Re: [PATCH 1/2][v2] mm: add notifier in pageblock isolation forballoon drivers

[Mel Gorman]

On Fri, Oct 02, 2009 at 01:44:58PM -0500, Robert Jennings wrote:
> Memory balloon drivers can allocate a large amount of memory which
> is not movable but could be freed to accomodate memory hotplug remove.
> 
> Prior to calling the memory hotplug notifier chain the memory in the
> pageblock is isolated.  If the migrate type is not MIGRATE_MOVABLE the
> isolation will not proceed, causing the memory removal for that page
> range to fail.
> 
> Rather than failing pageblock isolation if the the migrateteype is not

s/the the migrateteype/the migratetype/

> MIGRATE_MOVABLE, this patch checks if all of the pages in the pageblock
> are owned by a registered balloon driver (or other entity) using a
> notifier chain.  If all of the non-movable pages are owned by a balloon,
> they can be freed later through the memory notifier chain and the range
> can still be isolated in set_migratetype_isolate().
> 
> Signed-off-by: Robert Jennings <rcj@xxxxxxxxxxxxxxxxxx>
> 
> ---
>  drivers/base/memory.c  |   19 +++++++++++++++++++
>  include/linux/memory.h |   26 ++++++++++++++++++++++++++
>  mm/page_alloc.c        |   45 ++++++++++++++++++++++++++++++++++++++-------
>  3 files changed, 83 insertions(+), 7 deletions(-)
> 
> Index: b/drivers/base/memory.c
> ===================================================================
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -63,6 +63,20 @@ void unregister_memory_notifier(struct n
>  }
>  EXPORT_SYMBOL(unregister_memory_notifier);
>  
> +static BLOCKING_NOTIFIER_HEAD(memory_isolate_chain);
> +
> +int register_memory_isolate_notifier(struct notifier_block *nb)
> +{
> +	return blocking_notifier_chain_register(&memory_isolate_chain, nb);
> +}
> +EXPORT_SYMBOL(register_memory_isolate_notifier);
> +
> +void unregister_memory_isolate_notifier(struct notifier_block *nb)
> +{
> +	blocking_notifier_chain_unregister(&memory_isolate_chain, nb);
> +}
> +EXPORT_SYMBOL(unregister_memory_isolate_notifier);
> +
>  /*
>   * register_memory - Setup a sysfs device for a memory block
>   */
> @@ -157,6 +171,11 @@ int memory_notify(unsigned long val, voi
>  	return blocking_notifier_call_chain(&memory_chain, val, v);
>  }
>  
> +int memory_isolate_notify(unsigned long val, void *v)
> +{
> +	return blocking_notifier_call_chain(&memory_isolate_chain, val, v);
> +}
> +
>  /*
>   * MEMORY_HOTPLUG depends on SPARSEMEM in mm/Kconfig, so it is
>   * OK to have direct references to sparsemem variables in here.
> Index: b/include/linux/memory.h
> ===================================================================
> --- a/include/linux/memory.h
> +++ b/include/linux/memory.h
> @@ -50,6 +50,18 @@ struct memory_notify {
>  	int status_change_nid;
>  };
>  
> +/*
> + * During pageblock isolation, count the number of pages in the
> + * range [start_pfn, start_pfn + nr_pages)
> + */
> 

The comment could have been slightly better. The count of pages in the
range is nr_pages - memory_holes but what you're counting is the number
of pages owned by the balloon driver in the notification chain.

