Re: [RFC][Patch v10 1/2] mm: page_hinting: core infrastructure

[David Hildenbrand]

On 03.06.19 19:03, Nitesh Narayan Lal wrote:
> This patch introduces the core infrastructure for free page hinting in
> virtual environments. It enables the kernel to track the free pages which
> can be reported to its hypervisor so that the hypervisor could
> free and reuse that memory as per its requirement.
> 
> While the pages are getting processed in the hypervisor (e.g.,
> via MADV_FREE), the guest must not use them, otherwise, data loss
> would be possible. To avoid such a situation, these pages are
> temporarily removed from the buddy. The amount of pages removed
> temporarily from the buddy is governed by the backend(virtio-balloon
> in our case).
> 
> To efficiently identify free pages that can to be hinted to the
> hypervisor, bitmaps in a coarse granularity are used. Only fairly big
> chunks are reported to the hypervisor - especially, to not break up THP
> in the hypervisor - "MAX_ORDER - 2" on x86, and to save space. The bits
> in the bitmap are an indication whether a page *might* be free, not a
> guarantee. A new hook after buddy merging sets the bits.
> 
> Bitmaps are stored per zone, protected by the zone lock. A workqueue
> asynchronously processes the bitmaps, trying to isolate and report pages
> that are still free. The backend (virtio-balloon) is responsible for
> reporting these batched pages to the host synchronously. Once reporting/
> freeing is complete, isolated pages are returned back to the buddy.
> 
> There are still various things to look into (e.g., memory hotplug, more
> efficient locking, possible races when disabling).
> 
> Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx>
> ---
>  drivers/virtio/Kconfig       |   1 +
>  include/linux/page_hinting.h |  46 +++++++
>  mm/Kconfig                   |   6 +
>  mm/Makefile                  |   2 +
>  mm/page_alloc.c              |  17 +--
>  mm/page_hinting.c            | 236 +++++++++++++++++++++++++++++++++++
>  6 files changed, 301 insertions(+), 7 deletions(-)
>  create mode 100644 include/linux/page_hinting.h
>  create mode 100644 mm/page_hinting.c
> 
> diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> index 35897649c24f..5a96b7a2ed1e 100644
> --- a/drivers/virtio/Kconfig
> +++ b/drivers/virtio/Kconfig
> @@ -46,6 +46,7 @@ config VIRTIO_BALLOON
>  	tristate "Virtio balloon driver"
>  	depends on VIRTIO
>  	select MEMORY_BALLOON
> +	select PAGE_HINTING
>  	---help---
>  	 This driver supports increasing and decreasing the amount
>  	 of memory within a KVM guest.
> diff --git a/include/linux/page_hinting.h b/include/linux/page_hinting.h
> new file mode 100644
> index 000000000000..e65188fe1e6b
> --- /dev/null
> +++ b/include/linux/page_hinting.h
> @@ -0,0 +1,46 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_PAGE_HINTING_H
> +#define _LINUX_PAGE_HINTING_H
> +
> +/*
> + * Minimum page order required for a page to be hinted to the host.
> + */
> +#define PAGE_HINTING_MIN_ORDER		(MAX_ORDER - 2)
> +
> +/*
> + * struct page_hinting_cb: holds the callbacks to store, report and cleanup
> + * isolated pages.
> + * @prepare:		Callback responsible for allocating an array to hold
> + *			the isolated pages.
> + * @hint_pages:		Callback which reports the isolated pages synchornously
> + *			to the host.
> + * @cleanup:		Callback to free the the array used for reporting the
> + *			isolated pages.
> + * @max_pages:		Maxmimum pages that are going to be hinted to the host
> + *			at a time of granularity >= PAGE_HINTING_MIN_ORDER.
> + */
> +struct page_hinting_cb {
> +	int (*prepare)(void);
> +	void (*hint_pages)(struct list_head *list);
> +	void (*cleanup)(void);
> +	int max_pages;

If we allocate the array in virtio-balloon differently (e.g. similar to
bulk inflation/deflation of pfn's right now), we can most probably get
rid of prepare() and cleanup(), simplifying the code further.

