Re: [PATCH v9 6/8] mm: Introduce Reported pages

From: Alexander Duyck
Date: Mon Sep 09 2019 - 12:25:09 EST

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On Mon, 2019-09-09 at 17:42 +0300, Kirill A. Shutemov wrote:
> On Sat, Sep 07, 2019 at 10:25:53AM -0700, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxx>
> > ---
> > include/linux/mmzone.h | 52 +++++-
> > include/linux/page-flags.h | 11 +
> > include/linux/page_reporting.h | 178 ++++++++++++++++++++
> > mm/Kconfig | 5 +
> > mm/Makefile | 1
> > mm/memory_hotplug.c | 1
> > mm/page_alloc.c | 115 ++++++++++++-
> > mm/page_reporting.c | 358 ++++++++++++++++++++++++++++++++++++++++
> > 8 files changed, 711 insertions(+), 10 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 2ddf1f1971c0..4b2c44d7e266 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_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)
> > @@ -764,6 +780,8 @@ 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)
> > @@ -778,24 +796,48 @@ 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 */
> > static inline void move_to_free_list(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);
> > +
> > + /*
> > + * We must get the tail for our target list before moving the page on
> > + * the reported list as we will possibly be replacing the tail page of
> > + * the list with our current page if it is reported.
> > + */
> > + if (unlikely(PageReported(page)))
> > + move_page_to_reported_list(page, zone, migratetype);
> >
> > - list_move(&page->lru, &area->free_list[migratetype]);
> > + /*
> > + * To prevent unreported pages from being mixed with the reported
> > + * ones we add pages to the tail of the list. By doing this the function
> > + * above can either label them as included in the reported list or not
> > + * and the result will be consistent.
> > + */
> > + list_move_tail(&page->lru, tail);
> > }
> >
> > 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);
> > +
> > 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..836033ca237b
> > --- /dev/null
> > +++ b/include/linux/page_reporting.h
> > @@ -0,0 +1,178 @@
> > +/* 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>
> > +
> > +#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;
> > +};
> > +
> > +/* Boundary functions */
> > +struct list_head *__page_reporting_get_boundary(unsigned int order,
> > + int migratetype);
> > +void page_reporting_del_from_boundary(struct page *page);
> > +void page_reporting_add_to_boundary(struct page *page, int migratetype);
> > +void page_reporting_move_to_boundary(struct page *page, struct zone *zone,
> > + int migratetype);
> > +
> > +/* Reported page accessors, defined in page_alloc.c */
> > +struct page *get_unreported_page(struct zone *zone, unsigned int order,
> > + int migratetype);
> > +void free_reported_page(struct page *page, unsigned int order);
> > +
> > +/* 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);
> > +
> > +void __page_reporting_free_stats(struct zone *zone);
> > +void __page_reporting_request(struct zone *zone);
> > +
> > +static inline void __del_page_from_reported_list(struct page *page,
> > + struct zone *zone)
> > +{
> > + /* page_private will contain the page order, so just use it directly */
> > + zone->reported_pages[page_private(page) - PAGE_REPORTING_MIN_ORDER]--;
> > +
> > + /* clear the flag so we can report on it when it returns */
> > + __ClearPageReported(page);
> > +}
> > +#endif /* CONFIG_PAGE_REPORTING */
> > +
> > +/*
> > + * 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)
> > +{
> > +#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];
> > +}
> > +
> > +/*
> > + * Functions for adding/removing pages from reported end of list.
> > + * 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,
> > + int order,
> > + int migratetype)
> > +{
> > +#ifdef CONFIG_PAGE_REPORTING
>
> Instead of ifdefing full body of the helpers, can we have two sets of
> these helpres defined for CONFIG_PAGE_REPORTING and for
> !CONFIG_PAGE_REPORTING cases?
>
> Or can we get all this working with IS_ENABLED() instead of #ifdef?

I can probably split the helpers and just duplicate them with the ifdef
content stripped.

The problem with IS_ENABLED() is that we are calling functions that don't
exist if page reporting isn't enabled so that would cause build issues.

