[PATCH 3/7] ksm: shrink 32-bit rmap_item back to 32 bytes

From: Hugh Dickins
Date: Thu Feb 21 2013 - 03:22:48 EST

Think of struct rmap_item as an extension of struct page (restricted
to MADV_MERGEABLE areas): there may be a lot of them, we need to keep
them small, especially on 32-bit architectures of limited lowmem.

Siting "int nid" after "unsigned int checksum" works nicely on 64-bit,
making no change to its 64-byte struct rmap_item; but bloats the 32-bit
struct rmap_item from (nicely cache-aligned) 32 bytes to 36 bytes, which
rounds up to 40 bytes once allocated from slab. We'd better avoid that.

Hey, I only just remembered that the anon_vma pointer in struct rmap_item
has no purpose until the rmap_item is hung from a stable tree node (which
has its own nid field); and rmap_item's nid field no purpose than to say
which tree root to tell rb_erase() when unlinking from an unstable tree.

Double them up in a union. There's just one place where we set anon_vma
early (when we already hold mmap_sem): now we must remove tree_rmap_item
from its unstable tree there, before overwriting nid. No need to spatter
BUG()s around: we'd be seeing oopses if this were wrong.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
mm/ksm.c | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)

--- mmotm.orig/mm/ksm.c 2013-02-20 22:28:27.288001480 -0800
+++ mmotm/mm/ksm.c 2013-02-20 22:28:29.688001537 -0800
@@ -150,23 +150,25 @@ struct stable_node {
* struct rmap_item - reverse mapping item for virtual addresses
* @rmap_list: next rmap_item in mm_slot's singly-linked rmap_list
* @anon_vma: pointer to anon_vma for this mm,address, when in stable tree
+ * @nid: NUMA node id of unstable tree in which linked (may not match page)
* @mm: the memory structure this rmap_item is pointing into
* @address: the virtual address this rmap_item tracks (+ flags in low bits)
* @oldchecksum: previous checksum of the page at that virtual address
- * @nid: NUMA node id of unstable tree in which linked (may not match page)
* @node: rb node of this rmap_item in the unstable tree
* @head: pointer to stable_node heading this list in the stable tree
* @hlist: link into hlist of rmap_items hanging off that stable_node
struct rmap_item {
struct rmap_item *rmap_list;
- struct anon_vma *anon_vma; /* when stable */
+ union {
+ struct anon_vma *anon_vma; /* when stable */
+ int nid; /* when node of unstable tree */
+ };
struct mm_struct *mm;
unsigned long address; /* + low bits used for flags below */
unsigned int oldchecksum; /* when unstable */
- int nid;
union {
struct rb_node node; /* when node of unstable tree */
struct { /* when listed from stable tree */
@@ -1092,6 +1094,9 @@ static int try_to_merge_with_ksm_page(st
if (err)
goto out;

+ /* Unstable nid is in union with stable anon_vma: remove first */
+ remove_rmap_item_from_tree(rmap_item);
/* Must get reference to anon_vma while still holding mmap_sem */
rmap_item->anon_vma = vma->anon_vma;
@@ -1466,14 +1471,11 @@ static void cmp_and_merge_page(struct pa
kpage = try_to_merge_two_pages(rmap_item, page,
tree_rmap_item, tree_page);
- /*
- * As soon as we merge this page, we want to remove the
- * rmap_item of the page we have merged with from the unstable
- * tree, and insert it instead as new node in the stable tree.
- */
if (kpage) {
- remove_rmap_item_from_tree(tree_rmap_item);
+ /*
+ * The pages were successfully merged: insert new
+ * node in the stable tree and add both rmap_items.
+ */
stable_node = stable_tree_insert(kpage);
if (stable_node) {
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/