On 24.03.2014 [16:02:56 -0700], Nishanth Aravamudan wrote:

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]

Here is my solution to this. Comments appreciated!

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 in this environment. I believe
this is fine, as there are no valid hugepages and that won't change at
runtime.

Signed-off-by: Nishanth Aravamudan <redacted>

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index d19b30a..c7aa477 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c

@@ -1017,6 +1017,11 @@ static int __init init_hugetlbfs_fs(void)
 	int error;
 	int i;
 
+	if (!hugepages_supported()) {
+		printk(KERN_ERR "hugetlbfs: Disabling because there are no supported page 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 8c43cc4..0aea8de 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h

@@ -450,4 +450,14 @@ static inline spinlock_t *huge_pte_lock(struct hstate *h,
 	return ptl;
 }
 
+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;
+}
+
 #endif /* _LINUX_HUGETLB_H */

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c01cb9f..1c99585 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c

@@ -1949,11 +1949,7 @@ 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)) {

@@ -2069,6 +2065,9 @@ 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)

@@ -2122,6 +2121,9 @@ 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)

@@ -2147,6 +2149,8 @@ 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"

@@ -2163,6 +2167,8 @@ 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"

@@ -2177,6 +2183,9 @@ 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",