Re: [PATCH v14 3/6] mm: Introduce Reported pages

[Alexander Duyck]

On Wed, 2019-11-27 at 15:24 +0000, Mel Gorman wrote:
> On Tue, Nov 19, 2019 at 01:46:33PM -0800, 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.
> > 
> > To prevent the reported pages from leaking outside of the buddy lists I
> > have added a call to clear_reported_page to the del_page_from_free_list
> > function. As a result any reported page that is split, merged, or
> > allocated will have the flag cleared prior to the PageBuddy value being
> > cleared.
> > 
> > The process for reporting pages is fairly simple. Once the nr_free for
> > a given free area has exceeded the number of pages reported for that area
> > plus a certain high watermark value we will flag the zone as needing
> > reporting and schedule the worker thread to start reporting. That worker
> > thread will begin working from the lowest supported page reporting order
> > up to MAX_ORDER - 1 pulling unreported pages from the free list and
> > storing them in the scatterlist.
> > 
> > When processing each individual free list it is necessary for the worker
> > thread to release the zone lock when it needs to stop and report the full
> > scatterlist of pages. To reduce the work of the next iteration the worker
> > thread will rotate the free list so that the first unreported page in the
> > free list becomes the first entry in the list. Doing this we should only
> > have to walk an entire free list no more than twice assuming large
> > numbers of pages are not being added to the tail of the list.
> > 
> > It will then call a reporting function providing information on how many
> > entries are in the scatterlist. Once the function completes it will return
> > the pages to the tail of the free area from which they were allocated and
> > start over pulling more pages from the free areas until there are no
> > longer enough pages to report on to keep the worker busy.
> > 
> > The worker thread will work in a round-robin fashion making its way
> > though each zone requesting reporting, and through each reportable free
> > list within that zone. Once all free areas within the zone is below the
> > high watermark level for free pages to report the flag indicating that the
> > zone has requested reporting will be cleared, and if no zones are
> > requesting reporting the worker thread will exit.
> > 
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> 
> Ok, I'm ok with how this hooks into the allocator as the overhead is
> minimal. However, the patch itself still includes a number of
> optimisations instead of being a bare-boned implementation of the
> feature with optimisations layered on top. I think some of the
> optimisations are also broken but it's harder to be sure because both
> the feature and the optimisations are lumped together.

Well I can work on splitting them out if need be. 

> > ---
> >  include/linux/mmzone.h         |   12 +
> >  include/linux/page-flags.h     |   11 +
> >  include/linux/page_reporting.h |   31 ++++
> >  mm/Kconfig                     |   11 +
> >  mm/Makefile                    |    1 
> >  mm/memory_hotplug.c            |    2 
> >  mm/page_alloc.c                |   53 ++++++
> >  mm/page_reporting.c            |  337 ++++++++++++++++++++++++++++++++++++++++
> >  mm/page_reporting.h            |  125 +++++++++++++++
> >  9 files changed, 579 insertions(+), 4 deletions(-)
> >  create mode 100644 include/linux/page_reporting.h
> >  create mode 100644 mm/page_reporting.c
> >  create mode 100644 mm/page_reporting.h
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 8d93106490f3..9647499983b1 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -478,6 +478,14 @@ 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 */
> 
> Comment could be better. We know it's a pointer.

Okay, I can rework the comment. I'll call out specifically that I am
leaving it as a pointer because PAGE_REPORTING_MIN_ORDER might be variable
on some systems so I cannot allocate it statically.

> /*
>  * When a device is registered to track free pages, this array tracks
>  * the number of pages already reported at each order from
>  * PAGE_REPORTING_MIN_ORDER up to MAX_ORDER.
>  */
> 
> > @@ -550,6 +558,10 @@ 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.
> > +					 */
> >  };
> 
> Set if a *driver* enabled page reporting and has requested that unused
> pages are reported?
> 
> "flushing the data" would normally indicate that it's something to do
> with writeback.

