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>
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