[PATCH v2 3/3] mm/memory_hotplug: Refactor unregister_mem_sect_under_nodes

[osalvador]

From: Oscar Salvador <osalvador@xxxxxxx>

unregister_mem_sect_under_nodes() tries to allocate a nodemask_t
in order to check whithin the loop which nodes have already been unlinked,
so we do not repeat the operation on them.

NODEMASK_ALLOC calls kmalloc() if NODES_SHIFT > 8, otherwise
it just declares a nodemask_t variable whithin the stack.

Since kamlloc() can fail, we actually check whether NODEMASK_ALLOC failed or
not, and we return -ENOMEM accordingly.
remove_memory_section() does not check for the return value though.

The problem with this is that if we return -ENOMEM, it means that
unregister_mem_sect_under_nodes will not be able to remove the symlinks,
but since we do not check the return value, we go ahead and we call unregister_memory(),
which will remove all the mem_blks directories.

This will leave us with dangled symlinks.

The easiest way to overcome this is to fallback by calling sysfs_remove_link()
unconditionally in case NODEMASK_ALLOC failed.
This means that we will call sysfs_remove_link on nodes that have been already unlinked,
but nothing wrong happens as sysfs_remove_link() backs off somewhere down the chain in case
the link has already been removed.

I think that this is better than

a) dangled symlinks
b) having to recovery from such error in remove_memory_section

Since from now on we will not need to take care about return values, we can make the function void.

While at it, we can also drop the node_online() check, as a node can only be
offline if all the memory/cpus associated with it have been removed.

As we have a safe fallback, one thing that could also be done is to add __GFP_NORETRY
in the flags when calling NODEMASK_ALLOC, so we do not retry.

Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
---
 drivers/base/node.c  | 26 +++++++++++++++-----------
 include/linux/node.h |  5 ++---
 2 files changed, 17 insertions(+), 14 deletions(-)

diff --git a/drivers/base/node.c b/drivers/base/node.c
index dd3bdab230b2..0a3ca62687ea 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -449,35 +449,39 @@ int register_mem_sect_under_node(struct memory_block *mem_blk, void *arg)
 }
 
 /* unregister memory section under all nodes that it spans */
-int unregister_mem_sect_under_nodes(struct memory_block *mem_blk,
+void unregister_mem_sect_under_nodes(struct memory_block *mem_blk,
 				    unsigned long phys_index)
 {
 	NODEMASK_ALLOC(nodemask_t, unlinked_nodes, GFP_KERNEL);
 	unsigned long pfn, sect_start_pfn, sect_end_pfn;
 
-	if (!unlinked_nodes)
-		return -ENOMEM;
-	nodes_clear(*unlinked_nodes);
+	if (unlinked_nodes)
+		nodes_clear(*unlinked_nodes);
 
 	sect_start_pfn = section_nr_to_pfn(phys_index);
 	sect_end_pfn = sect_start_pfn + PAGES_PER_SECTION - 1;
 	for (pfn = sect_start_pfn; pfn <= sect_end_pfn; pfn++) {
-		int nid;
+		int nid = get_nid_for_pfn(pfn);;
 
-		nid = get_nid_for_pfn(pfn);
 		if (nid < 0)
 			continue;
-		if (!node_online(nid))
-			continue;
-		if (node_test_and_set(nid, *unlinked_nodes))
+		/*
+		 * It is possible that NODEMASK_ALLOC fails due to memory pressure.
+		 * If that happens, we fallback to call sysfs_remove_link unconditionally.
+		 * Nothing wrong will happen as sysfs_remove_link will back off
+		 * somewhere down the chain in case the link has already been removed.
+		 */
+		if (unlinked_nodes && node_test_and_set(nid, *unlinked_nodes))
 			continue;
+
 		sysfs_remove_link(&node_devices[nid]->dev.kobj,
 			 kobject_name(&mem_blk->dev.kobj));
 		sysfs_remove_link(&mem_blk->dev.kobj,
 			 kobject_name(&node_devices[nid]->dev.kobj));
 	}
-	NODEMASK_FREE(unlinked_nodes);
-	return 0;
+
+	if (unlinked_nodes)
+		NODEMASK_FREE(unlinked_nodes);
 }
 
 int link_mem_sections(int nid, unsigned long start_pfn, unsigned long end_pfn)
diff --git a/include/linux/node.h b/include/linux/node.h
index 257bb3d6d014..1203378e596a 100644
--- a/include/linux/node.h
+++ b/include/linux/node.h
@@ -72,7 +72,7 @@ extern int register_cpu_under_node(unsigned int cpu, unsigned int nid);
 extern int unregister_cpu_under_node(unsigned int cpu, unsigned int nid);
 extern int register_mem_sect_under_node(struct memory_block *mem_blk,
 						void *arg);
-extern int unregister_mem_sect_under_nodes(struct memory_block *mem_blk,
+extern void unregister_mem_sect_under_nodes(struct memory_block *mem_blk,
 					   unsigned long phys_index);
 
 #ifdef CONFIG_HUGETLBFS
@@ -105,10 +105,9 @@ static inline int register_mem_sect_under_node(struct memory_block *mem_blk,
 {
 	return 0;
 }
-static inline int unregister_mem_sect_under_nodes(struct memory_block *mem_blk,
+static inline void unregister_mem_sect_under_nodes(struct memory_block *mem_blk,
 						  unsigned long phys_index)
 {
-	return 0;
 }
 
 static inline void register_hugetlbfs_with_node(node_registration_func_t reg,
-- 
2.13.6