On Thu, Sep 03, 2015 at 01:36:45PM -0500, Eric Sandeen wrote: > On 9/3/15 1:22 PM, 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. > > Did you test it? > > # truncate --size=1g fsfile > # mkfs.ext4 fsfile > # mount -o loop -t ext3 fsfile > mount: wrong fs type, <snip> > # dmesg | tail > # EXT4-fs: (loop0): couldn't mount as ext3 due to feature incompatibilities > > > 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. > > Well by all means, let me do that for you. :) I had vaguely recalled that working, but I am glad to see that my recollection is wrong. Thanks for confirming it. > > 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 > > 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. > > I'm not wise in the ways of initramfs generation, so I'll defer on this one. > > -Eric > > > 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. > > -- > > 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
