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

[Vlastimil Babka]

On 03/31/2016 11:01 PM, Andrew Morton wrote:
> On Thu, 31 Mar 2016 15:13:41 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> > On 03/29/2016 03:06 PM, Vlastimil Babka wrote:
> > > On 03/25/2016 08:22 PM, Andrew Morton wrote:
> > >> Also, mm/mempolicy.c:offset_il_node() worries me:
> > >>
> > >> 	do {
> > >> 		nid = next_node(nid, pol->v.nodes);
> > >> 		c++;
> > >> 	} while (c <= target);
> > >>
> > >> Can't `nid' hit MAX_NUMNODES?
> > >
> > > AFAICS it can. interleave_nid() uses this and the nid is then used e.g.
> > > in node_zonelist() where it's used for NODE_DATA(nid). That's quite
> > > scary. It also predates git. Why don't we see crashes or KASAN finding this?
> >
> > Ah, I see. In offset_il_node(), nid is initialized to -1, and the number
> > of do-while iterations calling next_node() is up to the number of bits
> > set in the pol->v.nodes bitmap, so it can't reach past the last set bit
> > and return MAX_NUMNODES.
>
> Gack.  offset_il_node() should be dragged out, strangled, shot then burnt.

Ah, but you went with the much less amusing alternative of just fixing it.

> static unsigned offset_il_node(struct mempolicy *pol,
> 		struct vm_area_struct *vma, unsigned long off)
> {
> 	unsigned nnodes = nodes_weight(pol->v.nodes);
> 	unsigned target;
> 	int c;
> 	int nid = NUMA_NO_NODE;
>
> 	if (!nnodes)
> 		return numa_node_id();
> 	target = (unsigned int)off % nnodes;
> 	c = 0;
> 	do {
> 		nid = next_node(nid, pol->v.nodes);
> 		c++;
> 	} while (c <= target);
> 	return nid;
> }
>
> For starters it is relying upon next_node(-1, ...) behaving like
> first_node().  Fair enough I guess, but that isn't very clear.
>
> static inline int __next_node(int n, const nodemask_t *srcp)
> {
> 	return min_t(int,MAX_NUMNODES,find_next_bit(srcp->bits, MAX_NUMNODES, n+1));
> }
>
> will start from node 0 when it does the n+1.
>
> Also it is relying upon NUMA_NO_NODE having a value of -1.  That's just
> grubby - this code shouldn't "know" that NUMA_NO_NODE==-1.  It would have
> been better to use plain old "-1" here.

Yeah looks like a blind change of all "-1" to "NUMA_NO_NODE" happened at some point.

> Does this look clearer and correct?

Definitely.

> /*
>   * Do static interleaving for a VMA with known offset @n.  Returns the n'th
>   * node in pol->v.nodes (starting from n=0), wrapping around if n exceeds the
>   * number of present nodes.
>   */
> static unsigned offset_il_node(struct mempolicy *pol,
> 			       struct vm_area_struct *vma, unsigned long n)
> {
> 	unsigned nnodes = nodes_weight(pol->v.nodes);
> 	unsigned target;
> 	int i;
> 	int nid;
>
> 	if (!nnodes)
> 		return numa_node_id();
> 	target = (unsigned int)n % nnodes;
> 	nid = first_node(pol->v.nodes);
> 	for (i = 0; i < target; i++)
> 		nid = next_node(nid, pol->v.nodes);
> 	return nid;
> }
>
>
> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Subject: mm/mempolicy.c:offset_il_node() document and clarify
>
> This code was pretty obscure and was relying upon obscure side-effects of
> next_node(-1, ...) and was relying upon NUMA_NO_NODE being equal to -1.
>
> Clean that all up and document the function's intent.
>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Xishi Qiu <qiuxishi@xxxxxxxxxx>
> Cc: Joonsoo Kim <js1304@xxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Laura Abbott <lauraa@xxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>