[PATCH 2.6.17-rc1-mm3] add migratepage address space op to shmem

[Lee Schermerhorn <Lee.Schermerhorn@hp.com>]

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 migrations.

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@hp.com>

Index: linux-2.6.17-rc1-mm3/mm/shmem.c
===================================================================
--- linux-2.6.17-rc1-mm3.orig/mm/shmem.c	2006-04-19 17:29:09.000000000 -0400
+++ linux-2.6.17-rc1-mm3/mm/shmem.c	2006-04-19 17:29:36.000000000 -0400
@@ -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>
@@ -2165,6 +2167,7 @@ static struct address_space_operations s
 	.prepare_write	= shmem_prepare_write,
 	.commit_write	= simple_commit_write,
 #endif
+	.migratepage	= migrate_page,
 };
 
 static struct file_operations shmem_file_operations = {


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