Re: [RFC PATCH]mm: fix potential infinite loop in dissolve_free_huge_pages()

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

Hi Zhong,

On Thu, Jul 24, 2014 at 05:36:25PM +0800, Li Zhong wrote:
> It is possible for some platforms, such as powerpc to set HPAGE_SHIFT to
> 0 to indicate huge pages not supported. 
> 
> When this is the case, hugetlbfs could be disabled during boot time:
> hugetlbfs: disabling because there are no supported hugepage sizes
> 
> Then in dissolve_free_huge_pages(), order is kept maximum (64 for
> 64bits), and the for loop below won't end:
> for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << order)

At first I wonder that why could dissolve_free_huge_pages() is called
if the platform doesn't support hugetlbfs. But I found that the function
is called by memory hotplug code without checking hugepage support.

So it looks to me straightforward and self-descriptive to check
hugepage_supported() just before calling dissolve_free_huge_pages().

Thanks,
Naoya Horiguchi

> The fix below returns directly if the order isn't set to a correct
> value.
> 
> Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
> ---
>  mm/hugetlb.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 2024bbd..a950817 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1093,6 +1093,10 @@ void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
>  	for_each_hstate(h)
>  		if (order > huge_page_order(h))
>  			order = huge_page_order(h);
> +
> +	if (order == 8 * sizeof(void *))
> +		return;
> +
>  	VM_BUG_ON(!IS_ALIGNED(start_pfn, 1 << order));
>  	for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << order)
>  		dissolve_free_huge_page(pfn_to_page(pfn));
> 
> 
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