[PATCH v8 4/8] mm,memory_hotplug: Allocate memmap from the added memory range

From: Oscar Salvador
Date: Fri Apr 16 2021 - 06:22:12 EST

Physical memory hotadd has to allocate a memmap (struct page array) for
the newly added memory section. Currently, alloc_pages_node() is used
for those allocations.

This has some disadvantages:
a) an existing memory is consumed for that purpose
(eg: ~2MB per 128MB memory section on x86_64)
b) if the whole node is movable then we have off-node struct pages
which has performance drawbacks.
c) It might be there are no PMD_ALIGNED chunks so memmap array gets
populated with base pages.

This can be improved when CONFIG_SPARSEMEM_VMEMMAP is enabled.

Vmemap page tables can map arbitrary memory.
That means that we can simply use the beginning of each memory section and
map struct pages there.
struct pages which back the allocated space then just need to be treated
carefully.

Implementation wise we will reuse vmem_altmap infrastructure to override
the default allocator used by __populate_section_memmap.
Part of the implementation also relies on memory_block structure gaining
a new field which specifies the number of vmemmap_pages at the beginning.
This patch also introduces the following functions:

- mhp_init_memmap_on_memory:
Initializes vmemmap pages by calling move_pfn_range_to_zone(),
calls kasan_add_zero_shadow(), and onlines as many sections
as vmemmap pages fully span.
- mhp_deinit_memmap_on_memory:
Undoes what mhp_init_memmap_on_memory.

The new function memory_block_online() calls mhp_init_memmap_on_memory() before
doing the actual online_pages(). Should online_pages() fail, we clean up
by calling mhp_deinit_memmap_on_memory().
Adjusting of present_pages is done at the end once we know that online_pages()
succedeed.

On offline, memory_block_offline() needs to unaccount vmemmap pages from
present_pages() before calling offline_pages().
This is necessary because offline_pages() tears down some structures based
on the fact whether the node or the zone become empty.
If offline_pages() fails, we account back vmemmap pages.
If it succeeds, we call mhp_deinit_memmap_on_memory().

Hot-remove:

We need to be careful when removing memory, as adding and
removing memory needs to be done with the same granularity.
To check that this assumption is not violated, we check the
memory range we want to remove and if a) any memory block has
vmemmap pages and b) the range spans more than a single memory
block, we scream out loud and refuse to proceed.

If all is good and the range was using memmap on memory (aka vmemmap pages),
we construct an altmap structure so free_hugepage_table does the right
thing and calls vmem_altmap_free instead of free_pagetable.

Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
---
drivers/base/memory.c | 75 ++++++++++++++++--
include/linux/memory.h | 8 +-
include/linux/memory_hotplug.h | 17 +++-
include/linux/memremap.h | 2 +-
include/linux/mmzone.h | 7 +-
mm/Kconfig | 5 ++
mm/memory_hotplug.c | 171 ++++++++++++++++++++++++++++++++++++++---
mm/sparse.c | 2 -
8 files changed, 265 insertions(+), 22 deletions(-)

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index f209925a5d4e..179857d53982 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -173,16 +173,76 @@ static int memory_block_online(struct memory_block *mem)
{
unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
+ unsigned long nr_vmemmap_pages = mem->nr_vmemmap_pages;
+ struct zone *zone;
+ int ret;
+
+ zone = mhp_get_target_zone(start_pfn, nr_pages, mem->nid,
+ mem->online_type);
+
+ /*
+ * Although vmemmap pages have a different lifecycle than the pages
+ * they describe (they remain until the memory is unplugged), doing
+ * its initialization and accounting at hot-{online,offline} stage
+ * simplifies things a lot
+ */
+ if (nr_vmemmap_pages) {
+ ret = mhp_init_memmap_on_memory(start_pfn, nr_vmemmap_pages, zone);
+ if (ret)
+ return ret;
+ }
+
+ ret = online_pages(start_pfn + nr_vmemmap_pages,
+ nr_pages - nr_vmemmap_pages, zone);
+ if (ret) {
+ if (nr_vmemmap_pages)
+ mhp_deinit_memmap_on_memory(start_pfn, nr_vmemmap_pages);
+ return ret;
+ }
+
+ /*
+ * Account once onlining succeeded. If the page was unpopulated, it is
+ * now already properly populated.
+ */
+ if (nr_vmemmap_pages)
+ adjust_present_page_count(zone, nr_vmemmap_pages);

