Commit bc56620b authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds

ksm: shrink 32-bit rmap_item back to 32 bytes

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: default avatarHugh Dickins <>
Cc: Mel Gorman <>
Cc: Petr Holasek <>
Cc: Andrea Arcangeli <>
Cc: Izik Eidus <>
Cc: Johannes Weiner <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent b599cbdf
......@@ -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 */
......@@ -1094,6 +1096,9 @@ static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
if (err)
goto out;
/* Unstable nid is in union with stable anon_vma: remove first */
/* Must get reference to anon_vma while still holding mmap_sem */
rmap_item->anon_vma = vma->anon_vma;
......@@ -1468,14 +1473,11 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
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) {
* 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) {
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