[PATCH 3.2 21/46] hugetlb: ensure hugepage access is denied if hugepages are not supported

Ben Hutchings
2016-06-12T21:35:56Z

From: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>

commit 457c1b27ed56ec472d202731b12417bff023594a upstream.

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
....

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

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

HPAGE_SHIFT == 0 in this configuration, which indicates that hugepages
are not supported at boot-time, but this is only checked in
hugetlb_init(). Extract the check to a helper function, and use it in a
few relevant places.

This does make hugetlbfs not supported (not registered at all) in this
environment. I believe this is fine, as there are no valid hugepages
and that won't change at runtime.

[akpm@linux-foundation.org: use pr_info(), per Mel]
[akpm@linux-foundation.org: fix build when HPAGE_SHIFT is undefined]
Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2:
- Drop changes to hugetlb_show_meminfo()
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/hugetlbfs/inode.c | 5 +++++
include/linux/hugetlb.h | 10 ++++++++++
mm/hugetlb.c | 19 ++++++++++++++-----
3 files changed, 29 insertions(+), 5 deletions(-)

--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -979,6 +979,11 @@ static int __init init_hugetlbfs_fs(void
int error;
struct vfsmount *vfsmount;

+ if (!hugepages_supported()) {
+ pr_info("hugetlbfs: disabling because there are no supported hugepage sizes\n");
+ return -ENOTSUPP;
+ }
+
error = bdi_init(&hugetlbfs_backing_dev_info);
if (error)
return error;
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -344,6 +344,16 @@ static inline pgoff_t basepage_index(str
return __basepage_index(page);
}

+static inline bool hugepages_supported(void)
+{
+ /*
+ * 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
+ */
+ return HPAGE_SHIFT != 0;
+}
+
#else
struct hstate {};
#define alloc_huge_page_node(h, nid) NULL
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1889,11 +1889,7 @@ module_exit(hugetlb_exit);

static int __init hugetlb_init(void)
{
- /* 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)
+ if (!hugepages_supported())
return 0;

if (!size_to_hstate(default_hstate_size)) {
@@ -2010,6 +2006,9 @@ static int hugetlb_sysctl_handler_common
unsigned long tmp;
int ret;

+ if (!hugepages_supported())
+ return -ENOTSUPP;
+
tmp = h->max_huge_pages;

if (write && h->order >= MAX_ORDER)
@@ -2075,6 +2074,9 @@ int hugetlb_overcommit_handler(struct ct
unsigned long tmp;
int ret;

+ if (!hugepages_supported())
+ return -ENOTSUPP;
+
tmp = h->nr_overcommit_huge_pages;

if (write && h->order >= MAX_ORDER)
@@ -2100,6 +2102,8 @@ out:
void hugetlb_report_meminfo(struct seq_file *m)
{
struct hstate *h = &default_hstate;
+ if (!hugepages_supported())
+ return;
seq_printf(m,
"HugePages_Total: %5lu\n"
"HugePages_Free: %5lu\n"
@@ -2116,6 +2120,8 @@ void hugetlb_report_meminfo(struct seq_f
int hugetlb_report_node_meminfo(int nid, char *buf)
{
struct hstate *h = &default_hstate;
+ if (!hugepages_supported())
+ return 0;
return sprintf(buf,
"Node %d HugePages_Total: %5u\n"
"Node %d HugePages_Free: %5u\n"