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

[Mel Gorman <mgorman@suse.de>]

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?

-- 
Mel Gorman
SUSE Labs