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

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

[Lee Schermerhorn]

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

[Christoph Lameter]

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

[Andrew Morton]

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

This sounds to me like a bugfix-for-2.6.17 rather than a "PATCH
2.6.17-rc1-mm3"?

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

[Christoph Lameter]

On Thu, 20 Apr 2006, Andrew Morton wrote:

> >  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.
> 
> This sounds to me like a bugfix-for-2.6.17 rather than a "PATCH
> 2.6.17-rc1-mm3"?

Correct.
 

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