> > + /* flag page as reported */
> > + __SetPageReported(page);
> > +
> > + /* update areated page accounting */
> > + zone->reported_pages[order - PAGE_REPORTING_MIN_ORDER]++;
> > +
> > + /* update boundary of new migratetype and record it */
> > + page_reporting_add_to_boundary(page, migratetype);
> > +#endif
> > +}
> > +
> > +static inline void del_page_from_reported_list(struct page *page,
> > + struct zone *zone)
> > +{
> > +#ifdef CONFIG_PAGE_REPORTING
> > + /* push boundary back if we removed the upper boundary */
> > + if (test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
> > + page_reporting_del_from_boundary(page);
> > +
> > + __del_page_from_reported_list(page, zone);
> > +#endif
> > +}
> > +
> > +static inline void move_page_to_reported_list(struct page *page,
> > + struct zone *zone,
> > + int migratetype)
> > +{
> > +#ifdef CONFIG_PAGE_REPORTING
> > + page_reporting_move_to_boundary(page, zone, migratetype);
> > +#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
> > +}
> > +
> > +DECLARE_STATIC_KEY_FALSE(page_reporting_notify_enabled);
> > +/**
> > + * 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_branch_unlikely(&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;
>
> How is it not racy wrt page_reporting_fill()? Do we hold zone->lock or
> something?

Yes. This is called at the end of __free_one_page with the zone->lock
held.

> > +
> > + /* 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 a5dae9a7eb51..be1a5db50df5 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -237,6 +237,11 @@ config COMPACTION
> > linux-mm@xxxxxxxxxx
> >
> > #
> > +# support for unused page reporting
> > +config PAGE_REPORTING
> > + bool
> > +
> > +#
>
> Proper description for the config option?

I can add one. However the feature doesn't do anything without a caller
that makes use of it. I guess it would make sense to enable this for
something such as an out-of-tree module to later use.

