Re: [PATCH v5 4/6] mm: Introduce Reported pages

David Hildenbrand
2019-08-13T08:07:44Z

On 12.08.19 23:33, 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 "boundary" which represents 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 go 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.
>
> Doing this we should be able to make certain that we keep the reported
> 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 reports 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 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.
>
> The determination of when to start reporting is based on the tracking of
> the number of free pages in a given area versus the number of reported
> pages in that area. We keep track of the number of reported 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@linux.intel.com>
> ---
> include/linux/mmzone.h | 40 +++++
> include/linux/page-flags.h | 11 +
> include/linux/page_reporting.h | 138 ++++++++++++++++++
> mm/Kconfig | 5 +
> mm/Makefile | 1
> mm/memory_hotplug.c | 1
> mm/page_alloc.c | 136 +++++++++++++++++-
> mm/page_reporting.c | 308 ++++++++++++++++++++++++++++++++++++++++
> 8 files changed, 632 insertions(+), 8 deletions(-)
> create mode 100644 include/linux/page_reporting.h
> create mode 100644 mm/page_reporting.c
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 2f2b6f968ed3..b8ed926552b1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -462,6 +462,14 @@ 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 */
> @@ -537,6 +545,14 @@ enum zone_flags {
> ZONE_BOOSTED_WATERMARK, /* zone recently boosted watermarks.
> * Cleared when kswapd is woken.
> */
> + ZONE_PAGE_REPORTING_REQUESTED, /* zone enabled page reporting and has
> + * requested flushing the data out of
> + * higher order pages.
> + */
> + ZONE_PAGE_REPORTING_ACTIVE, /* zone enabled page reporting and is
> + * activly flushing the data out of
> + * higher order pages.
> + */
> };
>
> static inline unsigned long zone_managed_pages(struct zone *zone)
> @@ -757,6 +773,8 @@ static inline bool pgdat_is_empty(pg_data_t *pgdat)
> return !pgdat->node_start_pfn && !pgdat->node_spanned_pages;
> }
>
> +#include <linux/page_reporting.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)
> @@ -771,10 +789,16 @@ 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_unreported_tail(zone, order, migratetype);
>
> - list_add_tail(&page->lru, &area->free_list[migratetype]);
> - area->nr_free++;
> + /*
> + * To prevent the unreported pages from being interleaved with the
> + * reported ones while we are actively processing pages we will use
> + * the head of the reported 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 */
> @@ -783,12 +807,22 @@ 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 reported pages
> + * at the top of the free_list. It is cheaper to just process this
> + * page again than to walk around a page that is already reported.
> + */
> + clear_page_reported(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 Reported flag, if present, before resetting page type */
> + clear_page_reported(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 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/include/linux/page_reporting.h b/include/linux/page_reporting.h
> new file mode 100644
> index 000000000000..498bde6ea764
> --- /dev/null
> +++ b/include/linux/page_reporting.h
> @@ -0,0 +1,138 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_PAGE_REPORTING_H
> +#define _LINUX_PAGE_REPORTING_H
> +
> +#include <linux/mmzone.h>
> +#include <linux/jump_label.h>
> +#include <linux/pageblock-flags.h>
> +#include <asm/pgtable_types.h>
> +
> +#define PAGE_REPORTING_MIN_ORDER pageblock_order
> +#define PAGE_REPORTING_HWM 32
> +
> +#ifdef CONFIG_PAGE_REPORTING
> +struct page_reporting_dev_info {
> + /* function that alters pages to make them "reported" */
> + void (*report)(struct page_reporting_dev_info *phdev,
> + unsigned int nents);
> +
> + /* scatterlist containing pages to be processed */
> + struct scatterlist *sg;
> +
> + /*
> + * Upper limit on the number of pages that the react function
> + * expects to be placed into the batch list to be processed.
> + */
> + unsigned long capacity;
> +
> + /* work struct for processing reports */
> + struct delayed_work work;
> +
> + /*
> + * The number of zones requesting reporting, plus one additional if
> + * processing thread is active.
> + */
> + atomic_t refcnt;
> +};
> +
> +extern struct static_key page_reporting_notify_enabled;
> +
> +/* Boundary functions */
> +struct list_head *__page_reporting_get_boundary(unsigned int order,
> + int migratetype);
> +void page_reporting_del_from_boundary(struct page *page, struct zone *zone);
> +void page_reporting_add_to_boundary(struct page *page, struct zone *zone,
> + int migratetype);
> +
> +/* Hinted page accessors, defined in page_alloc.c */
> +struct page *get_unreported_page(struct zone *zone, unsigned int order,
> + int migratetype);
> +void put_reported_page(struct zone *zone, struct page *page);
> +
> +void __page_reporting_request(struct zone *zone);
> +void __page_reporting_free_stats(struct zone *zone);
> +
> +/* Tear-down and bring-up for page reporting devices */
> +void page_reporting_shutdown(struct page_reporting_dev_info *phdev);
> +int page_reporting_startup(struct page_reporting_dev_info *phdev);
> +#endif /* CONFIG_PAGE_REPORTING */
> +
> +static inline struct list_head *
> +get_unreported_tail(struct zone *zone, unsigned int order, int migratetype)
> +{
> +#ifdef CONFIG_PAGE_REPORTING
> + if (order >= PAGE_REPORTING_MIN_ORDER &&
> + test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
> + return __page_reporting_get_boundary(order, migratetype);
> +#endif
> + return &zone->free_area[order].free_list[migratetype];
> +}
> +
> +static inline void clear_page_reported(struct page *page,
> + struct zone *zone)
> +{
> +#ifdef CONFIG_PAGE_REPORTING
> + if (likely(!PageReported(page)))
> + return;
> +
> + /* push boundary back if we removed the upper boundary */
> + if (test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
> + page_reporting_del_from_boundary(page, zone);
> +
> + __ClearPageReported(page);
> +
> + /* page_private will contain the page order, so just use it directly */