Okay, I can work on the wording.

> >  
> >  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 1bf83c8fcaa7..a3a3b15b56a8 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
> 
> Only valid for buddy pages.....

Okay I can tweak the wording.

> > diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
> > new file mode 100644
> > index 000000000000..925a16b1d14b
> > --- /dev/null
> > +++ b/include/linux/page_reporting.h
> > @@ -0,0 +1,31 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _LINUX_PAGE_REPORTING_H
> > +#define _LINUX_PAGE_REPORTING_H
> > +
> > +#include <linux/mmzone.h>
> > +
> > +struct page_reporting_dev_info {
> > +	/* function that alters pages to make them "reported" */
> > +	void (*report)(struct page_reporting_dev_info *prdev,
> > +		       unsigned int nents);
> > +
> > +	/* pointer to scatterlist containing pages to be processed */
> > +	struct scatterlist *sg;
> > +
> 
> The choice of scatterlist is curious because it adds unnecessary complexity
> without needing the benefits scatterlist and sg_table provides. All you
> really need is an array of pages that is NULL terminated because you can
> get the order of the struct page when it's a buddy page.

The scatterlist is mostly about dealing with DMA mapping of the pages.
Most devices will need a way to push the pages out to the device doing the
reporting so I thought a scatterlist was the best way to gather the pages
together so that they could be mapped with one call.

> > +	/*
> > +	 * Upper limit on the number of pages that the report function
> > +	 * expects to be placed into the scatterlist to be processed.
> > +	 */
> > +	unsigned int capacity;
> > +
> 
> Instead of requiring the driver to specify capacity, why did you not
> simply make it PAGE_REPORTING_HWM and statically declare the array?

The problem is the device might not be able to support PAGE_REPORTING_HWM.
So I needed some way for it to communicate that. I could allocate the
array statically if that is what you prefer.

> > +	/* The number of zones requesting reporting */
> > +	atomic_t refcnt;
> > +
> 
> The refcnt is overkill. Simply loop over all zones until there is a
> clear pass. Despite being atomic, it actually requires the zone lock to
> avoid races in places like this
> 
> 		if (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags))
> 			refcnt = page_reporting_process_zone(prdev, zone);
> 
> The optimisation is small and it's relatively subtle.

I disagree. Without that we can easily get into states where a zone
requests reporting and we never get around to servicing it. I am using the
refcount to track that so that we don't shutdown the worker thread when
there is at least one zone requesting service.

> >  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 +1053,9 @@ 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);
> > @@ -1057,6 +1064,14 @@ static inline void __free_one_page(struct page *page,
> >  		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);
> >  }
> >  
> >  /*
> 
> If a reported page is merged with a larger buddy then the counts for the
> lower order needs to be updated or the counter gets out of sync. Then the
> reported status of the page needs to be cleared as the larger block may
> only be partially reported and the count is wrong. I know why you want
> the reported pages but it really should be in its own patch. This patch
> really should be just the bare essentials to support reporting of free
> pages even if it's sub-optimal. I know you also use reported_pages as
> part of a stop condition but a basic stop condition is simply to do a
> single pass.

Okay so there are actually a few things here to unpack.

First the comment is a bit out of date. The reason I don't need to bother
with reporting is because I process the lower orders before the higher
orders. As such we are still processing this zone if we are returning a
reported page and it is merged. We will reprocess the larger page in the
next pass.

Second, at the point where we call this function with the reported flag
set we have not yet set the page as reported. There is logic that will
check for that later and set the bits and increment the count if the page
becomes a buddy page and the order is still the same as the order we
originally freed it under.

Lastly, if you are concerned about us merging a reported page already on
the list the count is updated and the flag is cleared from the page when
it is deleted from the free list to be merged with the new page.

