hugetlb: ensure hugepage access is denied if hugepages are not supported

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>

3 files changed, 29 insertions, 5 deletions

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 204027520937..e19d4c0cacae 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -1030,6 +1030,11 @@ static int __init init_hugetlbfs_fs(void)
         int error;
         int i;
 
+        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;
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 5b337cf8fb86..b65166de1d9d 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -412,6 +412,16 @@ static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
         return &mm->page_table_lock;
 }
 
+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   /* CONFIG_HUGETLB_PAGE */
 struct hstate {};
 #define alloc_huge_page_node(h, nid) NULL
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 246192929a2d..c82290b9c1fc 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1981,11 +1981,7 @@ static int __init hugetlb_init(void)
 {
         int i;
 
-        /* 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)) {
@@ -2112,6 +2108,9 @@ static int hugetlb_sysctl_handler_common(bool obey_mempolicy,
         unsigned long tmp;
         int ret;
 
+        if (!hugepages_supported())
+                return -ENOTSUPP;
+
         tmp = h->max_huge_pages;
 
         if (write && h->order >= MAX_ORDER)
@@ -2165,6 +2164,9 @@ int hugetlb_overcommit_handler(struct ctl_table *table, int write,
         unsigned long tmp;
         int ret;
 
+        if (!hugepages_supported())
+                return -ENOTSUPP;
+
         tmp = h->nr_overcommit_huge_pages;
 
         if (write && h->order >= MAX_ORDER)
@@ -2190,6 +2192,8 @@ 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"
@@ -2206,6 +2210,8 @@ void hugetlb_report_meminfo(struct seq_file *m)
 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"
@@ -2220,6 +2226,9 @@ void hugetlb_show_meminfo(void)
         struct hstate *h;
         int nid;
 
+        if (!hugepages_supported())
+                return;
+
         for_each_node_state(nid, N_MEMORY)
                 for_each_hstate(h)
                         pr_info("Node %d hugepages_total=%u hugepages_free=%u hugepages_surp=%u hugepages_size=%lukB\n",