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

[Vlastimil Babka]

On 03/29/2016 11:52 AM, Vlastimil Babka wrote:
> 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.

Ah, so there's actually a dangerous path:
alloc_huge_page_node()
    dequeue_huge_page_node()
        list_for_each_entry(page, &h->hugepage_freelists[nid], lru)

hugepage_freelists is MAX_NUMNODES sized, so when nid is MAX_NUMNODES, 
we access past it.

However, look closer at how nid is obtained in alloc_migrate_target():

nodemask_t src = nodemask_of_node(page_to_nid(page));
nodemask_t dst;
nodes_complement(dst, src);

nid = next_node(page_to_nid(page), dst)

for nid to be MAX_NUMNODES, the original page has to be on node 
MAX_NUMNODES-1, otherwise the complement part means we hit the very next 
bit which is set.

It's actually a rather obfuscated way of doing:

nid = page_to_nid(page) + 1;

In that case the problem is in commit c8721bbbdd36 ("mm: memory-hotplug: 
enable memory hotplug to handle hugepage") from 3.12 and will likely 
affect only people that tune down MAX_NUMNODES to match their machine.

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