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

osalvador
2018-08-13T15:46:57Z

From: Oscar Salvador <osalvador@suse.de>

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@suse.de>
---
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