Re: [RFC][Patch v11 1/2] mm: page_hinting: core infrastructure
From: Alexander Duyck
Date: Wed Jul 10 2019 - 17:57:11 EST
On Wed, Jul 10, 2019 at 12:52 PM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> 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>
> ---
> include/linux/page_hinting.h | 45 +++++++
> mm/Kconfig | 6 +
> mm/Makefile | 1 +
> mm/page_alloc.c | 18 +--
> mm/page_hinting.c | 250 +++++++++++++++++++++++++++++++++++
> 5 files changed, 312 insertions(+), 8 deletions(-)
> create mode 100644 include/linux/page_hinting.h
> create mode 100644 mm/page_hinting.c
>
> diff --git a/include/linux/page_hinting.h b/include/linux/page_hinting.h
> new file mode 100644
> index 000000000000..4900feb796f9
> --- /dev/null
> +++ b/include/linux/page_hinting.h
> @@ -0,0 +1,45 @@
> +/* 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)
> +
Why use (MAX_ORDER - 2)? Is this just because of the issues I pointed
out earlier for is it due to something else? I'm just wondering if
this will have an impact on architectures outside of x86 as I had
chose pageblock_order which happened to be MAX_ORDER - 2 on x86, but I
don't know that the impact of doing that is on other architectures
versus the (MAX_ORDER - 2) approach you took here.
> +/*
> + * struct page_hinting_config: holds the information supplied by the balloon
> + * device to page hinting.
> + * @hint_pages: Callback which reports the isolated pages
> + * synchornously to the host.
> + * @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_config {
> + void (*hint_pages)(struct list_head *list);
> + int max_pages;
> +};
> +
> +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);
> +#ifdef CONFIG_PAGE_HINTING
> +void page_hinting_enqueue(struct page *page, int order);
> +int page_hinting_enable(const struct page_hinting_config *conf);
> +void page_hinting_disable(void);
> +#else
> +static inline void page_hinting_enqueue(struct page *page, int order)
> +{
> +}
> +
> +static inline int page_hinting_enable(const struct page_hinting_config *conf)
> +{
> + return -EOPNOTSUPP;
> +}
> +
> +static inline void page_hinting_disable(void)
> +{
> +}
> +#endif
> +#endif /* _LINUX_PAGE_HINTING_H */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index f0c76ba47695..e97fab429d9b 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -765,4 +765,10 @@ config GUP_BENCHMARK
> config ARCH_HAS_PTE_SPECIAL
> bool
>
> +# PAGE_HINTING will allow the guest to report the free pages to the
> +# host in fixed chunks as soon as the threshold is reached.
> +config PAGE_HINTING
> + bool
> + def_bool n
> + depends on X86_64
> endmenu
If there are no issue with using the term "PAGE_HINTING" I guess I
will update my patch set to use that term instead of aeration.
> diff --git a/mm/Makefile b/mm/Makefile
> index ac5e5ba78874..73be49177656 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -94,6 +94,7 @@ 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 d66bc8abe0af..8a44338bd04e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -69,6 +69,7 @@
> #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>
> @@ -874,10 +875,10 @@ 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);
> @@ -980,7 +981,8 @@ static inline void __free_one_page(struct page *page,
> migratetype);
> else
> add_to_free_area(page, &zone->free_area[order], migratetype);
> -
> + if (hint)
> + page_hinting_enqueue(page, order);
> }
I'm not sure I am a fan of the way the word "hint" is used here. At
first I thought this was supposed to be !hint since I thought hint
meant that it was a hinted page, not that we need to record that this
page has been freed. Maybe "record" or "report" might be a better word
to use here.
> /*
> @@ -1263,7 +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);
> @@ -1272,14 +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);
> }
>
> @@ -1369,7 +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);
> }
>
> @@ -2969,7 +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..0bfa09f8c3ed
> --- /dev/null
> +++ b/mm/page_hinting.c
> @@ -0,0 +1,250 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Page hinting core infrastructure to enable a VM to report free pages to its
> + * hypervisor.
> + *
> + * 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 zone_free_area: For a single zone across NUMA nodes, it holds the
> + * bitmap pointer to track the free pages and other required parameters
> + * used to recover these pages by scanning the bitmap.
