mm: make mem_map allocation continuous

vmemmap allocation currently has this layout:

 [ffffe20000000000-ffffe200001fffff] PMD ->ffff810001400000 on node 0
 [ffffe20000200000-ffffe200003fffff] PMD ->ffff810001800000 on node 0
 [ffffe20000400000-ffffe200005fffff] PMD ->ffff810001c00000 on node 0
 [ffffe20000600000-ffffe200007fffff] PMD ->ffff810002000000 on node 0
 [ffffe20000800000-ffffe200009fffff] PMD ->ffff810002400000 on node 0
...

note that there is a 2M hole between them - not optimal.

the root cause is that usemap (24 bytes) will be allocated after every 2M
mem_map, and it will push next vmemmap (2M) to the next (2M) alignment.

solution: try to allocate the mem_map continously.

after the patch, we get:

 [ffffe20000000000-ffffe200001fffff] PMD ->ffff810001400000 on node 0
 [ffffe20000200000-ffffe200003fffff] PMD ->ffff810001600000 on node 0
 [ffffe20000400000-ffffe200005fffff] PMD ->ffff810001800000 on node 0
 [ffffe20000600000-ffffe200007fffff] PMD ->ffff810001a00000 on node 0
 [ffffe20000800000-ffffe200009fffff] PMD ->ffff810001c00000 on node 0
...

which is the ideal layout.

and usemap will share a page because of they are allocated continuously too:

sparse_early_usemap_alloc: usemap = ffff810024e00000 size = 24
sparse_early_usemap_alloc: usemap = ffff810024e00080 size = 24
sparse_early_usemap_alloc: usemap = ffff810024e00100 size = 24
sparse_early_usemap_alloc: usemap = ffff810024e00180 size = 24
...

so we make the bootmem allocation more compact and use less memory
for usemap => mission accomplished ;-)

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 29 insertions, 3 deletions

diff --git a/mm/sparse.c b/mm/sparse.c
index 98d6b39c3472..458109b99e61 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -304,22 +304,48 @@ void __init sparse_init(void)
         unsigned long pnum;
         struct page *map;
         unsigned long *usemap;
+        unsigned long **usemap_map;
+        int size;
+
+        /*
+         * map is using big page (aka 2M in x86 64 bit)
+         * usemap is less one page (aka 24 bytes)
+         * so alloc 2M (with 2M align) and 24 bytes in turn will
+         * make next 2M slip to one more 2M later.
+         * then in big system, the memory will have a lot of holes...
+         * here try to allocate 2M pages continously.
+         *
+         * powerpc need to call sparse_init_one_section right after each
+         * sparse_early_mem_map_alloc, so allocate usemap_map at first.
+         */
+        size = sizeof(unsigned long *) * NR_MEM_SECTIONS;
+        usemap_map = alloc_bootmem(size);
+        if (!usemap_map)
+                panic("can not allocate usemap_map\n");
 
         for (pnum = 0; pnum < NR_MEM_SECTIONS; pnum++) {
                 if (!present_section_nr(pnum))
                         continue;
+                usemap_map[pnum] = sparse_early_usemap_alloc(pnum);
+        }
 
-                map = sparse_early_mem_map_alloc(pnum);
-                if (!map)
+        for (pnum = 0; pnum < NR_MEM_SECTIONS; pnum++) {
+                if (!present_section_nr(pnum))
                         continue;
 
-                usemap = sparse_early_usemap_alloc(pnum);
+                usemap = usemap_map[pnum];
                 if (!usemap)
                         continue;
 
+                map = sparse_early_mem_map_alloc(pnum);
+                if (!map)
+                        continue;
+
                 sparse_init_one_section(__nr_to_section(pnum), pnum, map,
                                                                 usemap);
         }
+
+        free_bootmem(__pa(usemap_map), size);
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG