[PATCH 19/22] [PATCH] add migratepage address space op to shmem

From: Chris Wright
Date: Wed May 17 2006 - 18:16:19 EST

-stable review patch. If anyone has any objections, please let us know.

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: http://free.linux.hp.com/~lts/Tools/

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@xxxxxx>
Acked-by: Christoph Lameter <clameter@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
mm/shmem.c | 1 +
1 file changed, 1 insertion(+)

--- linux-
+++ linux-
@@ -2172,6 +2172,7 @@ static struct address_space_operations s
.prepare_write = shmem_prepare_write,
.commit_write = simple_commit_write,
+ .migratepage = migrate_page,

static struct file_operations shmem_file_operations = {

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