Repeatable ZFS "kmem map too small" panic on 8.0-STABLE

[Artem Belevich]

> % sysctl hw.physmem vm.kmem_size vm.kmem_size_max vfs.zfs.arc_max
>
> hw.physmem:4102688768
> vm.kmem_size: 2147483648

Here's your problem -- kmem_size is for some reason only 2G.

Argh! I ran into that before. The code in sys/kern/kern_malloc.c
intentionally limits kmem_size to twice the physical memory size:

        /*
         * Limit kmem virtual size to twice the physical memory.
         * This allows for kmem map sparseness, but limits the size
         * to something sane. Be careful to not overflow the 32bit
         * ints while doing the check.
         */
        if (((vm_kmem_size / 2) / PAGE_SIZE) > cnt.v_page_count)
                vm_kmem_size = 2 * cnt.v_page_count * PAGE_SIZE;

So, either comment out these lines or just set vm.kmem_size to
slightly below 8G.

--Artem