- /dev/sda1 | ext2 | 100MB (/boot)
- /dev/sda2 | ext4 | 15GB (root)
- /dev/sda3 | swap | 1GB
- /dev/sda4 | ext4 | 104GB (/mnt/whatever)
EXT4-fs: dm-5: Filesystem with huge files cannot be mounted read-write without CONFIG_LSF.Fortunately, my root partition mounted cleanly and the error only applied to /dev/sda4. Googling around*, I discovered that the "huge_file" feature is implicit when running mkfs.ext4. I saw 'CONFIG_LSF' in the kernel config and the documentation said it was only necessary for files 2TB or larger -- clearly nothing I'd need to worry about -- so I didn't include it in the kernel.
Instead of recompiling my kernel, I figured I'd just reformat the partition without huge_file support. Yes, recompiling the kernel would've been a pain, but reformatting seemed like the more ideal/elegant solution -- especially considering this partition was barely 100GB. The default behavior for mke2fs.ext* can be found in /etc/mke2fs.conf (on gentoo, at least), and the culprit looks like this under the ext4 block:
features = has_journal,extents,huge_file,flex_bg,uninit_bg,dir_nlink,extra_isize