> + zone->reported_pages[page_private(page) - PAGE_REPORTING_MIN_ORDER]--;
> +#endif
> +}
> +
> +/* Free reported_pages and reset reported page tracking count to 0 */
> +static inline void page_reporting_reset(struct zone *zone)
> +{
> +#ifdef CONFIG_PAGE_REPORTING
> + if (zone->reported_pages)
> + __page_reporting_free_stats(zone);
> +#endif
> +}
> +
> +/**
> + * page_reporting_notify_free - Free page notification to start page processing
> + * @zone: Pointer to current zone of last page processed
> + * @order: Order of last page added to zone
> + *
> + * This function is meant to act as a screener for __page_reporting_request
> + * which will determine if a give zone has crossed over the high-water mark
> + * that will justify us beginning page treatment. If we have crossed that
> + * threshold then it will start the process of pulling some pages and
> + * placing them in the batch list for treatment.
> + */
> +static inline void page_reporting_notify_free(struct zone *zone, int order)
> +{
> +#ifdef CONFIG_PAGE_REPORTING
> + unsigned long nr_reported;
> +
> + /* Called from hot path in __free_one_page() */
> + if (!static_key_false(&page_reporting_notify_enabled))
> + return;
> +
> + /* Limit notifications only to higher order pages */
> + if (order < PAGE_REPORTING_MIN_ORDER)
> + return;
> +
> + /* Do not bother with tests if we have already requested reporting */
> + if (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags))
> + return;
> +
> + /* If reported_pages is not populated, assume 0 */
> + nr_reported = zone->reported_pages ?
> + zone->reported_pages[order - PAGE_REPORTING_MIN_ORDER] : 0;
> +
> + /* Only request it if we have enough to begin the page reporting */
> + if (zone->free_area[order].nr_free < nr_reported + PAGE_REPORTING_HWM)
> + return;
> +
> + /* This is slow, but should be called very rarely */
> + __page_reporting_request(zone);
> +#endif
> +}
> +#endif /*_LINUX_PAGE_REPORTING_H */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 1c9698509273..daa8c45e2af4 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -237,6 +237,11 @@ config COMPACTION
> linux-mm@kvack.org.
>
> #
> +# support for unused page reporting
> +config PAGE_REPORTING
> + bool
> +
> +#
> # support for page migration
> #
> config MIGRATION
> diff --git a/mm/Makefile b/mm/Makefile
> index d0b295c3b764..1e17ba0ed2f0 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -105,3 +105,4 @@ obj-$(CONFIG_PERCPU_STATS) += percpu-stats.o
> obj-$(CONFIG_ZONE_DEVICE) += memremap.o
> obj-$(CONFIG_HMM_MIRROR) += hmm.o
> obj-$(CONFIG_MEMFD_CREATE) += memfd.o
> +obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 5b8811945bbb..bd40beac293b 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1612,6 +1612,7 @@ 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);
> } else
> zone_pcp_update(zone);
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4b5812c3800e..d0d3fb12ba54 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -68,6 +68,7 @@
> #include <linux/lockdep.h>
> #include <linux/nmi.h>
> #include <linux/psi.h>
> +#include <linux/page_reporting.h>
>
> #include <asm/sections.h>
> #include <asm/tlbflush.h>
> @@ -915,7 +916,7 @@ static inline struct capture_control *task_capc(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);
> @@ -990,11 +991,20 @@ static inline void __free_one_page(struct page *page,
> done_merging:
> set_page_order(page, order);
>
> - if (is_shuffle_order(order) ? shuffle_add_to_tail() :
> - buddy_merge_likely(pfn, buddy_pfn, page, order))
> + if (reported ||
> + (is_shuffle_order(order) ? shuffle_add_to_tail() :
> + buddy_merge_likely(pfn, buddy_pfn, page, order)))
> add_to_free_list_tail(page, zone, order, migratetype);
> else
> add_to_free_list(page, zone, order, migratetype);
> +
> + /*
> + * No need to notify on a reported page as the total count of
> + * unreported pages will not have increased since we have essentially
> + * merged the reported page with one or more unreported pages.
> + */
> + if (!reported)
> + page_reporting_notify_free(zone, order);
> }
>
> /*
> @@ -1305,7 +1315,7 @@ 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);
> @@ -1321,7 +1331,7 @@ 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);
> }
>
> @@ -2183,6 +2193,122 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
> return NULL;
> }
>
> +#ifdef CONFIG_PAGE_REPORTING
> +/**
> + * get_unreported_page - Pull an unreported page from the free_list
> + * @zone: Zone to draw pages from
> + * @order: Order to draw pages from
> + * @mt: Migratetype to draw pages from
> + *
> + * This function will obtain a page from the free list. It will start by
> + * attempting to pull from the tail of the free list and if that is already
> + * reported on it will instead pull the head if that is unreported.
> + *
> + * The page will have the migrate type and order stored in the page
> + * metadata. While being processed the page will not be avaialble for
> + * allocation.
> + *
> + * Return: page pointer if raw page found, otherwise NULL
> + */
> +struct page *get_unreported_page(struct zone *zone, unsigned int order, int mt)
> +{
> + struct list_head *tail = get_unreported_tail(zone, order, mt);
> + struct free_area *area = &(zone->free_area[order]);
> + struct list_head *list = &area->free_list[mt];
> + struct page *page;
> +
> + /* zone lock should be held when this function is called */
> + lockdep_assert_held(&zone->lock);
> +
> + /* Find a page of the appropriate size in the preferred list */
> + page = list_last_entry(tail, struct page, lru);
> + list_for_each_entry_from_reverse(page, list, lru) {
> + /* If we entered this loop then the "raw" list isn't empty */
> +
> + /* If the page is reported try the head of the list */
> + if (PageReported(page)) {
> + page = list_first_entry(list, struct page, lru);
> +
> + /*
> + * If both the head and tail are reported then reset
> + * the boundary so that we read as an empty list
> + * next time and bail out.
> + */
> + if (PageReported(page)) {
> + page_reporting_add_to_boundary(page, zone, mt);
> + break;
> + }
> + }
> +
> + del_page_from_free_list(page, zone, order);
> +
> + /* record migratetype and order within page */
> + set_pcppage_migratetype(page, mt);
> + set_page_private(page, order);