> > @@ -3228,6 +3243,36 @@ int __isolate_free_page(struct page *page, unsigned int order)
> >  	return 1UL << order;
> >  }
> >  
> > +#ifdef CONFIG_PAGE_REPORTING
> > +/**
> > + * __free_isolated_page - Return a now-isolated page back where we got it
> > + * @page: Page that was isolated
> > + * @order: Order of the isolated page
> > + *
> > + * This function is meant to return a page pulled from the free lists via
> > + * __isolate_free_page back to the free lists they were pulled from.
> > + */
> 
> Isolated within mm has special meaning already. __putback_reported_page?

I wanted to get away from calling it a reported page because it isn't at
the point where we are calling this function. We don't set the reported
bit on the page until we have placed it back into the free list and
verified it wasn't merged.

I'm open to suggestions on how to name a function that is meant to be the
inverse of __isolate_free_page. All I did is swap the isolate and free
verb/adjective.

> > +void __free_isolated_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);
> > +
> > +	/*
> > +	 * Return isolated page to tail of freelist and don't bother with
> > +	 * triggering the page reporting notifiers since this page was
> > +	 * previously on the freelist and has likely already been reported.
> > +	 */
> > +	__free_one_page(page, pfn, zone, order, mt, true);
> > +}
> > +#endif /* CONFIG_PAGE_REPORTING */
> > +
> >  /*
> >   * Update NUMA hit/miss statistics
> >   *
> > diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> > new file mode 100644
> > index 000000000000..4844f0aa2904
> > --- /dev/null
> > +++ b/mm/page_reporting.c
> > @@ -0,0 +1,337 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#include <linux/mm.h>
> > +#include <linux/mmzone.h>
> > +#include <linux/page_reporting.h>
> > +#include <linux/gfp.h>
> > +#include <linux/export.h>
> > +#include <linux/delay.h>
> > +#include <linux/scatterlist.h>
> > +
> > +#include "page_reporting.h"
> > +#include "internal.h"
> > +
> > +static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly;
> > +
> > +#define for_each_reporting_migratetype_order(_order, _type) \
> > +	for (_order = PAGE_REPORTING_MIN_ORDER; _order < MAX_ORDER; _order++) \
> > +		for (_type = 0; _type < MIGRATE_TYPES; _type++) \
> > +			if (!is_migrate_isolate(_type))
> > +
> 
> This only has one user so simply put it in place. As this stands, _order
> and _type can be evaluated multiple times which potentially leads to
> chaos

Good point. Back when I started I had several spots using it. I guess I
have consolidated all of them now.

> > +static void page_reporting_populate_metadata(struct zone *zone)
> > +{
> > +	size_t size;
> > +	int node;
> > +
> > +	/*
> > +	 * 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)
> > +		return;
> > +
> > +	node = zone_to_nid(zone);
> > +	size = (MAX_ORDER - PAGE_REPORTING_MIN_ORDER) * sizeof(unsigned long);
> > +	zone->reported_pages = kzalloc_node(size, GFP_KERNEL, node);
> > +}
> > +
> 
> reported_pages is not necessarily updated by a CPU local to the zone.
> There is not much point worrying about its locality.

Okay, I can drop that.

> > +static void
> > +page_reporting_drain(struct page_reporting_dev_info *prdev, struct zone *zone)
> > +{
> > +	struct scatterlist *sg = prdev->sg;
> > +
> > +	/*
> > +	 * Drain the now reported pages back into their respective
> > +	 * free lists/areas. We assume at least one page is populated.
> > +	 */
> > +	do {
> > +		unsigned int order = get_order(sg->length);
> > +		struct page *page = sg_page(sg);
> > +
> > +		__free_isolated_page(page, order);
> > +
> > +		/*
> > +		 * If page was not comingled with another page we can
> > +		 * consider the result to be "reported" since the page
> > +		 * hasn't been modified, otherwise we will need to
> > +		 * report on the new larger page when we make our way
> > +		 * up to that higher order.
> > +		 */
> > +		if (PageBuddy(page) && page_order(page) == order)
> > +			mark_page_reported(page, zone, order);
> > +	} 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 cannot obtain a
> > + * full scatterlist of pages, in that case we will switch to idle.
> > + */
> 
> Document the return value because it's unclear why the number of populated
> elements in sg is not tracked in page_reporting_dev_info. It seems
> unnecessarily fragile.

