[v1,1/8] mm: balloon: update comment about isolation/migration/compaction
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Message ID 20181119101616.8901-2-david@redhat.com
State In Next
Commit 31960af8bd83977a9d70cf53d2ad1c645de81c7c
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  • [v1,1/8] mm: balloon: update comment about isolation/migration/compaction
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Commit Message

David Hildenbrand Nov. 19, 2018, 10:16 a.m. UTC
Commit b1123ea6d3b3 ("mm: balloon: use general non-lru movable page
feature") reworked balloon handling to make use of the general
non-lru movable page feature. The big comment block in
balloon_compaction.h contains quite some outdated information. Let's fix
this.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/balloon_compaction.h | 26 +++++++++-----------------
 1 file changed, 9 insertions(+), 17 deletions(-)

Comments

Michael S. Tsirkin Nov. 21, 2018, 1:56 a.m. UTC | #1
On Mon, Nov 19, 2018 at 11:16:09AM +0100, David Hildenbrand wrote:
> Commit b1123ea6d3b3 ("mm: balloon: use general non-lru movable page
> feature") reworked balloon handling to make use of the general
> non-lru movable page feature. The big comment block in
> balloon_compaction.h contains quite some outdated information. Let's fix
> this.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  include/linux/balloon_compaction.h | 26 +++++++++-----------------
>  1 file changed, 9 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 53051f3d8f25..cbe50da5a59d 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -4,15 +4,18 @@
>   *
>   * Common interface definitions for making balloon pages movable by compaction.
>   *
> - * Despite being perfectly possible to perform ballooned pages migration, they
> - * make a special corner case to compaction scans because balloon pages are not
> - * enlisted at any LRU list like the other pages we do compact / migrate.
> + * Balloon page migration makes use of the general non-lru movable page
> + * feature.
> + *
> + * page->private is used to reference the responsible balloon device.
> + * page->mapping is used in context of non-lru page migration to reference
> + * the address space operations for page isolation/migration/compaction.
>   *
>   * As the page isolation scanning step a compaction thread does is a lockless
>   * procedure (from a page standpoint), it might bring some racy situations while
>   * performing balloon page compaction. In order to sort out these racy scenarios
>   * and safely perform balloon's page compaction and migration we must, always,
> - * ensure following these three simple rules:
> + * ensure following these simple rules:
>   *
>   *   i. when updating a balloon's page ->mapping element, strictly do it under
>   *      the following lock order, independently of the far superior
> @@ -21,19 +24,8 @@
>   *	      +--spin_lock_irq(&b_dev_info->pages_lock);
>   *	            ... page->mapping updates here ...
>   *
> - *  ii. before isolating or dequeueing a balloon page from the balloon device
> - *      pages list, the page reference counter must be raised by one and the
> - *      extra refcount must be dropped when the page is enqueued back into
> - *      the balloon device page list, thus a balloon page keeps its reference
> - *      counter raised only while it is under our special handling;
> - *
> - * iii. after the lockless scan step have selected a potential balloon page for
> - *      isolation, re-test the PageBalloon mark and the PagePrivate flag
> - *      under the proper page lock, to ensure isolating a valid balloon page
> - *      (not yet isolated, nor under release procedure)
> - *
> - *  iv. isolation or dequeueing procedure must clear PagePrivate flag under
> - *      page lock together with removing page from balloon device page list.
> + *  ii. isolation or dequeueing procedure must remove the page from balloon
> + *      device page list under b_dev_info->pages_lock.
>   *
>   * The functions provided by this interface are placed to help on coping with
>   * the aforementioned balloon page corner case, as well as to ensure the simple
> -- 
> 2.17.2

Patch
diff mbox series

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 53051f3d8f25..cbe50da5a59d 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -4,15 +4,18 @@ 
  *
  * Common interface definitions for making balloon pages movable by compaction.
  *
- * Despite being perfectly possible to perform ballooned pages migration, they
- * make a special corner case to compaction scans because balloon pages are not
- * enlisted at any LRU list like the other pages we do compact / migrate.
+ * Balloon page migration makes use of the general non-lru movable page
+ * feature.
+ *
+ * page->private is used to reference the responsible balloon device.
+ * page->mapping is used in context of non-lru page migration to reference
+ * the address space operations for page isolation/migration/compaction.
  *
  * As the page isolation scanning step a compaction thread does is a lockless
  * procedure (from a page standpoint), it might bring some racy situations while
  * performing balloon page compaction. In order to sort out these racy scenarios
  * and safely perform balloon's page compaction and migration we must, always,
- * ensure following these three simple rules:
+ * ensure following these simple rules:
  *
  *   i. when updating a balloon's page ->mapping element, strictly do it under
  *      the following lock order, independently of the far superior
@@ -21,19 +24,8 @@ 
  *	      +--spin_lock_irq(&b_dev_info->pages_lock);
  *	            ... page->mapping updates here ...
  *
- *  ii. before isolating or dequeueing a balloon page from the balloon device
- *      pages list, the page reference counter must be raised by one and the
- *      extra refcount must be dropped when the page is enqueued back into
- *      the balloon device page list, thus a balloon page keeps its reference
- *      counter raised only while it is under our special handling;
- *
- * iii. after the lockless scan step have selected a potential balloon page for
- *      isolation, re-test the PageBalloon mark and the PagePrivate flag
- *      under the proper page lock, to ensure isolating a valid balloon page
- *      (not yet isolated, nor under release procedure)
- *
- *  iv. isolation or dequeueing procedure must clear PagePrivate flag under
- *      page lock together with removing page from balloon device page list.
+ *  ii. isolation or dequeueing procedure must remove the page from balloon
+ *      device page list under b_dev_info->pages_lock.
  *
  * The functions provided by this interface are placed to help on coping with
  * the aforementioned balloon page corner case, as well as to ensure the simple