Commit 304dbdb7 authored by Lee Schermerhorn's avatar Lee Schermerhorn Committed by Linus Torvalds
Browse files

[PATCH] add migratepage address space op to shmem

Basic problem: pages of a shared memory segment can only be migrated once.

In 2.6.16 through 2.6.17-rc1, shared memory mappings do not have a
migratepage address space op.  Therefore, migrate_pages() falls back to
default processing.  In this path, it will try to pageout() dirty pages.
Once a shared memory page has been migrated it becomes dirty, so
migrate_pages() will try to page it out.  However, because the page count
is 3 [cache + current + pte], pageout() will return PAGE_KEEP because
is_page_cache_freeable() returns false.  This will abort all subsequent

This patch adds a migratepage address space op to shared memory segments to
avoid taking the default path.  We use the "migrate_page()" function
because it knows how to migrate dirty pages.  This allows shared memory
segment pages to migrate, subject to other conditions such as # pte's
referencing the page [page_mapcount(page)], when requested.

I think this is safe.  If we're migrating a shared memory page, then we
found the page via a page table, so it must be in memory.

Can be verified with memtoy and the shmem-mbind-test script, both
available at:

Signed-off-by: default avatarLee Schermerhorn <>
Acked-by: default avatarChristoph Lameter <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 3b5fd59f
......@@ -46,6 +46,8 @@
#include <linux/mempolicy.h>
#include <linux/namei.h>
#include <linux/ctype.h>
#include <linux/migrate.h>
#include <asm/uaccess.h>
#include <asm/div64.h>
#include <asm/pgtable.h>
......@@ -2173,6 +2175,7 @@ static struct address_space_operations shmem_aops = {
.prepare_write = shmem_prepare_write,
.commit_write = simple_commit_write,
.migratepage = migrate_page,
static struct file_operations shmem_file_operations = {
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