- return online_pages(start_pfn, nr_pages, mem->online_type, mem->nid);
+ return ret;
}

static int memory_block_offline(struct memory_block *mem)
{
unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
+ unsigned long nr_vmemmap_pages = mem->nr_vmemmap_pages;
+ struct zone *zone;
+ int ret;
+
+ zone = page_zone(pfn_to_page(start_pfn));

- return offline_pages(start_pfn, nr_pages);
+ /*
+ * Unaccount before offlining, such that unpopulated zone and kthreads
+ * can properly be torn down in offline_pages().
+ */
+ if (nr_vmemmap_pages)
+ adjust_present_page_count(zone, -nr_vmemmap_pages);
+
+ ret = offline_pages(start_pfn + nr_vmemmap_pages,
+ nr_pages - nr_vmemmap_pages);
+ if (ret) {
+ /* offline_pages() failed. Account back. */
+ if (nr_vmemmap_pages)
+ adjust_present_page_count(zone, nr_vmemmap_pages);
+ return ret;
+ }
+
+ /*
+ * Re-adjust present pages if offline_pages() fails.
+ */
+ if (nr_vmemmap_pages)
+ mhp_deinit_memmap_on_memory(start_pfn, nr_vmemmap_pages);
+
+ return ret;
}

/*
@@ -576,7 +636,8 @@ int register_memory(struct memory_block *memory)
return ret;
}

-static int init_memory_block(unsigned long block_id, unsigned long state)
+static int init_memory_block(unsigned long block_id, unsigned long state,
+ unsigned long nr_vmemmap_pages)
{
struct memory_block *mem;
int ret = 0;
@@ -593,6 +654,7 @@ static int init_memory_block(unsigned long block_id, unsigned long state)
mem->start_section_nr = block_id * sections_per_block;
mem->state = state;
mem->nid = NUMA_NO_NODE;
+ mem->nr_vmemmap_pages = nr_vmemmap_pages;

ret = register_memory(mem);

@@ -612,7 +674,7 @@ static int add_memory_block(unsigned long base_section_nr)
if (section_count == 0)
return 0;
return init_memory_block(memory_block_id(base_section_nr),
- MEM_ONLINE);
+ MEM_ONLINE, 0);
}

static void unregister_memory(struct memory_block *memory)
@@ -634,7 +696,8 @@ static void unregister_memory(struct memory_block *memory)
*
* Called under device_hotplug_lock.
*/
-int create_memory_block_devices(unsigned long start, unsigned long size)
+int create_memory_block_devices(unsigned long start, unsigned long size,
+ unsigned long vmemmap_pages)
{
const unsigned long start_block_id = pfn_to_block_id(PFN_DOWN(start));
unsigned long end_block_id = pfn_to_block_id(PFN_DOWN(start + size));
@@ -647,7 +710,7 @@ int create_memory_block_devices(unsigned long start, unsigned long size)
return -EINVAL;

for (block_id = start_block_id; block_id != end_block_id; block_id++) {
- ret = init_memory_block(block_id, MEM_OFFLINE);
+ ret = init_memory_block(block_id, MEM_OFFLINE, vmemmap_pages);
if (ret)
break;
}
diff --git a/include/linux/memory.h b/include/linux/memory.h
index 4da95e684e20..97e92e8b556a 100644
--- a/include/linux/memory.h
+++ b/include/linux/memory.h
@@ -29,6 +29,11 @@ struct memory_block {
int online_type; /* for passing data to online routine */
int nid; /* NID for this memory block */
struct device dev;
+ /*
+ * Number of vmemmap pages. These pages
+ * lay at the beginning of the memory block.
+ */
+ unsigned long nr_vmemmap_pages;
};

