Size of mem_section array depends on size of physical address space.

In preparation for boot-time switching between paging modes on x86-64
we need to make allocation of mem_section dynamic.

The patch allocates the array on the first call to
sparse_memory_present_with_active_regions().

Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
---
 include/linux/mmzone.h |  2 +-
 mm/page_alloc.c        | 10 ++++++++++
 mm/sparse.c            | 17 +++++++++++------
 3 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index fc14b8b3f6ce..c8eb668eab79 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1137,7 +1137,7 @@ struct mem_section {
 #define SECTION_ROOT_MASK      (SECTIONS_PER_ROOT - 1)
 
 #ifdef CONFIG_SPARSEMEM_EXTREME
-extern struct mem_section *mem_section[NR_SECTION_ROOTS];
+extern struct mem_section **mem_section;
 #else
 extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
 #endif
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6d30e914afb6..639fd2dce0c4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5681,6 +5681,16 @@ void __init 
sparse_memory_present_with_active_regions(int nid)
        unsigned long start_pfn, end_pfn;
        int i, this_nid;
 
+#ifdef CONFIG_SPARSEMEM_EXTREME
+       if (!mem_section) {
+               unsigned long size, align;
+
+               size = sizeof(struct mem_section) * NR_SECTION_ROOTS;
+               align = 1 << (INTERNODE_CACHE_SHIFT);
+               mem_section = memblock_virt_alloc(size, align);
+       }
+#endif
+
        for_each_mem_pfn_range(i, nid, &start_pfn, &end_pfn, &this_nid)
                memory_present(this_nid, start_pfn, end_pfn);
 }
diff --git a/mm/sparse.c b/mm/sparse.c
index 7b4be3fd5cac..a91dafb189d4 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -22,8 +22,7 @@
  * 1) mem_section      - memory sections, mem_map's for valid memory
  */
 #ifdef CONFIG_SPARSEMEM_EXTREME
-struct mem_section *mem_section[NR_SECTION_ROOTS]
-       ____cacheline_internodealigned_in_smp;
+struct mem_section **mem_section;
 #else
 struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT]
        ____cacheline_internodealigned_in_smp;
@@ -104,7 +103,7 @@ static inline int sparse_index_init(unsigned long 
section_nr, int nid)
 int __section_nr(struct mem_section* ms)
 {
        unsigned long root_nr;
-       struct mem_section* root;
+       struct mem_section* root = NULL;
 
        for (root_nr = 0; root_nr < NR_SECTION_ROOTS; root_nr++) {
                root = __nr_to_section(root_nr * SECTIONS_PER_ROOT);
@@ -115,7 +114,7 @@ int __section_nr(struct mem_section* ms)
                     break;
        }
 
-       VM_BUG_ON(root_nr == NR_SECTION_ROOTS);
+       VM_BUG_ON(!root);
 
        return (root_nr * SECTIONS_PER_ROOT) + (ms - root);
 }
@@ -333,11 +332,17 @@ sparse_early_usemaps_alloc_pgdat_section(struct 
pglist_data *pgdat,
 static void __init check_usemap_section_nr(int nid, unsigned long *usemap)
 {
        unsigned long usemap_snr, pgdat_snr;
-       static unsigned long old_usemap_snr = NR_MEM_SECTIONS;
-       static unsigned long old_pgdat_snr = NR_MEM_SECTIONS;
+       static unsigned long old_usemap_snr = 0;
+       static unsigned long old_pgdat_snr = 0;
        struct pglist_data *pgdat = NODE_DATA(nid);
        int usemap_nid;
 
+       /* First call */
+       if (!old_usemap_snr) {
+               old_usemap_snr = NR_MEM_SECTIONS;
+               old_pgdat_snr = NR_MEM_SECTIONS;
+       }
+
        usemap_snr = pfn_to_section_nr(__pa(usemap) >> PAGE_SHIFT);
        pgdat_snr = pfn_to_section_nr(__pa(pgdat) >> PAGE_SHIFT);
        if (usemap_snr == pgdat_snr)
-- 
2.13.2

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