Can you elaborate why you (similar to Nitesh) have to save/restore the
migratetype? I can't yet follow why that is necessary. You could simply
set it to MOVABLE (like e.g., undo_isolate_page_range() would do via
alloc_contig_range()). If a pageblock is completely free, it might even
make sense to set it to MOVABLE.

(mainly wondering if this is required here and what the rational is)

Now a theoretical issue:

You are allocating pages from all zones, including the MOVABLE zone. The
pages are currently unmovable (however, only temporarily allocated).

del_page_from_free_area() will clear PG_buddy, and leave the refcount of
the page set to zero (!). has_unmovable_pages() will skip any pages
either on the MOVABLE zone or with a refcount of zero. So all pages that
are being reported are detected as movable. The isolation of allocated
pages will work - which is not bad, but I wonder what the implications are.

As far as I can follow, alloc_contig_range() ->
__alloc_contig_migrate_range() -> isolate_migratepages_range() ->
isolate_migratepages_block() will choke on these pages ((!PageLRU(page)
-> !__PageMovable(page) -> fail), essentially making
alloc_contig_range() range fail. Same could apply to offline_pages().

I would have thought that there has to be something like a
reported_pages_drain_all(), that waits until all reports are over (or
even signals to the hypervisor to abort reporting). As the pages are
isolated, they won't be allocated for reporting again.

I have not yet understood completely the way all the details work. I am
currently looking into using alloc_contig_range() in a different
context, which would co-exist with free-page-reporting.
--

Thanks,

David / dhildenb