int arch_get_memory_phys_device(unsigned long start_pfn);
@@ -80,7 +85,8 @@ static inline int memory_notify(unsigned long val, void *v)
#else
extern int register_memory_notifier(struct notifier_block *nb);
extern void unregister_memory_notifier(struct notifier_block *nb);
-int create_memory_block_devices(unsigned long start, unsigned long size);
+int create_memory_block_devices(unsigned long start, unsigned long size,
+ unsigned long vmemmap_pages);
void remove_memory_block_devices(unsigned long start, unsigned long size);
extern void memory_dev_init(void);
extern int memory_notify(unsigned long val, void *v);
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 7288aa5ef73b..992d85211549 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -55,6 +55,14 @@ typedef int __bitwise mhp_t;
*/
#define MHP_MERGE_RESOURCE ((__force mhp_t)BIT(0))

+/*
+ * We want memmap (struct page array) to be self contained.
+ * To do so, we will use the beginning of the hot-added range to build
+ * the page tables for the memmap array that describes the entire range.
+ * Only selected architectures support it with SPARSE_VMEMMAP.
+ */
+#define MHP_MEMMAP_ON_MEMORY ((__force mhp_t)BIT(1))
+
/*
* Extended parameters for memory hotplug:
* altmap: alternative allocator for memmap array (optional)
@@ -99,9 +107,15 @@ static inline void zone_seqlock_init(struct zone *zone)
extern int zone_grow_free_lists(struct zone *zone, unsigned long new_nr_pages);
extern int zone_grow_waitqueues(struct zone *zone, unsigned long nr_pages);
extern int add_one_highpage(struct page *page, int pfn, int bad_ppro);
+extern void adjust_present_page_count(struct zone *zone, long nr_pages);
/* VM interface that may be used by firmware interface */
+extern struct zone *mhp_get_target_zone(unsigned long pfn, unsigned long nr_pages,
+ int nid, int online_type);
+extern int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
+ struct zone *zone);
+extern void mhp_deinit_memmap_on_memory(unsigned long pfn, unsigned long nr_pages);
extern int online_pages(unsigned long pfn, unsigned long nr_pages,
- int online_type, int nid);
+ struct zone *zone);
extern struct zone *test_pages_in_a_zone(unsigned long start_pfn,
unsigned long end_pfn);
extern void __offline_isolated_pages(unsigned long start_pfn,
@@ -359,6 +373,7 @@ extern struct zone *zone_for_pfn_range(int online_type, int nid, unsigned start_
extern int arch_create_linear_mapping(int nid, u64 start, u64 size,
struct mhp_params *params);
void arch_remove_linear_mapping(u64 start, u64 size);
+extern bool mhp_supports_memmap_on_memory(unsigned long size);
#endif /* CONFIG_MEMORY_HOTPLUG */

#endif /* __LINUX_MEMORY_HOTPLUG_H */
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index f5b464daeeca..45a79da89c5f 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -17,7 +17,7 @@ struct device;
* @alloc: track pages consumed, private to vmemmap_populate()
*/
struct vmem_altmap {
- const unsigned long base_pfn;
+ unsigned long base_pfn;
const unsigned long end_pfn;
const unsigned long reserve;
unsigned long free;
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 47946cec7584..76f4ca5ed230 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -427,6 +427,11 @@ enum zone_type {
* techniques might use alloc_contig_range() to hide previously
* exposed pages from the buddy again (e.g., to implement some sort
* of memory unplug in virtio-mem).
+ * 6. Memory-hotplug: when using memmap_on_memory and onlining the memory
+ * to the MOVABLE zone, the vmemmap pages are also placed in such
+ * zone. Such pages cannot be really moved around as they are
+ * self-stored in the range, but they are treated as movable when
+ * the range they describe is about to be offlined.
*
* In general, no unmovable allocations that degrade memory offlining
* should end up in ZONE_MOVABLE. Allocators (like alloc_contig_range())
@@ -1378,10 +1383,8 @@ static inline int online_section_nr(unsigned long nr)

