powerpc hugepage bug(s) when no valid hstates?

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

In KVM guests on Power, if the guest is not backed by hugepages, we see
the following in the guest:

AnonHugePages:         0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:         64 kB

This seems like a configuration issue -- why is a hstate of 64k being
registered?

I did some debugging and found that the following does trigger,
mm/hugetlb.c::hugetlb_init():

        /* Some platform decide whether they support huge pages at boot
         * time. On these, such as powerpc, HPAGE_SHIFT is set to 0 when
         * there is no such support
         */
        if (HPAGE_SHIFT == 0)
                return 0;

That check is only during init-time. So we don't support hugepages, but
none of the hugetlb APIs actually check this condition (HPAGE_SHIFT ==
0), so /proc/meminfo above falsely indicates there is a valid hstate (at
least one). But note that there is no /sys/kernel/mm/hugepages meaning
no hstate was actually registered.

Further, it turns out that huge_page_order(default_hstate) is 0, so
hugetlb_report_meminfo is doing:

1UL << (huge_page_order(h) + PAGE_SHIFT - 10)

which ends up just doing 1 << (PAGE_SHIFT - 10) and since the base page
size is 64k, we report a hugepage size of 64k... And allow the user to
allocate hugepages via the sysctl, etc.

What's the right thing to do here?

1) Should we add checks for HPAGE_SHIFT == 0 to all the hugetlb APIs? It
seems like HPAGE_SHIFT == 0 should be the equivalent, functionally, of
the config options being off. This seems like a lot of overhead, though,
to put everywhere, so maybe I can do it in an arch-specific macro, that
in asm-generic defaults to 0 (and so will hopefully be compiled out?).

2) What should hugetlbfs do when HPAGE_SHIFT == 0? Should it be
mountable? Obviously if it's mountable, we can't great files there
(since the fs will report insufficient space). [1]

Thanks,
Nish

[1]
Currently, I am seeing the following when I `mount -t hugetlbfs /none
/dev/hugetlbfs`, and then simply do a `ls /dev/hugetlbfs`. I think it's
related to the fact that hugetlbfs is properly not correctly setting
itself up in this state?:

Unable to handle kernel paging request for data at address 0x00000031
Faulting instruction address: 0xc000000000245710
Oops: Kernel access of bad area, sig: 11 [#1]
SMP NR_CPUS=2048 NUMA pSeries
Modules linked in: pseries_rng rng_core virtio_net virtio_pci virtio_ring virtio
CPU: 0 PID: 1807 Comm: ls Not tainted 3.14.0-rc7-00066-g774868c-dirty #14
task: c00000007e804520 ti: c00000007aed4000 task.ti: c00000007aed4000
NIP: c000000000245710 LR: c00000000024586c CTR: 0000000000000000
REGS: c00000007aed74f0 TRAP: 0300   Not tainted  (3.14.0-rc7-00066-g774868c-dirty)
MSR: 8000000000009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 24002484  XER: 00000000
CFAR: 00003fff91037760 DAR: 0000000000000031 DSISR: 40000000 SOFTE: 1
GPR00: c00000000024586c c00000007aed7770 c000000000d85420 c00000007d7a0010
GPR04: c000000000abcf20 c000000000ed7c78 0000000000000020 c000000000cbc880
GPR08: 0000000000000000 0000000000000000 0000000080000000 0000000000000002
GPR12: 0000000044002484 c00000000fe40000 0000000000000000 00000000100232f0
GPR16: 0000000000000001 0000000000000000 0000000000000000 c00000007d794a40
GPR20: 0000000000000000 0000000000000024 c00000007a49a200 c00000007a2bd000
GPR24: c00000007aed7bb8 c00000007d7a0090 0000000000014800 0000000000000000
GPR28: c00000007d7a0010 c00000007a49a210 c00000007d7a0150 0000000000000001
NIP [c000000000245710] .time_out_leases+0x30/0x100
LR [c00000000024586c] .__break_lease+0x8c/0x480
Call Trace:
[c00000007aed7770] [c0000000002434c0] .lease_alloc+0x20/0xe0 (unreliable)
[c00000007aed77f0] [c00000000024586c] .__break_lease+0x8c/0x480
[c00000007aed78e0] [c0000000001e0374] .do_dentry_open.isra.14+0xf4/0x370
[c00000007aed7980] [c0000000001e0624] .finish_open+0x34/0x60
[c00000007aed7a00] [c0000000001f519c] .do_last+0x56c/0xe40
[c00000007aed7b20] [c0000000001f5b68] .path_openat+0xf8/0x800
[c00000007aed7c40] [c0000000001f7810] .do_filp_open+0x40/0xb0
[c00000007aed7d70] [c0000000001e1f08] .do_sys_open+0x198/0x2e0
[c00000007aed7e30] [c00000000000a158] syscall_exit+0x0/0x98
Instruction dump: