[PATCH] mm: migrate: One less atomic operation

[PATCH] mm: migrate: One less atomic operation

[Jacobo Giralt]

>From 3754c8617ef4377ce2ca2e3b28bdc28f8de1aa0d Mon Sep 17 00:00:00 2001
From: Jacobo Giralt <jacobo.giralt@gmail.com>
Date: Sat, 5 Nov 2011 13:12:50 +0100
Subject: [PATCH] mm: migrate: One less atomic operation

migrate_page_move_mapping drops a reference from the
old page after unfreezing its counter. Both operations
can be merged into a single atomic operation by
directly unfreezing to one less reference.

The same applies to migrate_huge_page_move_mapping.

Signed-off-by: Jacobo Giralt <jacobo.giralt@gmail.com>
---
 mm/migrate.c |   10 ++++------
 1 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 33358f8..46d04a0 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -269,12 +269,12 @@ static int migrate_page_move_mapping(struct address_space *mapping,
 
 	radix_tree_replace_slot(pslot, newpage);
 
-	page_unfreeze_refs(page, expected_count);
 	/*
-	 * Drop cache reference from old page.
+	 * Drop cache reference from old page by unfreezing
+	 * to one less reference.
 	 * We know this isn't the last reference.
 	 */
-	__put_page(page);
+	page_unfreeze_refs(page, expected_count - 1);
 
 	/*
 	 * If moved to a different zone then also account
@@ -334,9 +334,7 @@ int migrate_huge_page_move_mapping(struct address_space *mapping,
 
 	radix_tree_replace_slot(pslot, newpage);
 
-	page_unfreeze_refs(page, expected_count);
-
-	__put_page(page);
+	page_unfreeze_refs(page, expected_count - 1);
 
 	spin_unlock_irq(&mapping->tree_lock);
 	return 0;
-- 
1.7.5.4

Re: [PATCH] mm: migrate: One less atomic operation

[Jacobo Giralt]

I think I used an old e-mail address for Nick (got it from git blame),
I'm changing it now, sorry for the noise!

Regards,
Jacobo Giralt.

2011/11/5 Jacobo Giralt <jacobo.giralt@gmail.com>:
> From 3754c8617ef4377ce2ca2e3b28bdc28f8de1aa0d Mon Sep 17 00:00:00 2001
> From: Jacobo Giralt <jacobo.giralt@gmail.com>
> Date: Sat, 5 Nov 2011 13:12:50 +0100
> Subject: [PATCH] mm: migrate: One less atomic operation
>
> migrate_page_move_mapping drops a reference from the
> old page after unfreezing its counter. Both operations
> can be merged into a single atomic operation by
> directly unfreezing to one less reference.
>
> The same applies to migrate_huge_page_move_mapping.
>
> Signed-off-by: Jacobo Giralt <jacobo.giralt@gmail.com>
> ---
>  mm/migrate.c |   10 ++++------
>  1 files changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 33358f8..46d04a0 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -269,12 +269,12 @@ static int migrate_page_move_mapping(struct address_space *mapping,
>
>        radix_tree_replace_slot(pslot, newpage);
>
> -       page_unfreeze_refs(page, expected_count);
>        /*
> -        * Drop cache reference from old page.
> +        * Drop cache reference from old page by unfreezing
> +        * to one less reference.
>         * We know this isn't the last reference.
>         */
> -       __put_page(page);
> +       page_unfreeze_refs(page, expected_count - 1);
>
>        /*
>         * If moved to a different zone then also account
> @@ -334,9 +334,7 @@ int migrate_huge_page_move_mapping(struct address_space *mapping,
>
>        radix_tree_replace_slot(pslot, newpage);
>
> -       page_unfreeze_refs(page, expected_count);
> -
> -       __put_page(page);
> +       page_unfreeze_refs(page, expected_count - 1);
>
>        spin_unlock_irq(&mapping->tree_lock);
>        return 0;
> --
> 1.7.5.4
>
>
>
>

Re: [PATCH] mm: migrate: One less atomic operation

[Andrew Morton]

On Sat, 05 Nov 2011 15:38:17 +0100
Jacobo Giralt <jacobo.giralt@gmail.com> wrote:

> >From 3754c8617ef4377ce2ca2e3b28bdc28f8de1aa0d Mon Sep 17 00:00:00 2001
> From: Jacobo Giralt <jacobo.giralt@gmail.com>
> Date: Sat, 5 Nov 2011 13:12:50 +0100
> Subject: [PATCH] mm: migrate: One less atomic operation
> 
> migrate_page_move_mapping drops a reference from the
> old page after unfreezing its counter. Both operations
> can be merged into a single atomic operation by
> directly unfreezing to one less reference.
> 
> The same applies to migrate_huge_page_move_mapping.
> 

Fair enough.

urgh, you made me look at stuff :(

page_unfreeze_refs() and set_page_refcounted() are fishily similar.

page_unfreeze_refs() should use set_page_count().

set_page_count() is defined in the wrong file.

set_page_refcounted() should use page_count().

iow, a bit of a mess.