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

[Li Zhong <zhong@linux.vnet.ibm.com>]

On Thu, 2014-07-24 at 08:45 -0400, Naoya Horiguchi wrote:
> 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().

Hi, Naoya,

Thank you for the review and suggestion.

I'll send a updated version. 

Thanks, Zhong

> 
> 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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> 


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