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

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.

Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>

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

> 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.
>
> Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index d19b30a..cc8fcc7 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 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 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",

On Thu, Apr 03, 2014 at 04:14:13PM -0700, Nishanth Aravamudan wrote:
> 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.
> 
> Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>

Acked-by: Mel Gorman <mgorman@suse.de>

This patch looks ok but the changelog misses important information from
the original report which is probably why it fell through the cracks.
Add the fact that you encountered a problem during mount to the changelog
and resend it directly to Andrew. This part from your original report;

	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
	....

It probably slipped through the cracks because from the changelog this
looks like a minor formatting issue and not a functional fix.

> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index d19b30a..cc8fcc7 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 hugepage sizes\n");
> +		return -ENOTSUPP;
> +	}
> +
>  	error = bdi_init(&hugetlbfs_backing_dev_info);
>  	if (error)
>  		return error;

KERN_ERR feels like overkill for this type of issue. KERN_INFO?