On Sat, 6 Feb 2010, Andi Kleen wrote:
> > If a hot-added node has not been initialized for the cache, your code is
> > picking an existing one in zonelist order which may be excluded by
> > current's cpuset. Thus, your code has a very real chance of having
> > kmem_getpages() return NULL because get_page_from_freelist() will reject
> > non-atomic ALLOC_CPUSET allocations for prohibited nodes. That isn't a
> > scenario that requires a "funny cpuset," it just has to not allow whatever
> > initialized node comes first in the zonelist.
>
> The point was that you would need to run whoever triggers the memory
> hotadd in a cpuset with limitations. That would be a clear
> don't do that if hurts(tm)
>
With a subset of memory nodes, yes. What else prohibits that except for
your new code?
There's a second issue with this approach that I eluded to above: you're
picking the first initialized node for the cache based solely on whether
it is allocated or not. kmem_getpages() may still return NULL when it
would return new slab for any other initialized node, so you're better off
trying them all.
In other words, my earlier (untested) suggestion:
diff --git a/mm/slab.c b/mm/slab.c
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3172,6 +3172,7 @@ static void *fallback_alloc(struct kmem_cache *cache, gfp_t flags)
gfp_t local_flags;
struct zoneref *z;
struct zone *zone;
+ nodemask_t allowed_nodes = NODE_MASK_NONE;
enum zone_type high_zoneidx = gfp_zone(flags);
void *obj = NULL;
int nid;
@@ -3197,6 +3198,7 @@ retry:
flags | GFP_THISNODE, nid);
if (obj)
break;
+ node_set(nid, allowed_nodes);
}
}
@@ -3210,7 +3212,15 @@ retry:
if (local_flags & __GFP_WAIT)
local_irq_enable();
kmem_flagcheck(cache, flags);
- obj = kmem_getpages(cache, local_flags, numa_node_id());
+ nid = numa_node_id();
+ if (cache->nodelists[nid])
+ obj = kmem_getpages(cache, local_flags, nid);
+ else
+ for_each_node_mask(nid, allowed_nodes) {
+ obj = kmem_getpages(cache, local_flags, nid);
+ if (obj)
+ break;
+ }
if (local_flags & __GFP_WAIT)
local_irq_disable();
if (obj) {
Anyway, I'll leave these otherwise unnecessary limitations to Pekka.
Thanks.
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