#ifdef CONFIG_MEMORY_HOTPLUG
void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn);
-#ifdef CONFIG_MEMORY_HOTREMOVE
void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn);
#endif
-#endif

static inline struct mem_section *__pfn_to_section(unsigned long pfn)
{
diff --git a/mm/Kconfig b/mm/Kconfig
index 24c045b24b95..febf805000f8 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -183,6 +183,11 @@ config MEMORY_HOTREMOVE
depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
depends on MIGRATION

+config MHP_MEMMAP_ON_MEMORY
+ def_bool y
+ depends on MEMORY_HOTPLUG && SPARSEMEM_VMEMMAP
+ depends on ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
+
# Heavily threaded applications may benefit from splitting the mm-wide
# page_table_lock, so that faults on different parts of the user address
# space can be handled with less contention: split it at this NR_CPUS.
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index d05056b3c173..7efee41268b6 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -42,6 +42,8 @@
#include "internal.h"
#include "shuffle.h"

+static bool memmap_on_memory;
+
/*
* online_page_callback contains pointer to current page onlining function.
* Initially it is generic_online_page(). If it is required it could be
@@ -641,7 +643,12 @@ EXPORT_SYMBOL_GPL(generic_online_page);
static void online_pages_range(unsigned long start_pfn, unsigned long nr_pages)
{
const unsigned long end_pfn = start_pfn + nr_pages;
- unsigned long pfn;
+ unsigned long pfn = start_pfn;
+
+ while (!IS_ALIGNED(pfn, MAX_ORDER_NR_PAGES)) {
+ (*online_page_callback)(pfn_to_page(pfn), pageblock_order);
+ pfn += pageblock_nr_pages;
+ }

/*
* Online the pages in MAX_ORDER - 1 aligned chunks. The callback might
@@ -649,7 +656,7 @@ static void online_pages_range(unsigned long start_pfn, unsigned long nr_pages)
* later). We account all pages as being online and belonging to this
* zone ("present").
*/
- for (pfn = start_pfn; pfn < end_pfn; pfn += MAX_ORDER_NR_PAGES)
+ for (; pfn < end_pfn; pfn += MAX_ORDER_NR_PAGES)
(*online_page_callback)(pfn_to_page(pfn), MAX_ORDER - 1);

/* mark all involved sections as online */
@@ -829,7 +836,11 @@ struct zone * zone_for_pfn_range(int online_type, int nid, unsigned start_pfn,
return default_zone_for_pfn(nid, start_pfn, nr_pages);
}

-static void adjust_present_page_count(struct zone *zone, long nr_pages)
+/*
+ * This function should only be called by memory_block_{online,offline},
+ * and {online,offline}_pages.
+ */
+void adjust_present_page_count(struct zone *zone, long nr_pages)
{
unsigned long flags;

@@ -839,12 +850,64 @@ static void adjust_present_page_count(struct zone *zone, long nr_pages)
pgdat_resize_unlock(zone->zone_pgdat, &flags);
}

