Sep 03, 2015 at 06:22:25PM +0000, Richard Yao wrote: > > > What happens with this patch if /dev/$DEVICE is ext4 formatted and someone runs > > > `mount -t ext3 /dev/$DEVICE $MNT`? > > > > > > This should fail with the ext3 driver, but it looks like it will work fine with > > > CONFIG_EXT4_USE_FOR_EXT23 because ext3_fs_type maps to ext4_mount. My system is > > > not built with CONFIG_EXT4_USE_FOR_EXT23 (long story short: it uses a RHEL6 > > > derived config on Linux 4.1) and I do not have time to rebuild it to verify my > > > suspicion, but I imagine there are others on the list that could trivially check > > > this. > > > > On 4.2 with CONFIG_EXT4_USE_FOR_EXT23: > > > > # mke2fs -T ext4 /dev/sda > > # mount /dev/sda /mnt -t ext3 > > mount: wrong fs type, bad option, bad superblock on /dev/sda, > > missing codepage or helper program, or other error > > In some cases useful info is found in syslog - try > > dmesg | tail or so > > > > # lsmod|grep ext > > ext4 630784 0 > > jbd2 126976 1 ext4 > > mbcache 20480 1 ext4 > > # dmesg > > <snip> > > [74559.632979] EXT4-fs (sda): couldn't mount as ext3 due to feature incompatibilities > > > > > Also, new kernels are typically drop-in replacements on older userlands. An edge > > > case that no one appears to have mentioned is the possibility of using a newer > > > kernel on an older system where the initramfs generator might only include ext3, > > > which this would break. It might not be terrible to write a small dummy ext3 > > > module whose only purpose is to depend on ext4 and load it into the kernel on > > > those systems. That way initramfs software that properly grabs module > > > dependencies will include the ext4 module and `modprobe ext3` will do what it > > > > Well, if it goes looking for ext3.ko directly it will fail, but... > > > > # modinfo ext3 > > filename: /lib/modules/4.2.0-mcsum/kernel/fs/ext4/ext4.ko > > license: GPL > > description: Fourth Extended Filesystem > > author: Remy Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore Ts'o and others > > alias: fs-ext4 > > alias: ext3 > > alias: fs-ext3 > > <snip> > > > > I don't know about RHEL initrd scripts, but Ubuntu's use some modprobe trickery > > which ensures that it picks up the correct ext4.ko. It does something similar > > to this: > > > > # modprobe --ignore-install --quiet --show-depends ext3 | sed -e 's/^insmod //g' > > /lib/modules/4.2.0-mcsum/kernel/fs/mbcache.ko > > /lib/modules/4.2.0-mcsum/kernel/fs/jbd2/jbd2.ko > > /lib/modules/4.2.0-mcsum/kernel/fs/ext4/ext4.ko > > > > to pick up the modules for the initramfs. Not sure what the other distros > > do, though. > > Unfortunately, the genkernel team was not aware of this when it wrote support > for including kernel modules and I suspect others writing initramfs archive > generators did not either. > > https://gitweb.gentoo.org/proj/genkernel.git/tree/gen_initramfs.sh#n638 > https://gitweb.gentoo.org/proj/genkernel.git/tree/gen_moddeps.sh > https://gitweb.gentoo.org/proj/genkernel.git/tree/defaults/modules_load > > The way that works is that the module names are specified in a MODULES_* > variable and then we search for dependencies based on what is specified in > modules.dep. If you have an old enough version of the initramfs genreator, ext4 > is not specified and it will not recognize the alias. > > I admit that this is likely a very rare edge case. I do not feel too strongly > about breaking the older initramfs software. I just wanted to make sure others > knew that they were. Hmmm, is there no modules.alias on Gentoo? That seems unlikely to me, but I haven't run it in a while. It's unfortunate that it doesn't get parsed as part of initrd generation. <shrug> I guess the initrd would break if you were trying to install a 4.3 kernel onto a pre-2010ish Gentoo rootfs, unless that file gets updated(?) --D > > > > always did in terms of making ext3 file systems mountable. I suppose that we > > > could use aliases, but given that there is a compatibility shim for CONFIG_EXT3 > > > to avoid surprises, a dummy module seems reasonable. > > > > > > These are the only two things that I see preventing ext4 from being a drop-in > > > replacement for ext3. > > > > > > That said, my only connection with ext3/ext4 is that I am one of the genkernel > > > developers (Gentoo's Linux initramfs/kernel genreation framework/scripts), so > > > my opinion might not matter much here, but I am also in favor of killing ext3 in > > > favor of CONFIG_EXT4_USE_FOR_EXT23. > > > > :) > > > > --D > > > > > -- > > > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
