[Xen-devel] [PATCH] page-alloc: Clamp get_free_buddy() to online nodes

[Andrew Cooper <andrew.cooper3@citrix.com>]

d->node_affinity defaults to NODE_MASK_ALL which has bits set outside of
node_online_map.  This in turn causes the loop in get_free_buddy() to waste
effort iterating over offline nodes.

Always clamp d->node_affinity to node_online_map when in use.

This in turn involves implementing nodes_copy() which was previously missing.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Julien Grall <julien.grall@arm.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>

This patch hides the issue identified in the "UBSAN report in find_next_bit()"
so probably doesn't want applying until that is resolved.

A lower overhead option would be to do:

nodes_and(nodemask, node_online_map, d ? d->node_affinity : node_online_map);

however this doesn't work because the nodeset_t API has a hidden &(param)
throughout the API.  I've got half a mind to undo this nonsense and have
nodemask_t work in exactly the same way as cpumask_t.
---
 xen/common/page_alloc.c    | 7 ++++++-
 xen/include/xen/nodemask.h | 6 ++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 7825fd8c42..cec1b15d5b 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -811,11 +811,16 @@ static struct page_info *get_free_buddy(unsigned int zone_lo,
                                         const struct domain *d)
 {
     nodeid_t first_node, node = MEMF_get_node(memflags), req_node = node;
-    nodemask_t nodemask = d ? d->node_affinity : node_online_map;
+    nodemask_t nodemask;
     unsigned int j, zone, nodemask_retry = 0;
     struct page_info *pg;
     bool use_unscrubbed = (memflags & MEMF_no_scrub);
 
+    /* Clamp nodemask to node_online_map and optionally d->node_affinity. */
+    nodes_copy(nodemask, node_online_map);
+    if ( d )
+        nodes_and(nodemask, nodemask, d->node_affinity);
+
     if ( node == NUMA_NO_NODE )
     {
         if ( d != NULL )
diff --git a/xen/include/xen/nodemask.h b/xen/include/xen/nodemask.h
index e287399352..e83cfe2439 100644
--- a/xen/include/xen/nodemask.h
+++ b/xen/include/xen/nodemask.h
@@ -189,6 +189,12 @@ static inline int __nodes_weight(const nodemask_t *srcp, int nbits)
 	return bitmap_weight(srcp->bits, nbits);
 }
 
+#define nodes_copy(dst, src) __nodes_copy(&(dst), &(src))
+static inline void __nodes_copy(nodemask_t *dst, nodemask_t *src)
+{
+	return bitmap_copy(dst->bits, src->bits, MAX_NUMNODES);
+}
+
 #define nodes_shift_right(dst, src, n) \
 			__nodes_shift_right(&(dst), &(src), (n), MAX_NUMNODES)
 static inline void __nodes_shift_right(nodemask_t *dstp,
-- 
2.11.0


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