Re: [PATCH] Make is_mem_section_removable more conformable with offlining code

[Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com>]

2010/9/2 Michal Hocko <mhocko@suse.cz>:
> What about this? Just compile tested.
>
> ---
> From a2aaeafbaeb5b195b699df25060128b9e547949c Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Fri, 20 Aug 2010 15:39:16 +0200
> Subject: [PATCH] Make is_mem_section_removable more conformable with offlining code
>
> Currently is_mem_section_removable checks whether each pageblock from
> the given pfn range is of MIGRATE_MOVABLE type or if it is free. If both
> are false then the range is considered non removable.
>
> On the other hand, offlining code (more specifically
> set_migratetype_isolate) doesn't care whether a page is free and instead
> it just checks the migrate type of the page and whether the page's zone
> is movable.
>
> This can lead into a situation when we can mark a node as not removable
> just because a pageblock is MIGRATE_RESERVE and it is not free but still
> movable.
>
> Let's make a common helper is_page_removable which unifies both tests
> at one place.
>
> Do not rely on any of MIGRATE_* types as all others than MIGRATE_MOVABLE
> may be tricky. MIGRATE_RESERVE can be anything that just happened to
> fallback to that allocation, MIGRATE_RECLAIMABLE can be unmovable
> because slab (or what ever) has this page currently in use. If we tried
> to remove those pages and the isolation failed then those blocks
> would get to the MIRAGTE_MOVABLE list and we will end up with the
> unmovable pages in the movable zone.
>
> Let's, instead, check just whether a pageblock contains free or LRU
> pages.
>
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>  include/linux/mmzone.h |   24 ++++++++++++++++++++++++
>  mm/memory_hotplug.c    |   19 +------------------
>  mm/page_alloc.c        |    5 +----
>  3 files changed, 26 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 6e6e626..0bd941b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -669,6 +669,30 @@ unsigned long __init node_memmap_size_bytes(int, unsigned long, unsigned long);
>  */
>  #define zone_idx(zone)         ((zone) - (zone)->zone_pgdat->node_zones)
>
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +/*
> + * A free or LRU pages block are removable
> + * Do not use MIGRATE_MOVABLE because it can be insufficient and
> + * other MIGRATE types are tricky.
> + */
> +static inline bool is_page_removable(struct page *page)
> +{
> +       int page_block = 1 << pageblock_order;
> +       for (page_block > 0) {

for ?

> +               if (PageBuddy(page)) {
> +                       page_block -= page_order(page);
> +               }else if (PageLRU(page))
> +                       page_block--;
> +               else
> +                       return false;
> +       }
> +
> +       return true;
> +}

Hmm. above for is intending to check all pages in the block ?
I'll look into details, tomorrow.

Thanks,
-Kame

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