-int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
- int online_type, int nid)
+struct zone *mhp_get_target_zone(unsigned long pfn, unsigned long nr_pages,
+ int nid, int online_type)
+{
+ return zone_for_pfn_range(online_type, nid, pfn, nr_pages);
+}
+
+int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
+ struct zone *zone)
+{
+ unsigned long end_pfn = pfn + nr_pages;
+ int ret;
+
+ /*
+ * Initialize vmemmap pages with the corresponding node, zone links set.
+ */
+ move_pfn_range_to_zone(zone, pfn, nr_pages, NULL, MIGRATE_UNMOVABLE);
+
+ ret = kasan_add_zero_shadow(__va(PFN_PHYS(pfn)), PFN_PHYS(nr_pages));
+ if (ret) {
+ remove_pfn_range_from_zone(zone, pfn, nr_pages);
+ return ret;
+ }
+
+ /*
+ * It might be that the vmemmap_pages fully span sections. If that is
+ * the case, mark those sections online here as otherwise they will be
+ * left offline.
+ */
+ if (nr_pages >= PAGES_PER_SECTION)
+ online_mem_sections(pfn, ALIGN_DOWN(end_pfn, PAGES_PER_SECTION));
+
+ return ret;
+}
+
+void mhp_deinit_memmap_on_memory(unsigned long pfn, unsigned long nr_pages)
+{
+ unsigned long end_pfn = pfn + nr_pages;
+ /*
+ * The pages associated with this vmemmap have been offlined, so
+ * we can reset its state here.
+ */
+ remove_pfn_range_from_zone(page_zone(pfn_to_page(pfn)), pfn, nr_pages);
+ kasan_remove_zero_shadow(__va(PFN_PHYS(pfn)), PFN_PHYS(nr_pages));
+
+ /*
+ * It might be that the vmemmap_pages fully span sections. If that is
+ * the case, mark those sections offline here as otherwise they will be
+ * left online.
+ */
+ if (nr_pages >= PAGES_PER_SECTION)
+ offline_mem_sections(pfn, ALIGN_DOWN(end_pfn, PAGES_PER_SECTION));
+}
+
+int __ref online_pages(unsigned long pfn, unsigned long nr_pages, struct zone *zone)
{
unsigned long flags;
- struct zone *zone;
int need_zonelists_rebuild = 0;
+ int nid;
int ret;
struct memory_notify arg;

@@ -860,8 +923,9 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,

mem_hotplug_begin();

+ nid = zone_to_nid(zone);
+
/* associate pfn range with the zone */
- zone = zone_for_pfn_range(online_type, nid, pfn, nr_pages);
move_pfn_range_to_zone(zone, pfn, nr_pages, NULL, MIGRATE_ISOLATE);

arg.start_pfn = pfn;
@@ -1075,6 +1139,45 @@ static int online_memory_block(struct memory_block *mem, void *arg)
return device_online(&mem->dev);
}

+bool mhp_supports_memmap_on_memory(unsigned long size)
+{
+ unsigned long nr_vmemmap_pages = size / PAGE_SIZE;
+ unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page);
+ unsigned long remaining_size = size - vmemmap_size;
+
+ /*
+ * Besides having arch support and the feature enabled at runtime, we
+ * need a few more assumptions to hold true:
+ *
+ * a) We span a single memory block: memory onlining/offlinin;g happens
+ * in memory block granularity. We don't want the vmemmap of online
+ * memory blocks to reside on offline memory blocks. In the future,
+ * we might want to support variable-sized memory blocks to make the
+ * feature more versatile.
+ *
+ * b) The vmemmap pages span complete PMDs: We don't want vmemmap code
+ * to populate memory from the altmap for unrelated parts (i.e.,
+ * other memory blocks)
+ *
+ * c) The vmemmap pages (and thereby the pages that will be exposed to
+ * the buddy) have to cover full pageblocks: memory onlining/offlining
+ * code requires applicable ranges to be page-aligned, for example, to
+ * set the migratetypes properly.
+ *
+ * TODO: Although we have a check here to make sure that vmemmap pages
+ * fully populate a PMD, it is not the right place to check for
+ * this. A much better solution involves improving vmemmap code
+ * to fallback to base pages when trying to populate vmemmap using
+ * altmap as an alternative source of memory, and we do not exactly
+ * populate a single PMD.
+ */
+ return memmap_on_memory &&
+ IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
+ size == memory_block_size_bytes() &&
+ IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
+ IS_ALIGNED(remaining_size, (pageblock_nr_pages << PAGE_SHIFT));
+}
+
/*
* NOTE: The caller must call lock_device_hotplug() to serialize hotplug
* and online/offline operations (triggered e.g. by sysfs).
@@ -1084,6 +1187,7 @@ static int online_memory_block(struct memory_block *mem, void *arg)
int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
{
struct mhp_params params = { .pgprot = pgprot_mhp(PAGE_KERNEL) };
+ struct vmem_altmap mhp_altmap = {};
u64 start, size;
bool new_node = false;
int ret;
@@ -1110,13 +1214,26 @@ int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
goto error;
new_node = ret;

