sys-fs/zfs-0.6.3-r1 (and possibly earlier versions; I don’t know) requires that the grsecurity/PaX kernel configuration setting CONFIG_PAX_USERCOPY_SLABS be disabled. I’m filing this report to track this issue on the pattern of <https://bugs.gentoo.org/532992>.
Upstream issue report: <https://github.com/zfsonlinux/zfs/issues/2977>.
Correction: `sys-fs/zfs-kmod`, not `sys-fs/zfs`.
This was fixed in 0.6.5.2.