Re: [PATCH v10 3/6] mm: Introduce Reported pages

Michael S. Tsirkin
2019-09-23T08:16:07Z

On Wed, Sep 18, 2019 at 10:52:49AM -0700, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>
> In order to pave the way for free page reporting 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 Reported Buddy, which is essentially
> meant to just be the Uptodate flag used in conjunction with the Buddy
> page type.
>
> It adds a set of pointers we shall call "reported_boundary" which
> represent the upper boundary between the unreported and reported 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 verify that it went through the
> reporting 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.
>
> One limitation to this approach is that it is essentially a linear search
> and in the case of the free lists we can have pages added to either the
> head or the tail of the list. In order to place limits on this we only
> allow pages to be added before the reported_boundary instead of adding
> to the tail itself. 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 reported pages
> that were likely evicted from the guest memory.
>
> Since we will only be reporting one zone at a time we keep the boundary
> limited to being defined for just the zone we are currently reporting pages
> from. 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 reporting and the boundaries are active.
>
> We store the index of the boundary pointer used to track the reported page
> in the page->index value. Doing this we can avoid unnecessary computation
> to determine the index value again. There should be no issues with this as
> the value is unused when the page is in the buddy allocator, and is reset
> as soon as the page is removed from the free list.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
> include/linux/mmzone.h | 16 ++++
> include/linux/page-flags.h | 11 +++
> mm/Kconfig | 11 +++
> mm/compaction.c | 5 +
> mm/memory_hotplug.c | 2
> mm/page_alloc.c | 67 +++++++++++++++--
> mm/page_reporting.h | 178 ++++++++++++++++++++++++++++++++++++++++++++
> 7 files changed, 283 insertions(+), 7 deletions(-)
> create mode 100644 mm/page_reporting.h
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 270a7b493174..53922c30b8d8 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -463,6 +463,14 @@ struct zone {
> seqlock_t span_seqlock;
> #endif
>
> +#ifdef CONFIG_PAGE_REPORTING
> + /*
> + * Pointer to reported page tracking statistics array. The size of
> + * the array is MAX_ORDER - PAGE_REPORTING_MIN_ORDER. NULL when
> + * unused page reporting is not present.
> + */
> + unsigned long *reported_pages;
> +#endif
> int initialized;
>
> /* Write-intensive fields used from the page allocator */
> @@ -538,6 +546,14 @@ enum zone_flags {
> ZONE_BOOSTED_WATERMARK, /* zone recently boosted watermarks.
> * Cleared when kswapd is woken.
> */
> + ZONE_PAGE_REPORTING_ACTIVE, /* zone enabled page reporting and is
> + * activly flushing the data out of
> + * higher order pages.
> + */
> + ZONE_PAGE_REPORTING_REQUESTED, /* zone enabled page reporting and has
> + * requested flushing the data out of
> + * higher order pages.
> + */
> };
>
> static inline unsigned long zone_managed_pages(struct zone *zone)
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index f91cb8898ff0..759a3b3956f2 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -163,6 +163,9 @@ enum pageflags {
>
> /* non-lru isolated movable page */
> PG_isolated = PG_reclaim,
> +
> + /* Buddy pages. Used to track which pages have been reported */
> + PG_reported = PG_uptodate,
> };
>
> #ifndef __GENERATING_BOUNDS_H
> @@ -432,6 +435,14 @@ static inline bool set_hwpoison_free_buddy_page(struct page *page)
> #endif
>
> /*
> + * PageReported() is used to track reported free pages within the Buddy
> + * allocator. We can use the non-atomic version of the test and set
> + * operations as both should be shielded with the zone lock to prevent
> + * any possible races on the setting or clearing of the bit.
> + */
> +__PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
> +
> +/*
> * On an anonymous page mapped into a user virtual memory area,
> * page->mapping points to its anon_vma, not to a struct address_space;
> * with the PAGE_MAPPING_ANON bit set to distinguish it. See rmap.h.