> + * @bitmap: Pointer to the bitmap in PAGE_HINTING_MIN_ORDER
> + * granularity.
> + * @base_pfn: Starting PFN value for the zone whose bitmap is stored.
> + * @end_pfn: Indicates the last PFN value for the zone.
> + * @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 zone_free_area {
> + unsigned long *bitmap;
> + unsigned long base_pfn;
> + unsigned long end_pfn;
> + atomic_t free_pages;
> + unsigned long nbits;
> +} free_area[MAX_NR_ZONES];
> +
You still haven't addressed the NUMA issue I pointed out with v10. You
are only able to address the first set of zones with this setup. As
such you can end up missing large sections of memory if it is split
over multiple nodes.
> +static void init_hinting_wq(struct work_struct *work);
> +static DEFINE_MUTEX(page_hinting_init);
> +const struct page_hinting_config *page_hitning_conf;
> +struct work_struct hinting_work;
> +atomic_t page_hinting_active;
> +
> +void free_area_cleanup(int nr_zones)
> +{
I'm not sure why you are passing nr_zones as an argument here. Won't
this always be MAX_NR_ZONES?
> + int zone_idx;
> +
> + for (zone_idx = 0; zone_idx < nr_zones; zone_idx++) {
> + bitmap_free(free_area[zone_idx].bitmap);
> + free_area[zone_idx].base_pfn = 0;
> + free_area[zone_idx].end_pfn = 0;
> + free_area[zone_idx].nbits = 0;
> + atomic_set(&free_area[zone_idx].free_pages, 0);
> + }
> +}
> +
> +int page_hinting_enable(const struct page_hinting_config *conf)
> +{
> + unsigned long bitmap_size = 0;
> + int zone_idx = 0, ret = -EBUSY;
> + struct zone *zone;
> +
> + mutex_lock(&page_hinting_init);
> + if (!page_hitning_conf) {
> + for_each_populated_zone(zone) {
So for_each_populated_zone will go through all of the NUMA nodes. So
if I am not mistaken you will overwrite the free_area values of all
the previous nodes with the last node in the system. So if we have a
setup that has all the memory in the first node, and none in the
second it would effectively disable free page hinting would it not?
> + zone_idx = zone_idx(zone);
> +#ifdef CONFIG_ZONE_DEVICE
> + if (zone_idx == ZONE_DEVICE)
> + continue;
> +#endif
> + spin_lock(&zone->lock);
> + if (free_area[zone_idx].base_pfn) {
> + free_area[zone_idx].base_pfn =
> + min(free_area[zone_idx].base_pfn,
> + zone->zone_start_pfn);
> + free_area[zone_idx].end_pfn =
> + max(free_area[zone_idx].end_pfn,
> + zone->zone_start_pfn +
> + zone->spanned_pages);
> + } else {
> + free_area[zone_idx].base_pfn =
> + zone->zone_start_pfn;
> + free_area[zone_idx].end_pfn =
> + zone->zone_start_pfn +
> + zone->spanned_pages;
> + }
> + spin_unlock(&zone->lock);
> + }
> +
> + for (zone_idx = 0; zone_idx < MAX_NR_ZONES; zone_idx++) {
> + unsigned long pages = free_area[zone_idx].end_pfn -
> + free_area[zone_idx].base_pfn;
> + bitmap_size = (pages >> PAGE_HINTING_MIN_ORDER) + 1;
> + if (!bitmap_size)
> + continue;
> + free_area[zone_idx].bitmap = bitmap_zalloc(bitmap_size,
> + GFP_KERNEL);
> + if (!free_area[zone_idx].bitmap) {
> + free_area_cleanup(zone_idx);
> + mutex_unlock(&page_hinting_init);
> + return -ENOMEM;
> + }
> + free_area[zone_idx].nbits = bitmap_size;
> + }
So this is the bit that still needs to address hotplug right? I would
imagine you need to reallocate this if the spanned_pages range changes
correct?
