Date:   Thu, 25 Jul 2019 07:46:56 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        kvm@...r.kernel.org, mst@...hat.com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     yang.zhang.wz@...il.com, pagupta@...hat.com, riel@...riel.com,
        konrad.wilk@...cle.com, lcapitulino@...hat.com,
        wei.w.wang@...el.com, aarcange@...hat.com, pbonzini@...hat.com,
        dan.j.williams@...el.com, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 4/5] mm: Introduce Hinted pages


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@...ux.intel.com>
>>
>> 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@...ux.intel.com>
>> ---
>>  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?


> Matthew, what's your take?
>
-- 
Thanks
Nitesh

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