Will do in terms of documenting the return value.

As far as tracking the number of elements it mostly just has to do with
how this code has evolved. Originally this was just going straight to the
report function. However that leads to partial requests and I wanted to
avoid those since the cost for each call into the hypervisor can be high.

> > +static unsigned int
> > +page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone,
> > +		     unsigned int order, unsigned int mt, unsigned int nents)
> > +{
> > +	struct list_head *list = &zone->free_area[order].free_list[mt];
> > +	unsigned int page_len = PAGE_SIZE << order;
> > +	struct scatterlist *sg = prdev->sg;
> > +	struct page *page, *next;
> > +
> > +	/*
> > +	 * Perform early check, if free area is empty there is
> > +	 * nothing to process so we can skip this free_list.
> > +	 */
> > +	if (list_empty(list))
> > +		return nents;
> > +
> > +	spin_lock_irq(&zone->lock);
> > +
> > +	/* loop through free list adding unreported pages to sg list */
> > +	list_for_each_entry_safe(page, next, list, lru) {
> > +		/* We are going to skip over the reported pages. */
> > +		if (PageReported(page))
> > +			continue;
> > +
> > +		/* Attempt to add page to sg list */
> > +		if (nents < prdev->capacity) {
> > +			if (!__isolate_free_page(page, order))
> > +				break;
> > +
> > +			sg_set_page(&sg[nents++], page, page_len, 0);
> > +			continue;
> > +		}
> > +
> > +		/*
> > +		 * Make the first non-reported entry in the free list
> > +		 * the new head of the free list before we exit.
> > +		 */
> > +		if (!list_is_first(&page->lru, list))
> > +			list_rotate_to_front(&page->lru, list);
> > +
> > +		/* release lock before waiting on report processing*/
> > +		spin_unlock_irq(&zone->lock);
> > +
> > +		/* begin processing pages in local list */
> > +		prdev->report(prdev, nents);
> > +
> > +		/* reset number of entries */
> > +		nents = 0;
> > +
> > +		/* reacquire zone lock and resume processing free lists */
> > +		spin_lock_irq(&zone->lock);
> > +
> > +		/* flush reported pages from the sg list */
> > +		page_reporting_drain(prdev, zone);
> > +
> > +		/*
> > +		 * Reset next to first entry, the old next isn't valid
> > +		 * since we dropped the lock to report the pages
> > +		 */
> > +		next = list_first_entry(list, struct page, lru);
> > +	}
> > +
> > +	spin_unlock_irq(&zone->lock);
> > +
> > +	return nents;
> > +}
> > +
> > +static int
> > +page_reporting_process_zone(struct page_reporting_dev_info *prdev,
> > +			    struct zone *zone)
> > +{
> > +	unsigned int order, mt, nents = 0;
> > +	unsigned long watermark;
> > +	int refcnt = -1;
> > +
> > +	page_reporting_populate_metadata(zone);
> > +
> > +	/* Generate minimum watermark to be able to guarantee progress */
> > +	watermark = low_wmark_pages(zone) +
> > +		    (prdev->capacity << PAGE_REPORTING_MIN_ORDER);
> > +
> > +	/*
> > +	 * Cancel request if insufficient free memory or if we failed
> > +	 * to allocate page reporting statistics for the zone.
> > +	 */
> > +	if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA) ||
> > +	    !zone->reported_pages) {
> > +		spin_lock_irq(&zone->lock);
> > +		goto zone_not_ready;
> > +	}
> 
> The zone lock is acquired just for the zone bit and the refcnt? That
> seems drastic overkill because the bit handling is already racy and the
> refcnt is overkill.