> > # support for page migration
> > #
> > config MIGRATION
> > diff --git a/mm/Makefile b/mm/Makefile
> > index d996846697ef..fc4fa17b6c83 100644
> > --- a/mm/Makefile
> > +++ b/mm/Makefile
> > @@ -107,3 +107,4 @@ 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 49f7bf91c25a..cb71a7190682 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -1613,6 +1613,7 @@ 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 f85dc1561b85..615aea24c082 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>
> > @@ -916,7 +917,7 @@ 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);
> > @@ -991,11 +992,20 @@ static inline void __free_one_page(struct page *page,
> > done_merging:
> > set_page_order(page, order);
> >
> > - if (is_shuffle_order(order) ? shuffle_pick_tail() :
> > - buddy_merge_likely(pfn, buddy_pfn, page, order))
> > + if (reported ||
> > + (is_shuffle_order(order) ? shuffle_pick_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);
> > }
> >
> > /*
> > @@ -1306,7 +1316,7 @@ 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);
> > @@ -1322,7 +1332,7 @@ 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);
> > }
> >
> > @@ -2184,6 +2194,101 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
> > return NULL;
> > }
> >
> > +#ifdef CONFIG_PAGE_REPORTING
> > +/**
> > + * 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);
> > +}
> > +
> > +/**
> > + * 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, mt);
> > + break;
> > + }
> > + }
> > +
> > + del_page_from_free_list(page, zone, order);
> > +
> > + /*
> > + * Page will not be available for allocation while we are
> > + * processing it so update the freepage state.
> > + */
> > + __mod_zone_freepage_state(zone, -(1 << order), mt);
> > +
> > + return page;
> > + }
> > +
> > + return NULL;
> > +}
> > +#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.c b/mm/page_reporting.c
> > new file mode 100644
> > index 000000000000..a59ef53eb0b8
> > --- /dev/null
> > +++ b/mm/page_reporting.c
> > @@ -0,0 +1,358 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#include <linux/mm.h>
> > +#include <linux/mmzone.h>
> > +#include <linux/page-isolation.h>
> > +#include <linux/gfp.h>
> > +#include <linux/export.h>
> > +#include <linux/delay.h>
> > +#include <linux/slab.h>
> > +#include <linux/scatterlist.h>
> > +#include "internal.h"
> > +
> > +static struct page_reporting_dev_info __rcu *ph_dev_info __read_mostly;
> > +struct list_head **boundary __read_mostly;
> > +
> > +static inline struct list_head **get_boundary_ptr(unsigned int order,
> > + unsigned int migratetype)
> > +{
> > + return boundary +
> > + (order - PAGE_REPORTING_MIN_ORDER) * MIGRATE_TYPES + migratetype;
> > +}
> > +
> > +static void page_reporting_reset_boundary(struct zone *zone, unsigned int order,
> > + unsigned int migratetype)
> > +{
> > + struct list_head **tail = get_boundary_ptr(order, migratetype);
> > +
> > + *tail = &zone->free_area[order].free_list[migratetype];
> > +}
> > +
> > +#define for_each_reporting_migratetype_order(_order, _type) \
> > + for (_order = MAX_ORDER; _order-- != PAGE_REPORTING_MIN_ORDER;) \
> > + for (_type = MIGRATE_TYPES; _type--;) \
> > + if (!is_migrate_isolate(_type))
> > +
> > +static int page_reporting_populate_metadata(struct zone *zone)
> > +{
> > + unsigned int order, mt;
> > +
> > + /*
> > + * We need to make sure we have somewhere to store the tracking
> > + * data for how many reported pages are in the zone. To do that
> > + * we need to make certain zone->reported_pages is populated.
> > + */
> > + if (!zone->reported_pages) {
> > + zone->reported_pages =
> > + kcalloc(MAX_ORDER - PAGE_REPORTING_MIN_ORDER,
> > + sizeof(unsigned long),
> > + GFP_KERNEL);
> > + if (!zone->reported_pages)
> > + return -ENOMEM;
> > + }
> > +
> > + /* Update boundary data to reflect the zone we are currently working */
> > + for_each_reporting_migratetype_order(order, mt)
> > + page_reporting_reset_boundary(zone, order, mt);
> > +
> > + return 0;
> > +}
> > +
> > +struct list_head *__page_reporting_get_boundary(unsigned int order,
> > + int migratetype)
> > +{
> > + return *get_boundary_ptr(order, migratetype);
> > +}
> > +
> > +void page_reporting_del_from_boundary(struct page *page)
> > +{
> > + unsigned int order = page_private(page);
> > + int mt = get_pcppage_migratetype(page);
> > + struct list_head **tail = get_boundary_ptr(order, mt);
> > +
> > + if (*tail == &page->lru)
> > + *tail = page->lru.next;
> > +}
> > +
> > +void page_reporting_add_to_boundary(struct page *page, int migratetype)
> > +{
> > + unsigned int order = page_private(page);
> > + struct list_head **tail = get_boundary_ptr(order, migratetype);
> > +
> > + *tail = &page->lru;
> > + set_pcppage_migratetype(page, migratetype);
> > +}
> > +
> > +void page_reporting_move_to_boundary(struct page *page, struct zone *zone,
> > + int dest_mt)
> > +{
> > + /*
> > + * We essentially have two options available to us. The first is to
> > + * move the page from the boundary list on one migratetype to the
> > + * list for the new migratetype assuming reporting is still active.
> > + *
> > + * The other option is to clear the reported state of the page as
> > + * we will not be adding it to the group of pages that were already
> > + * reported. It is cheaper to just rereport such pages then go
> > + * through and do a special search to skip over them. If the page
> > + * is being moved into isolation we can defer this until the page
> > + * comes out of isolation since we do not scan the isolated
> > + * migratetype.
> > + */
> > + if (test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags)) {
> > + page_reporting_del_from_boundary(page);
> > + page_reporting_add_to_boundary(page, dest_mt);
> > + } else if (!is_migrate_isolate(dest_mt)) {
> > + __del_page_from_reported_list(page, zone);
> > + }
> > +}
> > +
> > +static unsigned int page_reporting_fill(struct zone *zone,
> > + struct page_reporting_dev_info *phdev)
> > +{
> > + struct scatterlist *sg = phdev->sg;
> > + unsigned int order, mt, count = 0;
> > +
> > + sg_init_table(phdev->sg, phdev->capacity);
> > +
> > + for_each_reporting_migratetype_order(order, mt) {
