bootmem/sparsemem: remove limit constraint in alloc_bootmem_section

While testing AMS (Active Memory Sharing) / CMO (Cooperative Memory
Overcommit) on powerpc, we tripped the following:

  kernel BUG at mm/bootmem.c:483!
  cpu 0x0: Vector: 700 (Program Check) at [c000000000c03940]
      pc: c000000000a62bd8: .alloc_bootmem_core+0x90/0x39c
      lr: c000000000a64bcc: .sparse_early_usemaps_alloc_node+0x84/0x29c
      sp: c000000000c03bc0
     msr: 8000000000021032
    current = 0xc000000000b0cce0
    paca    = 0xc000000001d80000
      pid   = 0, comm = swapper
  kernel BUG at mm/bootmem.c:483!
  enter ? for help
  [c000000000c03c80] c000000000a64bcc
  .sparse_early_usemaps_alloc_node+0x84/0x29c
  [c000000000c03d50] c000000000a64f10 .sparse_init+0x12c/0x28c
  [c000000000c03e20] c000000000a474f4 .setup_arch+0x20c/0x294
  [c000000000c03ee0] c000000000a4079c .start_kernel+0xb4/0x460
  [c000000000c03f90] c000000000009670 .start_here_common+0x1c/0x2c

This is

        BUG_ON(limit && goal + size > limit);

and after some debugging, it seems that

	goal = 0x7ffff000000
	limit = 0x80000000000

and sparse_early_usemaps_alloc_node ->
sparse_early_usemaps_alloc_pgdat_section calls

	return alloc_bootmem_section(usemap_size() * count, section_nr);

This is on a system with 8TB available via the AMS pool, and as a quirk
of AMS in firmware, all of that memory shows up in node 0.  So, we end
up with an allocation that will fail the goal/limit constraints.

In theory, we could "fall-back" to alloc_bootmem_node() in
sparse_early_usemaps_alloc_node(), but since we actually have HOTREMOVE
defined, we'll BUG_ON() instead.  A simple solution appears to be to
unconditionally remove the limit condition in alloc_bootmem_section,
meaning allocations are allowed to cross section boundaries (necessary
for systems of this size).

Johannes Weiner pointed out that if alloc_bootmem_section() no longer
guarantees section-locality, we need check_usemap_section_nr() to print
possible cross-dependencies between node descriptors and the usemaps
allocated through it.  That makes the two loops in
sparse_early_usemaps_alloc_node() identical, so re-factor the code a
bit.

[akpm@linux-foundation.org: code simplification]
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Anton Blanchard <anton@au1.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ben Herrenschmidt <benh@kernel.crashing.org>
Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: <stable@vger.kernel.org>	[3.3.1]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 19 deletions

diff --git a/mm/sparse.c b/mm/sparse.c
index 61d7cde23111..a8bc7d364deb 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -353,29 +353,21 @@ static void __init sparse_early_usemaps_alloc_node(unsigned long**usemap_map,
 
         usemap = sparse_early_usemaps_alloc_pgdat_section(NODE_DATA(nodeid),
                                                                  usemap_count);
-        if (usemap) {
-                for (pnum = pnum_begin; pnum < pnum_end; pnum++) {
-                        if (!present_section_nr(pnum))
-                                continue;
-                        usemap_map[pnum] = usemap;
-                        usemap += size;
+        if (!usemap) {
+                usemap = alloc_bootmem_node(NODE_DATA(nodeid), size * usemap_count);
+                if (!usemap) {
+                        printk(KERN_WARNING "%s: allocation failed\n", __func__);
+                        return;
                 }
-                return;
         }
 
-        usemap = alloc_bootmem_node(NODE_DATA(nodeid), size * usemap_count);
-        if (usemap) {
-                for (pnum = pnum_begin; pnum < pnum_end; pnum++) {
-                        if (!present_section_nr(pnum))
-                                continue;
-                        usemap_map[pnum] = usemap;
-                        usemap += size;
-                        check_usemap_section_nr(nodeid, usemap_map[pnum]);
-                }
-                return;
+        for (pnum = pnum_begin; pnum < pnum_end; pnum++) {
+                if (!present_section_nr(pnum))
+                        continue;
+                usemap_map[pnum] = usemap;
+                usemap += size;
+                check_usemap_section_nr(nodeid, usemap_map[pnum]);
         }
-
-        printk(KERN_WARNING "%s: allocation failed\n", __func__);
 }
 
 #ifndef CONFIG_SPARSEMEM_VMEMMAP