+ /*
+ * Self hosted memmap array
+ */
+ if (mhp_flags & MHP_MEMMAP_ON_MEMORY) {
+ if (!mhp_supports_memmap_on_memory(size)) {
+ ret = -EINVAL;
+ goto error;
+ }
+ mhp_altmap.free = PHYS_PFN(size);
+ mhp_altmap.base_pfn = PHYS_PFN(start);
+ params.altmap = &mhp_altmap;
+ }
+
/* call arch's memory hotadd */
ret = arch_add_memory(nid, start, size, &params);
if (ret < 0)
goto error;

/* create memory block devices after memory was added */
- ret = create_memory_block_devices(start, size);
+ ret = create_memory_block_devices(start, size, mhp_altmap.alloc);
if (ret) {
arch_remove_memory(nid, start, size, NULL);
goto error;
@@ -1762,6 +1879,14 @@ static int check_memblock_offlined_cb(struct memory_block *mem, void *arg)
return 0;
}

+static int get_nr_vmemmap_pages_cb(struct memory_block *mem, void *arg)
+{
+ /*
+ * If not set, continue with the next block.
+ */
+ return mem->nr_vmemmap_pages;
+}
+
static int check_cpu_on_node(pg_data_t *pgdat)
{
int cpu;
@@ -1836,6 +1961,9 @@ EXPORT_SYMBOL(try_offline_node);
static int __ref try_remove_memory(int nid, u64 start, u64 size)
{
int rc = 0;
+ struct vmem_altmap mhp_altmap = {};
+ struct vmem_altmap *altmap = NULL;
+ unsigned long nr_vmemmap_pages;

BUG_ON(check_hotplug_memory_range(start, size));

@@ -1848,6 +1976,31 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
if (rc)
return rc;

+ /*
+ * We only support removing memory added with MHP_MEMMAP_ON_MEMORY in
+ * the same granularity it was added - a single memory block.
+ */
+ if (memmap_on_memory) {
+ nr_vmemmap_pages = walk_memory_blocks(start, size, NULL,
+ get_nr_vmemmap_pages_cb);
+ if (nr_vmemmap_pages) {
+ if (size != memory_block_size_bytes()) {
+ pr_warn("Refuse to remove %#llx - %#llx,"
+ "wrong granularity\n",
+ start, start + size);
+ return -EINVAL;
+ }
+
+ /*
+ * Let remove_pmd_table->free_hugepage_table do the
+ * right thing if we used vmem_altmap when hot-adding
+ * the range.
+ */
+ mhp_altmap.alloc = nr_vmemmap_pages;
+ altmap = &mhp_altmap;
+ }
+ }
+
/* remove memmap entry */
firmware_map_remove(start, start + size, "System RAM");

@@ -1859,7 +2012,7 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)

mem_hotplug_begin();

- arch_remove_memory(nid, start, size, NULL);
+ arch_remove_memory(nid, start, size, altmap);

if (IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK)) {
memblock_free(start, size);
diff --git a/mm/sparse.c b/mm/sparse.c
index 7bd23f9d6cef..8e96cf00536b 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -623,7 +623,6 @@ void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
}
}

-#ifdef CONFIG_MEMORY_HOTREMOVE
/* Mark all memory sections within the pfn range as offline */
void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
{
@@ -644,7 +643,6 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
ms->section_mem_map &= ~SECTION_IS_ONLINE;
}
}
-#endif

#ifdef CONFIG_SPARSEMEM_VMEMMAP
static struct page * __meminit populate_section_memmap(unsigned long pfn,
--
2.16.3