mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y

Size of the mem_section[] array depends on the size of the physical address space.

In preparation for boot-time switching between paging modes on x86-64
we need to make the allocation of mem_section[] dynamic, because otherwise
we waste a lot of RAM: with CONFIG_NODE_SHIFT=10, mem_section[] size is 32kB
for 4-level paging and 2MB for 5-level paging mode.

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

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@suse.de>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20170929140821.37654-2-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 26 insertions, 7 deletions

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index c8f89417740b..e796edf1296f 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1150,13 +1150,17 @@ 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
 
 static inline struct mem_section *__nr_to_section(unsigned long nr)
 {
+#ifdef CONFIG_SPARSEMEM_EXTREME
+        if (!mem_section)
+                return NULL;
+#endif
         if (!mem_section[SECTION_NR_TO_ROOT(nr)])
                 return NULL;
         return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 77e4d3c5c57b..8dfd13f724d9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5646,6 +5646,16 @@ 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 83b3bf6461af..b00a97398795 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;
@@ -100,7 +99,7 @@ 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);
@@ -111,7 +110,7 @@ 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);
 }
@@ -329,11 +328,17 @@ again:
 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;
+        static unsigned long old_pgdat_snr;
         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)