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SubjectRe: [PATCH 01/11] mm: page_alloc: handle MIGRATE_ISOLATE in free_pcppages_bulk()
On Fri, Nov 18, 2011 at 05:43:08PM +0100, Marek Szyprowski wrote:
> From: Michal Nazarewicz <mina86@mina86.com>
>
> If page is on PCP list while pageblock it belongs to gets isolated,
> the page's private still holds the old migrate type. This means
> that free_pcppages_bulk() will put the page on a freelist of the
> old migrate type instead of MIGRATE_ISOLATE.
>
> This commit changes that by explicitly checking whether page's
> pageblock's migrate type is MIGRATE_ISOLATE and if it is, overwrites
> page's private data.
>
> Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
> mm/page_alloc.c | 12 ++++++++++++
> 1 files changed, 12 insertions(+), 0 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9dd443d..58d1a2e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -628,6 +628,18 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> page = list_entry(list->prev, struct page, lru);
> /* must delete as __free_one_page list manipulates */
> list_del(&page->lru);
> +
> + /*
> + * When page is isolated in set_migratetype_isolate()
> + * function it's page_private is not changed since the
> + * function has no way of knowing if it can touch it.
> + * This means that when a page is on PCP list, it's
> + * page_private no longer matches the desired migrate
> + * type.
> + */
> + if (get_pageblock_migratetype(page) == MIGRATE_ISOLATE)
> + set_page_private(page, MIGRATE_ISOLATE);
> +

How much of a problem is this in practice? It's adding overhead to the
free path for what should be a very rare case which is undesirable. I
know we are already calling get_pageblock_migrate() when freeing
pages but it should be unnecessary to call it again. I'd go as far
to say that it would be preferable to drain the per-CPU lists after
you set pageblocks MIGRATE_ISOLATE. The IPIs also have overhead but it
will be incurred for the rare rather than the common case.

--
Mel Gorman
SUSE Labs


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