> > + struct page *page;
> > +
> > + /*
> > + * Pull pages from free list until we have drained
> > + * it or we have reached capacity.
> > + */
> > + while ((page = get_unreported_page(zone, order, mt))) {
> > + sg_set_page(&sg[count], page, PAGE_SIZE << order, 0);
> > +
> > + if (++count == phdev->capacity)
> > + return count;
> > + }
> > + }
> > +
> > + /* mark end of scatterlist due to underflow */
> > + if (count)
> > + sg_mark_end(&sg[count - 1]);
> > +
> > + /*
> > + * If there are no longer enough free pages to fully populate
> > + * the scatterlist, then we can just shut it down for this zone.
> > + */
> > + __clear_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags);
> > + atomic_dec(&phdev->refcnt);
> > +
> > + return count;
> > +}
> > +
> > +static void page_reporting_drain(struct page_reporting_dev_info *phdev)
> > +{
> > + struct scatterlist *sg = phdev->sg;
> > +
> > + /*
> > + * Drain the now reported pages back into their respective
> > + * free lists/areas. We assume at least one page is populated.
> > + */
> > + do {
> > + free_reported_page(sg_page(sg), get_order(sg->length));
> > + } while (!sg_is_last(sg++));
> > +}
> > +
> > +/*
> > + * The page reporting cycle consists of 4 stages, fill, report, drain, and idle.
> > + * We will cycle through the first 3 stages until we fail to obtain any
> > + * pages, in that case we will switch to idle.
> > + */
> > +static void page_reporting_cycle(struct zone *zone,
> > + struct page_reporting_dev_info *phdev)
> > +{
> > + /*
> > + * Guarantee boundaries and stats are populated before we
> > + * start placing reported pages in the zone.
> > + */
> > + if (page_reporting_populate_metadata(zone))
> > + return;
> > +
> > + spin_lock_irq(&zone->lock);
> > +
> > + /* set bit indicating boundaries are present */
> > + __set_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags);
> > +
> > + do {
> > + /* Pull pages out of allocator into a scaterlist */
> > + unsigned int nents = page_reporting_fill(zone, phdev);
> > +
> > + /* no pages were acquired, give up */
> > + if (!nents)
> > + break;
> > +
> > + spin_unlock_irq(&zone->lock);
> > +
> > + /* begin processing pages in local list */
> > + phdev->report(phdev, nents);
> > +
> > + spin_lock_irq(&zone->lock);
> > +
> > + /*
> > + * We should have a scatterlist of pages that have been
> > + * processed. Return them to their original free lists.
> > + */
> > + page_reporting_drain(phdev);
> > +
> > + /* keep pulling pages till there are none to pull */
> > + } while (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags));
> > +
> > + /* processing of the zone is complete, we can disable boundaries */
> > + __clear_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags);
> > +
> > + spin_unlock_irq(&zone->lock);
> > +}
> > +
> > +static void page_reporting_process(struct work_struct *work)
> > +{
> > + struct delayed_work *d_work = to_delayed_work(work);
> > + struct page_reporting_dev_info *phdev =
> > + container_of(d_work, struct page_reporting_dev_info, work);
> > + struct zone *zone = first_online_pgdat()->node_zones;
> > +
> > + do {
> > + if (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags))
> > + page_reporting_cycle(zone, phdev);
> > +
> > + /* Move to next zone, if at end of list start over */
> > + zone = next_zone(zone) ? : first_online_pgdat()->node_zones;
> > +
> > + /*
> > + * As long as refcnt has not reached zero there are still
> > + * zones to be processed.
> > + */
> > + } while (atomic_read(&phdev->refcnt));
> > +}
> > +
> > +/* request page reporting on this zone */
> > +void __page_reporting_request(struct zone *zone)
> > +{
> > + struct page_reporting_dev_info *phdev;
> > +
> > + rcu_read_lock();
> > +
> > + /*
> > + * We use RCU to protect the ph_dev_info pointer. In almost all
> > + * cases this should be present, however in the unlikely case of
> > + * a shutdown this will be NULL and we should exit.
> > + */
> > + phdev = rcu_dereference(ph_dev_info);
> > + if (unlikely(!phdev))
> > + goto out;
> > +
> > + /*
> > + * We can use separate test and set operations here as there
> > + * is nothing else that can set or clear this bit while we are
> > + * holding the zone lock. The advantage to doing it this way is
> > + * that we don't have to dirty the cacheline unless we are
> > + * changing the value.
> > + */
> > + __set_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags);
> > +
> > + /*
> > + * Delay the start of work to allow a sizable queue to
> > + * build. For now we are limiting this to running no more
> > + * than 10 times per second.
> > + */
> > + if (!atomic_fetch_inc(&phdev->refcnt))
> > + schedule_delayed_work(&phdev->work, HZ / 10);
> > +out:
> > + rcu_read_unlock();
> > +}
> > +
> > +void __page_reporting_free_stats(struct zone *zone)
> > +{
> > + /* free reported_page statisitics */
> > + kfree(zone->reported_pages);
> > + zone->reported_pages = NULL;
> > +}
> > +
> > +static DEFINE_MUTEX(page_reporting_mutex);
> > +DEFINE_STATIC_KEY_FALSE(page_reporting_notify_enabled);
> > +
> > +void page_reporting_shutdown(struct page_reporting_dev_info *phdev)
> > +{
> > + mutex_lock(&page_reporting_mutex);
> > +
> > + if (rcu_access_pointer(ph_dev_info) == phdev) {
> > + /* Disable page reporting notification */
> > + static_branch_disable(&page_reporting_notify_enabled);
> > + RCU_INIT_POINTER(ph_dev_info, NULL);
> > + synchronize_rcu();
> > +
> > + /* Flush any existing work, and lock it out */
> > + cancel_delayed_work_sync(&phdev->work);
> > +
> > + /* Free scatterlist */
> > + kfree(phdev->sg);
> > + phdev->sg = NULL;
> > +
> > + /* Free boundaries */
> > + kfree(boundary);
> > + boundary = NULL;
> > + }
> > +
> > + mutex_unlock(&page_reporting_mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(page_reporting_shutdown);
> > +
> > +int page_reporting_startup(struct page_reporting_dev_info *phdev)
> > +{
> > + struct zone *zone;
> > + int err = 0;
> > +
> > + /* No point in enabling this if it cannot handle any pages */
> > + if (!phdev->capacity)
> > + return -EINVAL;
>
> Looks like a usage error. Maybe WARN_ON()?

