[PATCH] sparsemem memory model

Sparsemem abstracts the use of discontiguous mem_maps[].  This kind of
mem_map[] is needed by discontiguous memory machines (like in the old
CONFIG_DISCONTIGMEM case) as well as memory hotplug systems.  Sparsemem
replaces DISCONTIGMEM when enabled, and it is hoped that it can eventually
become a complete replacement.

A significant advantage over DISCONTIGMEM is that it's completely separated
from CONFIG_NUMA.  When producing this patch, it became apparent in that NUMA
and DISCONTIG are often confused.

Another advantage is that sparse doesn't require each NUMA node's ranges to be
contiguous.  It can handle overlapping ranges between nodes with no problems,
where DISCONTIGMEM currently throws away that memory.

Sparsemem uses an array to provide different pfn_to_page() translations for
each SECTION_SIZE area of physical memory.  This is what allows the mem_map[]
to be chopped up.

In order to do quick pfn_to_page() operations, the section number of the page
is encoded in page->flags.  Part of the sparsemem infrastructure enables
sharing of these bits more dynamically (at compile-time) between the
page_zone() and sparsemem operations.  However, on 32-bit architectures, the
number of bits is quite limited, and may require growing the size of the
page->flags type in certain conditions.  Several things might force this to
occur: a decrease in the SECTION_SIZE (if you want to hotplug smaller areas of
memory), an increase in the physical address space, or an increase in the
number of used page->flags.

One thing to note is that, once sparsemem is present, the NUMA node
information no longer needs to be stored in the page->flags.  It might provide
speed increases on certain platforms and will be stored there if there is
room.  But, if out of room, an alternate (theoretically slower) mechanism is
used.

This patch introduces CONFIG_FLATMEM.  It is used in almost all cases where
there used to be an #ifndef DISCONTIG, because SPARSEMEM and DISCONTIGMEM
often have to compile out the same areas of code.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Martin Bligh <mbligh@aracnet.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Signed-off-by: Bob Picco <bob.picco@hp.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 35 insertions, 3 deletions

diff --git a/mm/Kconfig b/mm/Kconfig
index 5127441561b4..cd379936cac6 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -6,6 +6,7 @@ choice
         prompt "Memory model"
         depends on SELECT_MEMORY_MODEL
         default DISCONTIGMEM_MANUAL if ARCH_DISCONTIGMEM_DEFAULT
+        default SPARSEMEM_MANUAL if ARCH_SPARSEMEM_DEFAULT
         default FLATMEM_MANUAL
 
 config FLATMEM_MANUAL
@@ -17,7 +18,15 @@ config FLATMEM_MANUAL
           only have one option here: FLATMEM.  This is normal
           and a correct option.
 
-          If unsure, choose this option over any other.
+          Some users of more advanced features like NUMA and
+          memory hotplug may have different options here.
+          DISCONTIGMEM is an more mature, better tested system,
+          but is incompatible with memory hotplug and may suffer
+          decreased performance over SPARSEMEM.  If unsure between
+          "Sparse Memory" and "Discontiguous Memory", choose
+          "Discontiguous Memory".
+
+          If unsure, choose this option (Flat Memory) over any other.
 
 config DISCONTIGMEM_MANUAL
         bool "Discontigious Memory"
@@ -35,15 +44,38 @@ config DISCONTIGMEM_MANUAL
 
           If unsure, choose "Flat Memory" over this option.
 
+config SPARSEMEM_MANUAL
+        bool "Sparse Memory"
+        depends on ARCH_SPARSEMEM_ENABLE
+        help
+          This will be the only option for some systems, including
+          memory hotplug systems.  This is normal.
+
+          For many other systems, this will be an alternative to
+          "Discontigious Memory".  This option provides some potential
+          performance benefits, along with decreased code complexity,
+          but it is newer, and more experimental.
+
+          If unsure, choose "Discontiguous Memory" or "Flat Memory"
+          over this option.
+
 endchoice
 
 config DISCONTIGMEM
         def_bool y
         depends on (!SELECT_MEMORY_MODEL && ARCH_DISCONTIGMEM_ENABLE) || DISCONTIGMEM_MANUAL
 
+config SPARSEMEM
+        def_bool y
+        depends on SPARSEMEM_MANUAL
+
 config FLATMEM
         def_bool y
-        depends on !DISCONTIGMEM || FLATMEM_MANUAL
+        depends on (!DISCONTIGMEM && !SPARSEMEM) || FLATMEM_MANUAL
+
+config FLAT_NODE_MEM_MAP
+        def_bool y
+        depends on !SPARSEMEM
 
 #
 # Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
@@ -56,4 +88,4 @@ config NEED_MULTIPLE_NODES
 
 config HAVE_MEMORY_PRESENT
         def_bool y
-        depends on ARCH_HAVE_MEMORY_PRESENT
+        depends on ARCH_HAVE_MEMORY_PRESENT || SPARSEMEM