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 balloon_compaction.h contains quite some outdated information.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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