I'm not sure I follow.

> > +
> > +	sg_init_table(prdev->sg, prdev->capacity);
> > +
> > +	/* Process each free list starting from lowest order/mt */
> > +	for_each_reporting_migratetype_order(order, mt)
> > +		nents = page_reporting_cycle(prdev, zone, order, mt, nents);
> > +
> > +	/* mark end of sg list and report the remainder */
> > +	if (nents) {
> > +		sg_mark_end(&prdev->sg[nents - 1]);
> > +		prdev->report(prdev, nents);
> > +	}
> > +
> > +	spin_lock_irq(&zone->lock);
> > +
> > +	/* flush any remaining pages out from the last report */
> > +	if (nents)
> > +		page_reporting_drain(prdev, zone);
> > +
> > +	/* check to see if values are low enough for us to stop for now */
> > +	for (order = PAGE_REPORTING_MIN_ORDER; order < MAX_ORDER; order++) {
> > +		if (pages_unreported(zone, order) < PAGE_REPORTING_HWM)
> > +			continue;
> > +#ifdef CONFIG_MEMORY_ISOLATION
> > +		/*
> > +		 * Do not allow a free_area with isolated pages to request
> > +		 * that we continue with page reporting. Keep the reporting
> > +		 * light until the isolated pages have been cleared.
> > +		 */
> > +		if (!free_area_empty(&zone->free_area[order], MIGRATE_ISOLATE))
> > +			continue;
> > +#endif
> > +		goto zone_not_complete;
> > +	}
> > +
> > +zone_not_ready:
> > +	/*
> > +	 * 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);
> > +	refcnt = atomic_dec_return(&prdev->refcnt);
> 
> If a request comes in while processing is already happening then the
> subsequent request is lost.

How can a request come in when we are holding the zone lock? Basically the
notify call can only occur while holding the zone lock. That is one of the
reasons for holding the lock is to protect the flag and keep it consistent
with the page counts as we clear it.

The only case where we are not checking the counts is for the case above
where we clear it due to us being too close to the watermark. In that case
we want to just ignore the request anyway.

> > +zone_not_complete:
> > +	spin_unlock_irq(&zone->lock);
> > +
> > +	return refcnt;
> > +}
> > +
> > +static void page_reporting_process(struct work_struct *work)
> > +{
> > +	struct delayed_work *d_work = to_delayed_work(work);
> > +	struct page_reporting_dev_info *prdev =
> > +		container_of(d_work, struct page_reporting_dev_info, work);
> > +	struct zone *zone = first_online_pgdat()->node_zones;
> > +	int refcnt = -1;
> > +
> > +	do {
> > +		if (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags))
> > +			refcnt = page_reporting_process_zone(prdev, zone);
> > +
> > +		/* 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 (refcnt);
> > +}
> > +
> 
> This is being done from a workqueue context and potentially this runs
> forever if the refcnt remains elevated. It's dangerous and should be in
> it's own patch. In the basic implementation, just do a single pass of
> all zones.

So I already have a modification that I have for the next version of the
patch set where instead of doing a do/while I simply reschedule the
delayed work to run again in 200ms if we still have work outstanding.

What I can do is split that out so that it is in the second patch with
optimizations.

> > +/* request page reporting on this zone */
> > +void __page_reporting_request(struct zone *zone)
> > +{
> > +	struct page_reporting_dev_info *prdev;
> > +
> > +	rcu_read_lock();
> > +
> > +	/*
> > +	 * We use RCU to protect the pr_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.
> > +	 */
> > +	prdev = rcu_dereference(pr_dev_info);
> > +	if (unlikely(!prdev))
> > +		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);
> > +
> 
> The comment implies that the bit should have been tested first.

It was in the page_reporting_notify_free() call. I can update the comment.