I can do that. We shouldn't really be accessing this with that setup
anyway.

> > +
> > + mutex_lock(&page_reporting_mutex);
> > +
> > + /* nothing to do if already in use */
> > + if (rcu_access_pointer(ph_dev_info)) {
> > + err = -EBUSY;
> > + goto err_out;
> > + }
>
> Again, it's from "something went horribly wrong" category.
> Maybe WARN_ON()?

That one I am not so sure about. Right now we only have one user for the
page reporting interface. My concern is if we ever have more than one we
may experience collisions. The device driver requesting this should
display an error message if it is not able tor register the interface.

> > +
> > + boundary = kcalloc(MAX_ORDER - PAGE_REPORTING_MIN_ORDER,
> > + sizeof(struct list_head *) * MIGRATE_TYPES,
> > + GFP_KERNEL);
>
> Could you comment here on why this size of array is allocated?
> The calculation is not obvious to a reader.

Would something like the following work for you?
/*
* Allocate space to store the boundaries for the zone we are
* actively reporting on. We will need to store one boundary
* pointer per migratetype, and then we need to have one of these
* arrays per order for orders greater than or equal to
* PAGE_REPORTING_MIN_ORDER.
*/

> > + if (!boundary) {
> > + err = -ENOMEM;
> > + goto err_out;
> > + }
> > +
> > + /* allocate scatterlist to store pages being reported on */
> > + phdev->sg = kcalloc(phdev->capacity, sizeof(*phdev->sg), GFP_KERNEL);
> > + if (!phdev->sg) {
> > + err = -ENOMEM;
> > +
> > + kfree(boundary);
> > + boundary = NULL;
> > +
> > + goto err_out;
> > + }
> > +
> > +
> > + /* initialize refcnt and work structures */
> > + atomic_set(&phdev->refcnt, 0);
> > + INIT_DELAYED_WORK(&phdev->work, &page_reporting_process);
> > +
> > + /* assign device, and begin initial flush of populated zones */
> > + rcu_assign_pointer(ph_dev_info, phdev);
> > + for_each_populated_zone(zone) {
> > + spin_lock_irq(&zone->lock);
> > + __page_reporting_request(zone);
> > + spin_unlock_irq(&zone->lock);
> > + }
> > +
> > + /* enable page reporting notification */
> > + static_branch_enable(&page_reporting_notify_enabled);
> > +err_out:
> > + mutex_unlock(&page_reporting_mutex);
> > +
> > + return err;
> > +}
> > +EXPORT_SYMBOL_GPL(page_reporting_startup);
> >
> >

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