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Subject[PATCH 5/5] mm/memory_hotplug: Clean up node_states_check_changes_offline
Date
From: Oscar Salvador <osalvador@suse.de>

This patch, as the previous one, gets rid of the wrong if statements.
While at it, I realized that the comments are sometimes very confusing,
to say the least, and wrong.
For example:

---
zone_last = ZONE_MOVABLE;

/*
* check whether node_states[N_HIGH_MEMORY] will be changed
* If we try to offline the last present @nr_pages from the node,
* we can determind we will need to clear the node from
* node_states[N_HIGH_MEMORY].
*/

for (; zt <= zone_last; zt++)
present_pages += pgdat->node_zones[zt].present_pages;
if (nr_pages >= present_pages)
arg->status_change_nid = zone_to_nid(zone);
else
arg->status_change_nid = -1;
---

In case the node gets empry, it must be removed from N_MEMORY.
We already check N_HIGH_MEMORY a bit above within the CONFIG_HIGHMEM
ifdef code.
Not to say that status_change_nid is for N_MEMORY, and not for
N_HIGH_MEMORY.

So I re-wrote some of the comments to what I think is better.

Signed-off-by: Oscar Salvador <osalvador@suse.de>
---
mm/memory_hotplug.c | 71 +++++++++++++++++++++--------------------------------
1 file changed, 28 insertions(+), 43 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index ab3c1de18c5d..15ecf3d7a554 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1485,51 +1485,36 @@ static void node_states_check_changes_offline(unsigned long nr_pages,
{
struct pglist_data *pgdat = zone->zone_pgdat;
unsigned long present_pages = 0;
- enum zone_type zt, zone_last = ZONE_NORMAL;
+ enum zone_type zt;

/*
- * If we have HIGHMEM or movable node, node_states[N_NORMAL_MEMORY]
- * contains nodes which have zones of 0...ZONE_NORMAL,
- * set zone_last to ZONE_NORMAL.
- *
- * If we don't have HIGHMEM nor movable node,
- * node_states[N_NORMAL_MEMORY] contains nodes which have zones of
- * 0...ZONE_MOVABLE, set zone_last to ZONE_MOVABLE.
+ * Check whether node_states[N_NORMAL_MEMORY] will be changed.
+ * If the memory to be offline is within the range
+ * [0..ZONE_NORMAL], and it is the last present memory there,
+ * the zones in that range will become empty after the offlining,
+ * thus we can determine that we need to clear the node from
+ * node_states[N_NORMAL_MEMORY].
*/
- if (N_MEMORY == N_NORMAL_MEMORY)
- zone_last = ZONE_MOVABLE;
-
- /*
- * check whether node_states[N_NORMAL_MEMORY] will be changed.
- * If the memory to be offline is in a zone of 0...zone_last,
- * and it is the last present memory, 0...zone_last will
- * become empty after offline , thus we can determind we will
- * need to clear the node from node_states[N_NORMAL_MEMORY].
- */
- for (zt = 0; zt <= zone_last; zt++)
+ for (zt = 0; zt <= ZONE_NORMAL; zt++)
present_pages += pgdat->node_zones[zt].present_pages;
- if (zone_idx(zone) <= zone_last && nr_pages >= present_pages)
+ if (zone_idx(zone) <= ZONE_NORMAL && nr_pages >= present_pages)
arg->status_change_nid_normal = zone_to_nid(zone);
else
arg->status_change_nid_normal = -1;

#ifdef CONFIG_HIGHMEM
/*
- * If we have movable node, node_states[N_HIGH_MEMORY]
- * contains nodes which have zones of 0...ZONE_HIGHMEM,
- * set zone_last to ZONE_HIGHMEM.
- *
- * If we don't have movable node, node_states[N_NORMAL_MEMORY]
- * contains nodes which have zones of 0...ZONE_MOVABLE,
- * set zone_last to ZONE_MOVABLE.
+ * node_states[N_HIGH_MEMORY] contains nodes which
+ * have normal memory or high memory.
+ * Here we add the present_pages belonging to ZONE_HIGHMEM.
+ * If the zone is within the range of [0..ZONE_HIGHMEM), and
+ * we determine that the zones in that range become empty,
+ * we need to clear the node for N_HIGH_MEMORY.
*/
- zone_last = ZONE_HIGHMEM;
- if (N_MEMORY == N_HIGH_MEMORY)
- zone_last = ZONE_MOVABLE;
+ zt = ZONE_HIGHMEM;
+ present_pages += pgdat->node_zones[zt].present_pages;

- for (; zt <= zone_last; zt++)
- present_pages += pgdat->node_zones[zt].present_pages;
- if (zone_idx(zone) <= zone_last && nr_pages >= present_pages)
+ if (zone_idx(zone) <= zt && nr_pages >= present_pages)
arg->status_change_nid_high = zone_to_nid(zone);
else
arg->status_change_nid_high = -1;
@@ -1542,18 +1527,18 @@ static void node_states_check_changes_offline(unsigned long nr_pages,
#endif

/*
- * node_states[N_HIGH_MEMORY] contains nodes which have 0...ZONE_MOVABLE
+ * We have accounted the pages from [0..ZONE_NORMAL), and
+ * in case of CONFIG_HIGHMEM the pages from ZONE_HIGHMEM
+ * as well.
+ * Here we count the possible pages from ZONE_MOVABLE.
+ * If after having accounted all the pages, we see that the nr_pages
+ * to be offlined is over or equal to the accounted pages,
+ * we know that the node will become empty, and so, we can clear
+ * it for N_MEMORY as well.
*/
- zone_last = ZONE_MOVABLE;
+ zt = ZONE_MOVABLE;
+ present_pages += pgdat->node_zones[zt].present_pages;

- /*
- * check whether node_states[N_HIGH_MEMORY] will be changed
- * If we try to offline the last present @nr_pages from the node,
- * we can determind we will need to clear the node from
- * node_states[N_HIGH_MEMORY].
- */
- for (; zt <= zone_last; zt++)
- present_pages += pgdat->node_zones[zt].present_pages;
if (nr_pages >= present_pages)
arg->status_change_nid = zone_to_nid(zone);
else
--
2.13.6
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