> > +	/*
> > +	 * Delay the start of work to allow a sizable queue to
> > +	 * build. For now we are limiting this to running no more
> > +	 * than 5 times per second.
> > +	 */
> > +	if (!atomic_fetch_inc(&prdev->refcnt))
> > +		schedule_delayed_work(&prdev->work, HZ / 5);
> 
> The refcnt is incremented whether this zone has already been requested or
> not but only decremented when the bit is cleared so the refcnt potentially
> remains elevated forever.

So this is one of the reasons why all accesses to the flag are protected
by the zone lock. Basically page_reporting_notify_free() and this function
are both called with the zone lock held.

The only other caller of this function is page_reporting_register which
calls it for each populated zone. I suppose I could make it for every zone
since it is possible that someone unloaded the driver, unpopulated a zone,
and then reloaded the driver, but anyway the idea is that the zone lock is
keeping the flag and refcount consistent.

> > +out:
> > +	rcu_read_unlock();
> > +}
> > +
> > +static DEFINE_MUTEX(page_reporting_mutex);
> > +DEFINE_STATIC_KEY_FALSE(page_reporting_enabled);
> > +
> > +void page_reporting_unregister(struct page_reporting_dev_info *prdev)
> > +{
> > +	mutex_lock(&page_reporting_mutex);
> > +
> > +	if (rcu_access_pointer(pr_dev_info) == prdev) {
> > +		/* Disable page reporting notification */
> > +		RCU_INIT_POINTER(pr_dev_info, NULL);
> > +		synchronize_rcu();
> > +
> > +		/* Flush any existing work, and lock it out */
> > +		cancel_delayed_work_sync(&prdev->work);
> > +
> > +		/* Free scatterlist */
> > +		kfree(prdev->sg);
> > +		prdev->sg = NULL;
> > +	}
> > +
> > +	mutex_unlock(&page_reporting_mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(page_reporting_unregister);
> > +
> > +int page_reporting_register(struct page_reporting_dev_info *prdev)
> > +{
> > +	struct zone *zone;
> > +	int err = 0;
> > +
> > +	/* No point in enabling this if it cannot handle any pages */
> > +	if (WARN_ON(!prdev->capacity || prdev->capacity > PAGE_REPORTING_HWM))
> > +		return -EINVAL;
> > +
> > +	mutex_lock(&page_reporting_mutex);
> > +
> > +	/* nothing to do if already in use */
> > +	if (rcu_access_pointer(pr_dev_info)) {
> > +		err = -EBUSY;
> > +		goto err_out;
> > +	}
> > +
> > +	/* allocate scatterlist to store pages being reported on */
> > +	prdev->sg = kcalloc(prdev->capacity, sizeof(*prdev->sg), GFP_KERNEL);
> > +	if (!prdev->sg) {
> > +		err = -ENOMEM;
> > +		goto err_out;
> > +	}
> > +
> > +
> > +	/* initialize refcnt and work structures */
> > +	atomic_set(&prdev->refcnt, 0);
> > +	INIT_DELAYED_WORK(&prdev->work, &page_reporting_process);
> > +
> > +	/* assign device, and begin initial flush of populated zones */
> > +	rcu_assign_pointer(pr_dev_info, prdev);
> > +	for_each_populated_zone(zone) {
> > +		spin_lock_irq(&zone->lock);
> > +		__page_reporting_request(zone);
> > +		spin_unlock_irq(&zone->lock);
> > +	}
> > +
> > +	/* enable page reporting notification */
> > +	if (!static_key_enabled(&page_reporting_enabled)) {
> > +		static_branch_enable(&page_reporting_enabled);
> > +		pr_info("Unused page reporting enabled\n");
> > +	}
> > +err_out:
> > +	mutex_unlock(&page_reporting_mutex);
> > +
> > +	return err;
> > +}
> > +EXPORT_SYMBOL_GPL(page_reporting_register);
> > diff --git a/mm/page_reporting.h b/mm/page_reporting.h
> > new file mode 100644
> > index 000000000000..2ad31bbb0036
> > --- /dev/null
> > +++ b/mm/page_reporting.h
> > @@ -0,0 +1,125 @@
> > +/* 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
> 
> This is potentially the same as MAX_ORDER-1. Not sure if it matters or
> not.

