Re: [Xen-devel] [PATCH v1 1/8] mm: balloon: update comment about isolation/migration/compaction

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

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 balloon_compaction.h contains quite some outdated information.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@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

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

> ---
>  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

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