> diff --git a/mm/Kconfig b/mm/Kconfig
> index a5dae9a7eb51..0419b2a9be3e 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -237,6 +237,17 @@ config COMPACTION
> linux-mm@kvack.org.
>
> #
> +# support for unused page reporting
> +config PAGE_REPORTING
> + bool "Allow for reporting of unused pages"
> + def_bool n
> + help
> + Unused page reporting allows for the incremental acquisition of
> + unused pages from the buddy allocator for the purpose of reporting
> + those pages to another entity, such as a hypervisor, so that the
> + memory can be freed up for other uses.
> +
> +#
> # support for page migration
> #
> config MIGRATION
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ce08b39d85d4..60e064330b3a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -24,6 +24,7 @@
> #include <linux/page_owner.h>
> #include <linux/psi.h>
> #include "internal.h"
> +#include "page_reporting.h"
>
> #ifdef CONFIG_COMPACTION
> static inline void count_compact_event(enum vm_event_item item)
> @@ -1325,6 +1326,8 @@ static int next_search_order(struct compact_control *cc, int order)
> continue;
>
> spin_lock_irqsave(&cc->zone->lock, flags);
> + page_reporting_free_area_release(cc->zone, order,
> + MIGRATE_MOVABLE);
> freelist = &area->free_list[MIGRATE_MOVABLE];
> list_for_each_entry_reverse(freepage, freelist, lru) {
> unsigned long pfn;
> @@ -1681,6 +1684,8 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
> continue;
>
> spin_lock_irqsave(&cc->zone->lock, flags);
> + page_reporting_free_area_release(cc->zone, order,
> + MIGRATE_MOVABLE);
> freelist = &area->free_list[MIGRATE_MOVABLE];
> list_for_each_entry(freepage, freelist, lru) {
> unsigned long free_pfn;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 49f7bf91c25a..09c6f52e2bc5 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -41,6 +41,7 @@
>
> #include "internal.h"
> #include "shuffle.h"
> +#include "page_reporting.h"
>
> /*
> * online_page_callback contains pointer to current page onlining function.
> @@ -1613,6 +1614,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
> if (!populated_zone(zone)) {
> zone_pcp_reset(zone);
> build_all_zonelists(NULL);
> + page_reporting_reset_zone(zone);
> } else
> zone_pcp_update(zone);
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f8271ec8e06e..ed0128c65936 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -74,6 +74,7 @@
> #include <asm/div64.h>
> #include "internal.h"
> #include "shuffle.h"
> +#include "page_reporting.h"
>
> /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
> static DEFINE_MUTEX(pcp_batch_high_lock);
> @@ -891,10 +892,15 @@ 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_unreported_tail(zone, order, migratetype);
>
> - list_add_tail(&page->lru, &area->free_list[migratetype]);
> - area->nr_free++;
> + /*
> + * To prevent the unreported pages from slipping behind our iterator
> + * we will force them to be inserted in front of it. By doing this
> + * we should only need to make one pass through the freelist.
> + */
> + list_add_tail(&page->lru, tail);
> + zone->free_area[order].nr_free++;
> }
>
> /* Used for pages which are on another list */
> @@ -903,12 +909,20 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
> {
> struct free_area *area = &zone->free_area[order];
>
> + /* Make certain the page isn't occupying the boundary */
> + if (unlikely(PageReported(page)))
> + __del_page_from_reported_list(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)
> {
> + /* remove page from reported list, and clear reported state */
> + if (unlikely(PageReported(page)))
> + del_page_from_reported_list(page, zone, order);
> +
> list_del(&page->lru);
> __ClearPageBuddy(page);
> set_page_private(page, 0);
> @@ -972,7 +986,7 @@ static inline void del_page_from_free_list(struct page *page, struct zone *zone,
> static inline void __free_one_page(struct page *page,
> unsigned long pfn,
> struct zone *zone, unsigned int order,
> - int migratetype)
> + int migratetype, bool reported)
> {
> struct capture_control *capc = task_capc(zone);
> unsigned long uninitialized_var(buddy_pfn);
> @@ -1048,7 +1062,9 @@ static inline void __free_one_page(struct page *page,
> done_merging:
> set_page_order(page, order);
>
> - if (is_shuffle_order(order))