It doesn't. Basically what I am going for is the largest order that it is
safe to report on that will not break THP. If THP is not enabled then this
value should be MAX_ORDER - 1 if I am not mistaken.

> > +#define PAGE_REPORTING_HWM		32
> > +
> > +#ifdef CONFIG_PAGE_REPORTING
> > +/* Reported page accessors, defined in page_alloc.c */
> > +void __free_isolated_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;
> > +}
> > +
> > +DECLARE_STATIC_KEY_FALSE(page_reporting_enabled);
> > +void __page_reporting_request(struct zone *zone);
> > +
> > +static inline bool page_reported(struct page *page)
> > +{
> > +	return static_branch_unlikely(&page_reporting_enabled) &&
> > +	       PageReported(page);
> > +}
> > +
> > +static inline unsigned long
> > +pages_unreported(struct zone *zone, int order)
> > +{
> > +	unsigned long nr_free;
> > +	int report_order;
> > +
> > +	/* Limit notifications only to higher order pages */
> > +	report_order = order - PAGE_REPORTING_MIN_ORDER;
> > +	if (report_order < 0)
> > +		return 0;
> > +
> > +	nr_free = zone->free_area[order].nr_free;
> > +
> > +	/* Only subtract reported_pages count if it is present */
> > +	if (!zone->reported_pages)
> > +		return nr_free;
> > +
> > +	return nr_free - zone->reported_pages[report_order];
> > +}
> 
> Initially, this should be a full list traversal because I'm not
> convinced the reported_pages count is kept perfectly in sync. The
> optimisation confuses the review of the basic feature itself.

This is called per freed page in page_reporting_notify_free below. Are you
saying I have to traverse the entire free list per freed page?

If that is what you are wanting I would still probably want to limit it
such that we only walk the first PAGE_REPORTING_HWM worth of pages and if
those are not reported then just report that many.

> > +
> > +/**
> > + * 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)
> > +{
> > +	/* Called from hot path in __free_one_page() */
> > +	if (!static_branch_unlikely(&page_reporting_enabled))
> > +		return;
> > +
> > +	/* Do not bother with tests if we have already requested reporting */
> > +	if (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags))
> > +		return;
> > +
> > +	/* Determine if we have crossed reporting threshold */
> > +	if (pages_unreported(zone, order) < PAGE_REPORTING_HWM)
> > +		return;
> > +
> > +	/* This is slow, but should be called very rarely */
> > +	__page_reporting_request(zone);
> > +}
> > +
> > +/*
> > + * Functions for marking/clearing reported pages from the freelist.
> > + * All of them expect the zone lock to be held to maintain
> > + * consistency of the reported_pages count versus nr_free.
> > + */
> > +static inline void
> > +mark_page_reported(struct page *page, struct zone *zone, unsigned int order)
> > +{
> > +	/* flag page as reported */
> > +	__SetPageReported(page);
> > +
> > +	/* update areated page accounting */
> > +	zone->reported_pages[order - PAGE_REPORTING_MIN_ORDER]++;
> > +}
> > +
> > +static inline void
> > +clear_page_reported(struct page *page, struct zone *zone, unsigned int order)
> > +{
> > +	/* 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 */
> > +#define page_reported(_page)	false
> > +
> > +static inline void page_reporting_reset_zone(struct zone *zone)
> > +{
> > +}
> > +
> > +static inline void page_reporting_notify_free(struct zone *zone, int order)
> > +{
> > +}
> > +
> > +static inline void
> > +clear_page_reported(struct page *page, struct zone *zone, unsigned int order)
> > +{
> > +}
> > +#endif /* CONFIG_PAGE_REPORTING */
> > +#endif /*_MM_PAGE_REPORTING_H */
> >