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

[Christoph Lameter <clameter@sgi.com>]

On Thu, 20 Apr 2006, Lee Schermerhorn wrote:

> 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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