[RFC PATCH] hugetlb: ensure hugepage access is denied if hugepages are not supported

[Nishanth Aravamudan]

On 26.03.2014 [08:58:15 -0700], Nishanth Aravamudan wrote:
> 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 <nacc at linux.vnet.ibm.com>

Ping on this? The patch below fixes a pretty easy-to-reproduce bug in
guests under KVM guests on Power.

Thanks,
Nish

> 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",