> +#define MEM_ISOLATE_COUNT	(1<<0)
> +
> +struct memory_isolate_notify {
> +	unsigned long start_pfn;
> +	unsigned int nr_pages;
> +	unsigned int pages_found;
> +};
> +
>  struct notifier_block;
>  struct mem_section;
>  
> @@ -76,14 +88,28 @@ static inline int memory_notify(unsigned
>  {
>  	return 0;
>  }
> +static inline int register_memory_isolate_notifier(struct notifier_block *nb)
> +{
> +	return 0;
> +}
> +static inline void unregister_memory_isolate_notifier(struct notifier_block *nb)
> +{
> +}
> +static inline int memory_isolate_notify(unsigned long val, void *v)
> +{
> +	return 0;
> +}
>  #else
>  extern int register_memory_notifier(struct notifier_block *nb);
>  extern void unregister_memory_notifier(struct notifier_block *nb);
> +extern int register_memory_isolate_notifier(struct notifier_block *nb);
> +extern void unregister_memory_isolate_notifier(struct notifier_block *nb);
>  extern int register_new_memory(int, struct mem_section *);
>  extern int unregister_memory_section(struct mem_section *);
>  extern int memory_dev_init(void);
>  extern int remove_memory_block(unsigned long, struct mem_section *, int);
>  extern int memory_notify(unsigned long val, void *v);
> +extern int memory_isolate_notify(unsigned long val, void *v);
>  extern struct memory_block *find_memory_block(struct mem_section *);
>  #define CONFIG_MEM_BLOCK_SIZE	(PAGES_PER_SECTION<<PAGE_SHIFT)
>  enum mem_add_context { BOOT, HOTPLUG };
> Index: b/mm/page_alloc.c
> ===================================================================
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -48,6 +48,7 @@
>  #include <linux/page_cgroup.h>
>  #include <linux/debugobjects.h>
>  #include <linux/kmemleak.h>
> +#include <linux/memory.h>
>  #include <trace/events/kmem.h>
>  
>  #include <asm/tlbflush.h>
> @@ -4985,23 +4986,53 @@ void set_pageblock_flags_group(struct pa
>  int set_migratetype_isolate(struct page *page)
>  {
>  	struct zone *zone;
> -	unsigned long flags;
> +	unsigned long flags, pfn, iter;
> +	unsigned long immobile = 0;
> +	struct memory_isolate_notify arg;
> +	int notifier_ret;
>  	int ret = -EBUSY;
>  	int zone_idx;
>  
>  	zone = page_zone(page);
>  	zone_idx = zone_idx(zone);
> +
>  	spin_lock_irqsave(&zone->lock, flags);
> +	if (get_pageblock_migratetype(page) == MIGRATE_MOVABLE ||
> +	    zone_idx == ZONE_MOVABLE) {
> +		ret = 0;
> +		goto out;
> +	}
> +
> +	pfn = page_to_pfn(page);
> +	arg.start_pfn = pfn;
> +	arg.nr_pages = pageblock_nr_pages;
> +	arg.pages_found = 0;
> +
>  	/*
> -	 * In future, more migrate types will be able to be isolation target.
> +	 * The pageblock can be isolated even if the migrate type is
> +	 * not *_MOVABLE.  The memory isolation notifier chain counts
> +	 * the number of pages in this pageblock that can be freed later
> +	 * through the memory notifier chain.  If all of the pages are
> +	 * accounted for, isolation can continue.

This comment could have been clearer as well

* It may be possible to isolate a pageblock even if the migratetype is
* not MIGRATE_MOVABLE. The memory isolation notifier chain is used by
* balloon drivers to return the number of pages in a range that are held
* by the balloon driver to shrink memory. If all the pages are accounted
* for by balloons or are free, isolation can continue

>  	 */
> -	if (get_pageblock_migratetype(page) != MIGRATE_MOVABLE &&
> -	    zone_idx != ZONE_MOVABLE)
> +	notifier_ret = memory_isolate_notify(MEM_ISOLATE_COUNT, &arg);
> +	notifier_ret = notifier_to_errno(notifier_ret);
> +       	if (notifier_ret || !arg.pages_found)
>  		goto out;
> -	set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> -	move_freepages_block(zone, page, MIGRATE_ISOLATE);
> -	ret = 0;
> +
> +	for (iter = pfn; iter < (pfn + pageblock_nr_pages); iter++)
> +		if (page_count(pfn_to_page(iter)))
> +			immobile++;
> +

This part here is not safe when CONFIG_HOLES_IN_ZONE is set. You need to
make it something like

for (iter = pfn; iter < (pfn + pageblock_nr_pages); iter++) {
	if (!pfn_valid_within(pfn))
		continue;

	if (page_count(pfn_to_page(iter)))
		immobile++;
}

You shouldn't need to run pfn_valid() as you're always starting from a valid
page and never going outside MAX_ORDER_NR_PAGES in this iterator.

> +	if (arg.pages_found == immobile)
> +		ret = 0;
> +

Ok, so if all pages in a range that are in use match the count returned
by the balloon, then it's ok to isolate.

>  out:
> +	if (!ret) {
> +		set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> +		move_freepages_block(zone, page, MIGRATE_ISOLATE);
> +	}
> +
>  	spin_unlock_irqrestore(&zone->lock, flags);
>  	if (!ret)
>  		drain_all_pages();
> 

The patch looks more or less sane. It would be nice to have a follow-on patch
that clarified some details but it's not necessary. The pfn_valid_within()
should be done as a follow-on patch. I haven't actually tested this but
otherwise it looks ok. Once the pfn_valid_within() is sorted out, it has
my Ack.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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