Note that enabling flex_bg, uninit_bg, on a filesystem that was not created with them has no effect.: the block groups have already been laid out and initialized in the old way. extra_isize is a bit cheeky: if the fs was formatted as ext2 before there was such a thing, then it was formatted using 128 byte inodes, so there is no extra space to use. Newer versions of mkfs use 256 byte inodes on ext2 anyway so you can then enable using the extra space by setting the flag.
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