> + if (reported)
> + to_tail = true;
> + else if (is_shuffle_order(order))
> to_tail = shuffle_pick_tail();
> else
> to_tail = buddy_merge_likely(pfn, buddy_pfn, page, order);
> @@ -1367,7 +1383,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> if (unlikely(isolated_pageblocks))
> mt = get_pageblock_migratetype(page);
>
> - __free_one_page(page, page_to_pfn(page), zone, 0, mt);
> + __free_one_page(page, page_to_pfn(page), zone, 0, mt, false);
> trace_mm_page_pcpu_drain(page, 0, mt);
> }
> spin_unlock(&zone->lock);
> @@ -1383,7 +1399,7 @@ static void free_one_page(struct zone *zone,
> is_migrate_isolate(migratetype))) {
> migratetype = get_pfnblock_migratetype(page, pfn);
> }
> - __free_one_page(page, pfn, zone, order, migratetype);
> + __free_one_page(page, pfn, zone, order, migratetype, false);
> spin_unlock(&zone->lock);
> }
>
> @@ -2245,6 +2261,43 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
> return NULL;
> }
>
> +#ifdef CONFIG_PAGE_REPORTING
> +struct list_head **reported_boundary __read_mostly;
> +
> +/**
> + * free_reported_page - Return a now-reported page back where we got it
> + * @page: Page that was reported
> + * @order: Order of the reported page
> + *
> + * This function will pull the migratetype and order information out
> + * of the page and attempt to return it where it found it. If the page
> + * is added to the free list without changes we will mark it as being
> + * reported.
> + */
> +void free_reported_page(struct page *page, unsigned int order)
> +{
> + struct zone *zone = page_zone(page);
> + unsigned long pfn;
> + unsigned int mt;
> +
> + /* zone lock should be held when this function is called */
> + lockdep_assert_held(&zone->lock);
> +
> + pfn = page_to_pfn(page);
> + mt = get_pfnblock_migratetype(page, pfn);
> + __free_one_page(page, pfn, zone, order, mt, true);
> +
> + /*
> + * If page was not comingled with another page we can consider
> + * the result to be "reported" since part of the page hasn't been
> + * modified, otherwise we would need to report on the new larger
> + * page.
> + */
> + if (PageBuddy(page) && page_order(page) == order)
> + add_page_to_reported_list(page, zone, order, mt);
> +}
> +#endif /* CONFIG_PAGE_REPORTING */
> +
> /*
> * This array describes the order lists are fallen back to when
> * the free lists for the desirable migrate type are depleted
> diff --git a/mm/page_reporting.h b/mm/page_reporting.h
> new file mode 100644
> index 000000000000..c5e1bb58ad96
> --- /dev/null
> +++ b/mm/page_reporting.h
> @@ -0,0 +1,178 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _MM_PAGE_REPORTING_H
> +#define _MM_PAGE_REPORTING_H
> +
> +#include <linux/mmzone.h>
> +#include <linux/pageblock-flags.h>
> +#include <linux/page-isolation.h>
> +#include <linux/jump_label.h>
> +#include <linux/slab.h>
> +#include <asm/pgtable.h>
> +
> +#define PAGE_REPORTING_MIN_ORDER pageblock_order
> +#define PAGE_REPORTING_HWM 32
> +
> +#ifdef CONFIG_PAGE_REPORTING
> +/* Reported page accessors, defined in page_alloc.c */
> +void free_reported_page(struct page *page, unsigned int order);
> +
> +/* Free reported_pages and reset reported page tracking count to 0 */
> +static inline void page_reporting_reset_zone(struct zone *zone)
> +{
> + kfree(zone->reported_pages);
> + zone->reported_pages = NULL;
> +}
> +
> +/* Boundary functions */
> +static inline pgoff_t
> +get_reporting_index(unsigned int order, unsigned int migratetype)
> +{
> + /*
> + * We will only ever be dealing with pages greater-than or equal to
> + * PAGE_REPORTING_MIN_ORDER. Since that is the case we can avoid
> + * allocating unused space by limiting our index range to only the
> + * orders that are supported for page reporting.
> + */
> + return (order - PAGE_REPORTING_MIN_ORDER) * MIGRATE_TYPES + migratetype;
> +}
> +
> +extern struct list_head **reported_boundary __read_mostly;
> +
> +static inline void
> +page_reporting_reset_boundary(struct zone *zone, unsigned int order, int mt)
> +{
> + int index;
> +
> + if (order < PAGE_REPORTING_MIN_ORDER)
> + return;
> + if (!test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
> + return;
> +
> + index = get_reporting_index(order, mt);
> + reported_boundary[index] = &zone->free_area[order].free_list[mt];
> +}