> +};
> +
> +#ifdef CONFIG_PAGE_HINTING
> +void page_hinting_enqueue(struct page *page, int order);
> +void page_hinting_enable(const struct page_hinting_cb *cb);
> +void page_hinting_disable(void);
> +#else
> +static inline void page_hinting_enqueue(struct page *page, int order)
> +{
> +}
> +
> +static inline void page_hinting_enable(struct page_hinting_cb *cb)
> +{
> +}
> +
> +static inline void page_hinting_disable(void)
> +{
> +}
> +#endif
> +#endif /* _LINUX_PAGE_HINTING_H */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index ee8d1f311858..177d858de758 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -764,4 +764,10 @@ config GUP_BENCHMARK
>  config ARCH_HAS_PTE_SPECIAL
>  	bool
>  
> +# PAGE_HINTING will allow the guest to report the free pages to the
> +# host in regular interval of time.
> +config PAGE_HINTING
> +       bool
> +       def_bool n
> +       depends on X86_64
>  endmenu
> diff --git a/mm/Makefile b/mm/Makefile
> index ac5e5ba78874..bec456dfee34 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -41,6 +41,7 @@ obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
>  			   interval_tree.o list_lru.o workingset.o \
>  			   debug.o $(mmu-y)
>  
> +
>  # Give 'page_alloc' its own module-parameter namespace
>  page-alloc-y := page_alloc.o
>  page-alloc-$(CONFIG_SHUFFLE_PAGE_ALLOCATOR) += shuffle.o
> @@ -94,6 +95,7 @@ obj-$(CONFIG_Z3FOLD)	+= z3fold.o
>  obj-$(CONFIG_GENERIC_EARLY_IOREMAP) += early_ioremap.o
>  obj-$(CONFIG_CMA)	+= cma.o
>  obj-$(CONFIG_MEMORY_BALLOON) += balloon_compaction.o
> +obj-$(CONFIG_PAGE_HINTING) += page_hinting.o
>  obj-$(CONFIG_PAGE_EXTENSION) += page_ext.o
>  obj-$(CONFIG_CMA_DEBUGFS) += cma_debug.o
>  obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3b13d3914176..d12f69e0e402 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_hinting.h>
>  
>  #include <asm/sections.h>
>  #include <asm/tlbflush.h>
> @@ -873,10 +874,10 @@ compaction_capture(struct capture_control *capc, struct page *page,
>   * -- nyc
>   */
>  
> -static inline void __free_one_page(struct page *page,
> +inline void __free_one_page(struct page *page,
>  		unsigned long pfn,
>  		struct zone *zone, unsigned int order,
> -		int migratetype)
> +		int migratetype, bool hint)
>  {
>  	unsigned long combined_pfn;
>  	unsigned long uninitialized_var(buddy_pfn);
> @@ -951,6 +952,8 @@ static inline void __free_one_page(struct page *page,
>  done_merging:
>  	set_page_order(page, order);
>  
> +	if (hint)
> +		page_hinting_enqueue(page, order);
>  	/*
>  	 * If this is not the largest possible page, check if the buddy
>  	 * of the next-highest order is free. If it is, it's possible
> @@ -1262,7 +1265,7 @@ 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, true);
>  		trace_mm_page_pcpu_drain(page, 0, mt);
>  	}
>  	spin_unlock(&zone->lock);
> @@ -1271,14 +1274,14 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  static void free_one_page(struct zone *zone,
>  				struct page *page, unsigned long pfn,
>  				unsigned int order,
> -				int migratetype)
> +				int migratetype, bool hint)
>  {
>  	spin_lock(&zone->lock);
>  	if (unlikely(has_isolate_pageblock(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, hint);
>  	spin_unlock(&zone->lock);
>  }
>  
> @@ -1368,7 +1371,7 @@ static void __free_pages_ok(struct page *page, unsigned int order)
>  	migratetype = get_pfnblock_migratetype(page, pfn);
>  	local_irq_save(flags);
>  	__count_vm_events(PGFREE, 1 << order);
> -	free_one_page(page_zone(page), page, pfn, order, migratetype);
> +	free_one_page(page_zone(page), page, pfn, order, migratetype, true);
>  	local_irq_restore(flags);
>  }
>  
> @@ -2968,7 +2971,7 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn)
>  	 */
>  	if (migratetype >= MIGRATE_PCPTYPES) {
>  		if (unlikely(is_migrate_isolate(migratetype))) {
> -			free_one_page(zone, page, pfn, 0, migratetype);
> +			free_one_page(zone, page, pfn, 0, migratetype, true);
>  			return;
>  		}
>  		migratetype = MIGRATE_MOVABLE;
> diff --git a/mm/page_hinting.c b/mm/page_hinting.c
> new file mode 100644
> index 000000000000..7341c6462de2
> --- /dev/null
> +++ b/mm/page_hinting.c
> @@ -0,0 +1,236 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Page hinting support to enable a VM to report the freed pages back
> + * to the host.
> + *
> + * Copyright Red Hat, Inc. 2019
> + *
> + * Author(s): Nitesh Narayan Lal <nitesh@xxxxxxxxxx>
> + */
> +
> +#include <linux/mm.h>
> +#include <linux/slab.h>
> +#include <linux/page_hinting.h>
> +#include <linux/kvm_host.h>
> +
> +/*
> + * struct hinting_bitmap: holds the bitmap pointer which tracks the freed PFNs
> + * and other required parameters which could help in retrieving the original
> + * PFN value using the bitmap.
> + * @bitmap:		Pointer to the bitmap of free PFN.
> + * @base_pfn:		Starting PFN value for the zone whose bitmap is stored.
> + * @free_pages:		Tracks the number of free pages of granularity
> + *			PAGE_HINTING_MIN_ORDER.
> + * @nbits:		Indicates the total size of the bitmap in bits allocated
> + *			at the time of initialization.
> + */
> +struct hinting_bitmap {
> +	unsigned long *bitmap;
> +	unsigned long base_pfn;
> +	atomic_t free_pages;
> +	unsigned long nbits;
> +} bm_zone[MAX_NR_ZONES];
> +
> +static void init_hinting_wq(struct work_struct *work);
> +extern int __isolate_free_page(struct page *page, unsigned int order);
> +extern void __free_one_page(struct page *page, unsigned long pfn,
> +			    struct zone *zone, unsigned int order,
> +			    int migratetype, bool hint);
> +const struct page_hinting_cb *hcb;
> +struct work_struct hinting_work;
> +
> +static unsigned long find_bitmap_size(struct zone *zone)
> +{
> +	unsigned long nbits = ALIGN(zone->spanned_pages,
> +			    PAGE_HINTING_MIN_ORDER);
> +
> +	nbits = nbits >> PAGE_HINTING_MIN_ORDER;
> +	return nbits;

