This patch adds extra explanation of recently added vmap
node layer based on community feedback. No functional change.
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
---
mm/vmalloc.c | 60 ++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 46 insertions(+), 14 deletions(-)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 257981e37936..b8be601b056d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -765,9 +765,10 @@ static struct rb_root free_vmap_area_root = RB_ROOT;
static DEFINE_PER_CPU(struct vmap_area *, ne_fit_preload_node);
/*
- * An effective vmap-node logic. Users make use of nodes instead
- * of a global heap. It allows to balance an access and mitigate
- * contention.
+ * This structure defines a single, solid model where a list and
+ * rb-tree are part of one entity protected by the lock. Nodes are
+ * sorted in ascending order, thus for O(1) access to left/right
+ * neighbors a list is used as well as for sequential traversal.
*/
struct rb_list {
struct rb_root root;
@@ -775,16 +776,23 @@ struct rb_list {
spinlock_t lock;
};
+/*
+ * A fast size storage contains VAs up to 1M size. A pool consists
+ * of linked between each other ready to go VAs of certain sizes.
+ * An index in the pool-array corresponds to number of pages + 1.
+ */
+#define MAX_VA_SIZE_PAGES 256
+
struct vmap_pool {
struct list_head head;
unsigned long len;
};
/*
- * A fast size storage contains VAs up to 1M size.
+ * An effective vmap-node logic. Users make use of nodes instead
+ * of a global heap. It allows to balance an access and mitigate
+ * contention.
*/
-#define MAX_VA_SIZE_PAGES 256
-
static struct vmap_node {
/* Simple size segregated storage. */
struct vmap_pool pool[MAX_VA_SIZE_PAGES];
@@ -803,6 +811,11 @@ static struct vmap_node {
unsigned long nr_purged;
} single;
+/*
+ * Initial setup consists of one single node, i.e. a balancing
+ * is fully disabled. Later on, after vmap is initialized these
+ * parameters are updated based on a system capacity.
+ */
static struct vmap_node *vmap_nodes = &single;
static __read_mostly unsigned int nr_vmap_nodes = 1;
static __read_mostly unsigned int vmap_zone_size = 1;
@@ -2048,7 +2061,12 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
}
}
- /* Attach the pool back if it has been partly decayed. */
+ /*
+ * Attach the pool back if it has been partly decayed.
+ * Please note, it is supposed that nobody(other contexts)
+ * can populate the pool therefore a simple list replace
+ * operation takes place here.
+ */
if (!full_decay && !list_empty(&tmp_list)) {
spin_lock(&vn->pool_lock);
list_replace_init(&tmp_list, &vn->pool[i].head);
@@ -2257,16 +2275,14 @@ struct vmap_area *find_vmap_area(unsigned long addr)
* An addr_to_node_id(addr) converts an address to a node index
* where a VA is located. If VA spans several zones and passed
* addr is not the same as va->va_start, what is not common, we
- * may need to scan an extra nodes. See an example:
+ * may need to scan extra nodes. See an example:
*
- * <--va-->
+ * <----va---->
* -|-----|-----|-----|-----|-
* 1 2 0 1
*
- * VA resides in node 1 whereas it spans 1 and 2. If passed
- * addr is within a second node we should do extra work. We
- * should mention that it is rare and is a corner case from
- * the other hand it has to be covered.
+ * VA resides in node 1 whereas it spans 1, 2 an 0. If passed
+ * addr is within 2 or 0 nodes we should do extra work.
*/
i = j = addr_to_node_id(addr);
do {
@@ -2289,6 +2305,9 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
struct vmap_area *va;
int i, j;
+ /*
+ * Check the comment in the find_vmap_area() about the loop.
+ */
i = j = addr_to_node_id(addr);
do {
vn = &vmap_nodes[i];
@@ -4882,7 +4901,20 @@ static void vmap_init_nodes(void)
int i, n;
#if BITS_PER_LONG == 64
- /* A high threshold of max nodes is fixed and bound to 128. */
+ /*
+ * A high threshold of max nodes is fixed and bound to 128,
+ * thus a scale factor is 1 for systems where number of cores
+ * are less or equal to specified threshold.
+ *
+ * As for NUMA-aware notes. For bigger systems, for example
+ * NUMA with multi-sockets, where we can end-up with thousands
+ * of cores in total, a "sub-numa-clustering" should be added.
+ *
+ * In this case a NUMA domain is considered as a single entity
+ * with dedicated sub-nodes in it which describe one group or
+ * set of cores. Therefore a per-domain purging is supposed to
+ * be added as well as a per-domain balancing.
+ */
n = clamp_t(unsigned int, num_possible_cpus(), 1, 128);
if (n > 1) {
--
2.39.2
