Re: [PATCH v2 4/5] mm: Introduce Hinted pages

[David Hildenbrand]

On 25.07.19 13:46, Nitesh Narayan Lal wrote:
> 
> On 7/25/19 4:53 AM, David Hildenbrand wrote:
>> On 24.07.19 19:03, Alexander Duyck wrote:
>>> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>>
>>> In order to pave the way for free page hinting in virtualized environments
>>> we will need a way to get pages out of the free lists and identify those
>>> pages after they have been returned. To accomplish this, this patch adds
>>> the concept of a Hinted Buddy, which is essentially meant to just be the
>>> Offline page type used in conjunction with the Buddy page type.
>>>
>>> It adds a set of pointers we shall call "boundary" which represents the
>>> upper boundary between the unhinted and hinted pages. The general idea is
>>> that in order for a page to cross from one side of the boundary to the
>>> other it will need to go through the hinting process. Ultimately a
>>> free_list has been fully processed when the boundary has been moved from
>>> the tail all they way up to occupying the first entry in the list.
>>>
>>> Doing this we should be able to make certain that we keep the hinted
>>> pages as one contiguous block in each free list. This will allow us to
>>> efficiently manipulate the free lists whenever we need to go in and start
>>> sending hints to the hypervisor that there are new pages that have been
>>> freed and are no longer in use.
>>>
>>> An added advantage to this approach is that we should be reducing the
>>> overall memory footprint of the guest as it will be more likely to recycle
>>> warm pages versus trying to allocate the hinted pages that were likely
>>> evicted from the guest memory.
>>>
>>> Since we will only be hinting one zone at a time we keep the boundary
>>> limited to being defined for just the zone we are currently placing hinted
>>> pages into. Doing this we can keep the number of additional pointers needed
>>> quite small. To flag that the boundaries are in place we use a single bit
>>> in the zone to indicate that hinting and the boundaries are active.
>>>
>>> The determination of when to start hinting is based on the tracking of the
>>> number of free pages in a given area versus the number of hinted pages in
>>> that area. We keep track of the number of hinted pages per free_area in a
>>> separate zone specific area. We do this to avoid modifying the free_area
>>> structure as this can lead to false sharing for the highest order with the
>>> zone lock which leads to a noticeable performance degradation.
>>>
>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>> ---
>>>  include/linux/mmzone.h       |   40 +++++-
>>>  include/linux/page-flags.h   |    8 +
>>>  include/linux/page_hinting.h |  139 ++++++++++++++++++++
>>>  mm/Kconfig                   |    5 +
>>>  mm/Makefile                  |    1 
>>>  mm/memory_hotplug.c          |    1 
>>>  mm/page_alloc.c              |  136 ++++++++++++++++++-
>>>  mm/page_hinting.c            |  298 ++++++++++++++++++++++++++++++++++++++++++
>>>  8 files changed, 620 insertions(+), 8 deletions(-)
>>>  create mode 100644 include/linux/page_hinting.h
>>>  create mode 100644 mm/page_hinting.c
>>>
>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>> index f0c68b6b6154..42bdebb20484 100644
>>> --- a/include/linux/mmzone.h
>>> +++ b/include/linux/mmzone.h
>>> @@ -460,6 +460,14 @@ struct zone {
>>>  	seqlock_t		span_seqlock;
>>>  #endif
>>>  
>>> +#ifdef CONFIG_PAGE_HINTING
>>> +	/*
>>> +	 * Pointer to hinted page tracking statistics array. The size of
>>> +	 * the array is MAX_ORDER - PAGE_HINTING_MIN_ORDER. NULL when
>>> +	 * page hinting is not present.
>>> +	 */
>>> +	unsigned long		*hinted_pages;
>>> +#endif
>>>  	int initialized;
>>>  
>>>  	/* Write-intensive fields used from the page allocator */
>>> @@ -535,6 +543,14 @@ enum zone_flags {
>>>  	ZONE_BOOSTED_WATERMARK,		/* zone recently boosted watermarks.
>>>  					 * Cleared when kswapd is woken.
>>>  					 */
>>> +	ZONE_PAGE_HINTING_REQUESTED,	/* zone enabled page hinting and has
