Re: [PATCH] mm: fix invalid node in alloc_migrate_target()

[Vlastimil Babka]

On 03/26/2016 06:31 AM, Xishi Qiu wrote:
> On 2016/3/26 3:22, Andrew Morton wrote:
>
> > On Fri, 25 Mar 2016 14:56:04 +0800 Xishi Qiu <qiuxishi@xxxxxxxxxx> wrote:
> >
> > > It is incorrect to use next_node to find a target node, it will
> > > return MAX_NUMNODES or invalid node. This will lead to crash in
> > > buddy system allocation.
> > >
> > > ...
> > >
> > > --- a/mm/page_isolation.c
> > > +++ b/mm/page_isolation.c
> > > @@ -289,11 +289,11 @@ struct page *alloc_migrate_target(struct page *page, unsigned long private,
> > >   	 * now as a simple work-around, we use the next node for destination.
> > >   	 */
> > >   	if (PageHuge(page)) {
> > > -		nodemask_t src = nodemask_of_node(page_to_nid(page));
> > > -		nodemask_t dst;
> > > -		nodes_complement(dst, src);
> > > +		int node = next_online_node(page_to_nid(page));
> > > +		if (node == MAX_NUMNODES)
> > > +			node = first_online_node;
> > >   		return alloc_huge_page_node(page_hstate(compound_head(page)),
> > > -					    next_node(page_to_nid(page), dst));
> > > +					    node);
> > >   	}
> > >
> > >   	if (PageHighMem(page))
> >
> > Indeed.  Can you tell us more about this circumstances under which the
> > kernel will crash?  I need to decide which kernel version(s) need the
> > patch, but the changelog doesn't contain the info needed to make this
> > decision (it should).
>
> Hi Andrew,
>
> I read the code v4.4, and find the following path maybe trigger the bug.
>
> alloc_migrate_target()
> 	alloc_huge_page_node()  // the node may be offline or MAX_NUMNODES
> 		__alloc_buddy_huge_page_no_mpol()
> 			__alloc_buddy_huge_page()
> 				__hugetlb_alloc_buddy_huge_page()

The code in this functions seems to come from 099730d67417d ("mm, 
hugetlb: use memory policy when available") by Dave Hansen (adding to 
CC), which was indeed merged in 4.4-rc1.

However, alloc_pages_node() is only called in the block guarded by:

if (!IS_ENABLED(CONFIG_NUMA) || !vma) {

The rather weird "!IS_ENABLED(CONFIG_NUMA)" part comes from immediate 
followup commit e0ec90ee7e6f ("mm, hugetlbfs: optimize when NUMA=n")

So I doubt the code path here can actually happen. But it's fragile and 
confusing nevertheless.

> 					alloc_pages_node()
> 						__alloc_pages_node()
> 							VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
> 							VM_WARN_ON(!node_online(nid));
>
> Thanks,
> Xishi Qiu