I think we can simplify this to

return (zone->spanned_pages >> PAGE_HINTING_MIN_ORDER) + 1;

> +}
> +
> +void page_hinting_enable(const struct page_hinting_cb *callback)
> +{
> +	struct zone *zone;
> +	int idx = 0;
> +	unsigned long bitmap_size = 0;

You should probably protect enabling via a mutex and return -EINVAL or
similar if we already have a callback set (if we ever have different
drivers). But this has very little priority :)

> +
> +	for_each_populated_zone(zone) {
> +		spin_lock(&zone->lock);
> +		bitmap_size = find_bitmap_size(zone);
> +		bm_zone[idx].bitmap = bitmap_zalloc(bitmap_size, GFP_KERNEL);
> +		if (!bm_zone[idx].bitmap)
> +			return;
> +		bm_zone[idx].nbits = bitmap_size;
> +		bm_zone[idx].base_pfn = zone->zone_start_pfn;
> +		spin_unlock(&zone->lock);
> +		idx++;
> +	}
> +	hcb = callback;
> +	INIT_WORK(&hinting_work, init_hinting_wq);

There are also possible races when enabling, you will have to take care
of at one point.

> +}
> +EXPORT_SYMBOL_GPL(page_hinting_enable);
> +
> +void page_hinting_disable(void)
> +{
> +	struct zone *zone;
> +	int idx = 0;
> +
> +	cancel_work_sync(&hinting_work);
> +	hcb = NULL;
> +	for_each_populated_zone(zone) {
> +		spin_lock(&zone->lock);
> +		bitmap_free(bm_zone[idx].bitmap);
> +		bm_zone[idx].base_pfn = 0;
> +		bm_zone[idx].nbits = 0;
> +		atomic_set(&bm_zone[idx].free_pages, 0);
> +		spin_unlock(&zone->lock);
> +		idx++;
> +	}
> +}
> +EXPORT_SYMBOL_GPL(page_hinting_disable);
> +
> +static unsigned long pfn_to_bit(struct page *page, int zonenum)
> +{
> +	unsigned long bitnr;
> +
> +	bitnr = (page_to_pfn(page) - bm_zone[zonenum].base_pfn)
> +			 >> PAGE_HINTING_MIN_ORDER;
> +	return bitnr;
> +}
> +
> +static void release_buddy_pages(struct list_head *pages)

maybe "release_isolated_pages", not sure.