>>> +					 * requested flushing the data out of
>>> +					 * higher order pages.
>>> +					 */
>>> +	ZONE_PAGE_HINTING_ACTIVE,	/* zone enabled page hinting and is
>>> +					 * activly flushing the data out of
>>> +					 * higher order pages.
>>> +					 */
>>>  };
>>>  
>>>  static inline unsigned long zone_managed_pages(struct zone *zone)
>>> @@ -755,6 +771,8 @@ static inline bool pgdat_is_empty(pg_data_t *pgdat)
>>>  	return !pgdat->node_start_pfn && !pgdat->node_spanned_pages;
>>>  }
>>>  
>>> +#include <linux/page_hinting.h>
>>> +
>>>  /* Used for pages not on another list */
>>>  static inline void add_to_free_list(struct page *page, struct zone *zone,
>>>  				    unsigned int order, int migratetype)
>>> @@ -769,10 +787,16 @@ static inline void add_to_free_list(struct page *page, struct zone *zone,
>>>  static inline void add_to_free_list_tail(struct page *page, struct zone *zone,
>>>  					 unsigned int order, int migratetype)
>>>  {
>>> -	struct free_area *area = &zone->free_area[order];
>>> +	struct list_head *tail = get_unhinted_tail(zone, order, migratetype);
>>>  
>>> -	list_add_tail(&page->lru, &area->free_list[migratetype]);
>>> -	area->nr_free++;
>>> +	/*
>>> +	 * To prevent the unhinted pages from being interleaved with the
>>> +	 * hinted ones while we are actively processing pages we will use
>>> +	 * the head of the hinted pages to determine the tail of the free
>>> +	 * list.
>>> +	 */
>>> +	list_add_tail(&page->lru, tail);
>>> +	zone->free_area[order].nr_free++;
>>>  }
>>>  
>>>  /* Used for pages which are on another list */
>>> @@ -781,12 +805,22 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
>>>  {
>>>  	struct free_area *area = &zone->free_area[order];
>>>  
>>> +	/*
>>> +	 * Clear Hinted flag, if present, to avoid placing hinted pages
>>> +	 * at the top of the free_list. It is cheaper to just process this
>>> +	 * page again, then have to walk around a page that is already hinted.
>>> +	 */
>>> +	clear_page_hinted(page, zone);
>>> +
>>>  	list_move(&page->lru, &area->free_list[migratetype]);
>>>  }
>>>  
>>>  static inline void del_page_from_free_list(struct page *page, struct zone *zone,
>>>  					   unsigned int order)
>>>  {
>>> +	/* Clear Hinted flag, if present, before clearing the Buddy flag */
>>> +	clear_page_hinted(page, zone);
>>> +
>>>  	list_del(&page->lru);
>>>  	__ClearPageBuddy(page);
>>>  	set_page_private(page, 0);
>>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>>> index b848517da64c..b753dbf673cb 100644
>>> --- a/include/linux/page-flags.h
>>> +++ b/include/linux/page-flags.h
>>> @@ -745,6 +745,14 @@ static inline int page_has_type(struct page *page)
>>>  PAGE_TYPE_OPS(Offline, offline)
>>>  
>>>  /*
>>> + * PageHinted() is an alias for Offline, however it is not meant to be an
>>> + * exclusive value. It should be combined with PageBuddy() when seen as it
>>> + * is meant to indicate that the page has been scrubbed while waiting in
>>> + * the buddy system.
>>> + */
>>> +PAGE_TYPE_OPS(Hinted, offline)
>>
>> CCing Matthew
>>
>> I am still not sure if I like the idea of having two page types at a time.
>>
>> 1. Once we run out of page type bits (which can happen easily looking at
>> it getting more and more user - e.g., maybe for vmmap pages soon), we
>> might want to convert again back to a value-based, not bit-based type
>> detection. This will certainly make this switch harder.
>>
>> 2. It will complicate the kexec/kdump handling. I assume it can be fixed
>> some way - e.g., making the elf interface aware of the exact notion of
>> page type bits compared to mapcount values we have right now (e.g.,
>> PAGE_BUDDY_MAPCOUNT_VALUE). Not addressed in this series yet.
>>
>>
>> Can't we reuse one of the traditional page flags for that, not used
>> along with buddy pages? E.g., PG_dirty: Pages that were not hinted yet
>> are dirty.
> 
> Will it not conflict with the regular use case of PG_dirty bit somehow?

AFAIK it is primarily used for pagecache pages only, so never with pages
in the buddy. Unfortunately, page-flags.h lacks proper documentation.

-- 

Thanks,

David / dhildenb