So this seems to be costly.
I'm guessing it's the access to flags:

/* zone flags, see below */
unsigned long flags;

/* Primarily protects free_area */
spinlock_t lock;

which is in the same cache line as the lock.

> +
> +static inline void page_reporting_disable_boundaries(struct zone *zone)
> +{
> + /* zone lock should be held when this function is called */
> + lockdep_assert_held(&zone->lock);
> +
> + __clear_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags);
> +}
> +
> +static inline void
> +page_reporting_free_area_release(struct zone *zone, unsigned int order, int mt)
> +{
> + page_reporting_reset_boundary(zone, order, mt);
> +}
> +
> +/*
> + * Method for obtaining the tail of the free list. Using this allows for
> + * tail insertions of unreported pages into the region that is currently
> + * being scanned so as to avoid interleaving reported and unreported pages.
> + */
> +static inline struct list_head *
> +get_unreported_tail(struct zone *zone, unsigned int order, int migratetype)
> +{
> + if (order >= PAGE_REPORTING_MIN_ORDER &&
> + test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
> + return reported_boundary[get_reporting_index(order,
> + migratetype)];
> +
> + return &zone->free_area[order].free_list[migratetype];
> +}
> +
> +/*
> + * Functions for adding/removing reported pages to the freelist.
> + * All of them expect the zone lock to be held to maintain
> + * consistency of the reported list as a subset of the free list.
> + */
> +static inline void
> +add_page_to_reported_list(struct page *page, struct zone *zone,
> + unsigned int order, unsigned int mt)
> +{
> + /*
> + * Default to using index 0, this will be updated later if the zone
> + * is still being processed.
> + */
> + page->index = 0;
> +
> + /* flag page as reported */
> + __SetPageReported(page);
> +
> + /* update areated page accounting */
> + zone->reported_pages[order - PAGE_REPORTING_MIN_ORDER]++;
> +}
> +
> +static inline void page_reporting_pull_boundary(struct page *page)
> +{
> + struct list_head **tail = &reported_boundary[page->index];
> +
> + if (*tail == &page->lru)
> + *tail = page->lru.next;
> +}
> +
> +static inline void
> +__del_page_from_reported_list(struct page *page, struct zone *zone)
> +{
> + /*
> + * Since the page is being pulled from the list we need to update
> + * the boundary, after that we can just update the index so that
> + * the correct boundary will be checked in the future.
> + */
> + if (test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
> + page_reporting_pull_boundary(page);
> +}
> +
> +static inline void
> +del_page_from_reported_list(struct page *page, struct zone *zone,
> + unsigned int order)
> +{
> + __del_page_from_reported_list(page, zone);
> +
> + /* page_private will contain the page order, so just use it directly */
> + zone->reported_pages[order - PAGE_REPORTING_MIN_ORDER]--;
> +
> + /* clear the flag so we can report on it when it returns */
> + __ClearPageReported(page);
> +}
> +
> +#else /* CONFIG_PAGE_REPORTING */
> +static inline void page_reporting_reset_zone(struct zone *zone)
> +{
> +}
> +
> +static inline void
> +page_reporting_free_area_release(struct zone *zone, unsigned int order, int mt)
> +{
> +}
> +
> +static inline struct list_head *
> +get_unreported_tail(struct zone *zone, unsigned int order, int migratetype)
> +{
> + return &zone->free_area[order].free_list[migratetype];
> +}
> +
> +static inline void
> +add_page_to_reported_list(struct page *page, struct zone *zone,
> + int order, int migratetype)
> +{
> +}
> +
> +static inline void
> +__del_page_from_reported_list(struct page *page, struct zone *zone)
> +{
> +}
> +
> +static inline void
> +del_page_from_reported_list(struct page *page, struct zone *zone,
> + unsigned int order)
> +{
> +}
> +
> +static inline void
> +move_page_to_reported_list(struct page *page, struct zone *zone, int dest_mt)
> +{
> +}
> +#endif /* CONFIG_PAGE_REPORTING */
> +#endif /*_MM_PAGE_REPORTING_H */