> +{
> +	int mt = 0, zonenum, order;
> +	struct page *page, *next;
> +	struct zone *zone;
> +	unsigned long bitnr;
> +
> +	list_for_each_entry_safe(page, next, pages, lru) {
> +		zonenum = page_zonenum(page);
> +		zone = page_zone(page);
> +		bitnr = pfn_to_bit(page, zonenum);
> +		spin_lock(&zone->lock);
> +		list_del(&page->lru);
> +		order = page_private(page);
> +		set_page_private(page, 0);
> +		mt = get_pageblock_migratetype(page);
> +		__free_one_page(page, page_to_pfn(page), zone,
> +				order, mt, false);
> +		spin_unlock(&zone->lock);
> +	}
> +}
> +
> +static void bm_set_pfn(struct page *page)
> +{
> +	unsigned long bitnr = 0;
> +	int zonenum = page_zonenum(page);
> +	struct zone *zone = page_zone(page);
> +
> +	lockdep_assert_held(&zone->lock);
> +	bitnr = pfn_to_bit(page, zonenum);
> +	if (bm_zone[zonenum].bitmap &&
> +	    bitnr < bm_zone[zonenum].nbits &&
> +	    !test_and_set_bit(bitnr, bm_zone[zonenum].bitmap))
> +		atomic_inc(&bm_zone[zonenum].free_pages);
> +}
> +
> +static void scan_hinting_bitmap(int zonenum, int free_pages)
> +{
> +	unsigned long set_bit, start = 0;
> +	struct page *page;
> +	struct zone *zone;
> +	int scanned_pages = 0, ret = 0, order, isolated_cnt = 0;
> +	LIST_HEAD(isolated_pages);
> +
> +	ret = hcb->prepare();
> +	if (ret < 0)
> +		return;
> +	for (;;) {
> +		ret = 0;
> +		set_bit = find_next_bit(bm_zone[zonenum].bitmap,
> +					bm_zone[zonenum].nbits, start);
> +		if (set_bit >= bm_zone[zonenum].nbits)
> +			break;
> +		page = pfn_to_online_page((set_bit << PAGE_HINTING_MIN_ORDER) +
> +				bm_zone[zonenum].base_pfn);
> +		if (!page)
> +			continue;

You are not clearing the bit / decrementing the counter.

> +		zone = page_zone(page);
> +		spin_lock(&zone->lock);
> +
> +		if (PageBuddy(page) && page_private(page) >=
> +		    PAGE_HINTING_MIN_ORDER) {
> +			order = page_private(page);
> +			ret = __isolate_free_page(page, order);
> +		}
> +		clear_bit(set_bit, bm_zone[zonenum].bitmap);
> +		spin_unlock(&zone->lock);
> +		if (ret) {
> +			/*
> +			 * restoring page order to use it while releasing
> +			 * the pages back to the buddy.
> +			 */
> +			set_page_private(page, order);
> +			list_add_tail(&page->lru, &isolated_pages);
> +			isolated_cnt++;
> +			if (isolated_cnt == hcb->max_pages) {
> +				hcb->hint_pages(&isolated_pages);
> +				release_buddy_pages(&isolated_pages);
> +				isolated_cnt = 0;
> +			}
> +		}
> +		start = set_bit + 1;
> +		scanned_pages++;
> +	}
> +	if (isolated_cnt) {
> +		hcb->hint_pages(&isolated_pages);
> +		release_buddy_pages(&isolated_pages);
> +	}
> +	hcb->cleanup();
> +	if (scanned_pages > free_pages)
> +		atomic_sub((scanned_pages - free_pages),
> +			   &bm_zone[zonenum].free_pages);

This looks overly complicated. Can't we somehow simply decrement when
clearing a bit?

> +}
> +
> +static bool check_hinting_threshold(void)
> +{
> +	int zonenum = 0;
> +
> +	for (; zonenum < MAX_NR_ZONES; zonenum++) {
> +		if (atomic_read(&bm_zone[zonenum].free_pages) >=
> +				hcb->max_pages)
> +			return true;
> +	}
> +	return false;
> +}
> +
> +static void init_hinting_wq(struct work_struct *work)
> +{
> +	int zonenum = 0, free_pages = 0;
> +
> +	for (; zonenum < MAX_NR_ZONES; zonenum++) {
> +		free_pages = atomic_read(&bm_zone[zonenum].free_pages);
> +		if (free_pages >= hcb->max_pages) {
> +			/* Find a better way to synchronize per zone
> +			 * free_pages.
> +			 */
> +			atomic_sub(free_pages,
> +				   &bm_zone[zonenum].free_pages);

I can't follow yet why we need that information. Wouldn't it be enough
to just track the number of set bits in the bitmap and start hinting
depending on that count? (there are false positives, but do we really care?)

> +			scan_hinting_bitmap(zonenum, free_pages);
> +		}
> +	}
> +}
> +
> +void page_hinting_enqueue(struct page *page, int order)
> +{
> +	if (hcb && order >= PAGE_HINTING_MIN_ORDER)
> +		bm_set_pfn(page);
> +	else
> +		return;
> +
> +	if (check_hinting_threshold()) {
> +		int cpu = smp_processor_id();
> +
> +		queue_work_on(cpu, system_wq, &hinting_work);
> +	}
> +}
> 


-- 

Thanks,

David / dhildenb