> + page_hitning_conf = conf;
> + INIT_WORK(&hinting_work, init_hinting_wq);
> + ret = 0;
> + }
> + mutex_unlock(&page_hinting_init);
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(page_hinting_enable);
> +
> +void page_hinting_disable(void)
> +{
> + cancel_work_sync(&hinting_work);
> + page_hitning_conf = NULL;
> + free_area_cleanup(MAX_NR_ZONES);
> +}
> +EXPORT_SYMBOL_GPL(page_hinting_disable);
> +
> +static unsigned long pfn_to_bit(struct page *page, int zone_idx)
> +{
> + unsigned long bitnr;
> +
> + bitnr = (page_to_pfn(page) - free_area[zone_idx].base_pfn)
> + >> PAGE_HINTING_MIN_ORDER;
> + return bitnr;
> +}
> +
> +static void release_buddy_pages(struct list_head *pages)
> +{
> + int mt = 0, zone_idx, order;
> + struct page *page, *next;
> + unsigned long bitnr;
> + struct zone *zone;
> +
> + list_for_each_entry_safe(page, next, pages, lru) {
> + zone_idx = page_zonenum(page);
> + zone = page_zone(page);
> + bitnr = pfn_to_bit(page, zone_idx);
> + 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)
> +{
> + struct zone *zone = page_zone(page);
> + int zone_idx = page_zonenum(page);
> + unsigned long bitnr = 0;
> +
> + lockdep_assert_held(&zone->lock);
> + bitnr = pfn_to_bit(page, zone_idx);
> + /*
> + * TODO: fix possible underflows.
> + */
> + if (free_area[zone_idx].bitmap &&
> + bitnr < free_area[zone_idx].nbits &&
> + !test_and_set_bit(bitnr, free_area[zone_idx].bitmap))
> + atomic_inc(&free_area[zone_idx].free_pages);
> +}
> +
> +static void scan_zone_free_area(int zone_idx, int free_pages)
> +{
> + int ret = 0, order, isolated_cnt = 0;
> + unsigned long set_bit, start = 0;
> + LIST_HEAD(isolated_pages);
> + struct page *page;
> + struct zone *zone;
> +
> + for (;;) {
> + ret = 0;
> + set_bit = find_next_bit(free_area[zone_idx].bitmap,
> + free_area[zone_idx].nbits, start);
> + if (set_bit >= free_area[zone_idx].nbits)
> + break;
> + page = pfn_to_online_page((set_bit << PAGE_HINTING_MIN_ORDER) +
> + free_area[zone_idx].base_pfn);
> + if (!page)
> + continue;
> + 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, free_area[zone_idx].bitmap);
> + atomic_dec(&free_area[zone_idx].free_pages);
> + 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 == page_hitning_conf->max_pages) {
> + page_hitning_conf->hint_pages(&isolated_pages);
> + release_buddy_pages(&isolated_pages);
> + isolated_cnt = 0;
> + }
> + }
> + start = set_bit + 1;
> + }
> + if (isolated_cnt) {
> + page_hitning_conf->hint_pages(&isolated_pages);
> + release_buddy_pages(&isolated_pages);
> + }
> +}
> +
I really worry that this loop is going to become more expensive as the
size of memory increases. For example if we hint on just 16 pages we
would have to walk something like 4K bits, 512 longs, if a system had
64G of memory. Have you considered testing with a larger memory
footprint to see if it has an impact on performance?
> +static void init_hinting_wq(struct work_struct *work)
> +{
> + int zone_idx, free_pages;
> +
> + atomic_set(&page_hinting_active, 1);
> + for (zone_idx = 0; zone_idx < MAX_NR_ZONES; zone_idx++) {
> + free_pages = atomic_read(&free_area[zone_idx].free_pages);
> + if (free_pages >= page_hitning_conf->max_pages)
> + scan_zone_free_area(zone_idx, free_pages);
> + }
> + atomic_set(&page_hinting_active, 0);
> +}
> +
> +void page_hinting_enqueue(struct page *page, int order)
> +{
> + int zone_idx;
> +
> + if (!page_hitning_conf || order < PAGE_HINTING_MIN_ORDER)
> + return;
I would think it is going to be expensive to be jumping into this
function for every freed page. You should probably have an inline
taking care of the order check before you even get here since it would
be faster that way.
> +
> + bm_set_pfn(page);
> + if (atomic_read(&page_hinting_active))
> + return;
So I would think this piece is racy. Specifically if you set a PFN
that is somewhere below the PFN you are currently processing in your
scan it is going to remain unset until you have another page freed
after the scan is completed. I would worry you can end up with a batch
free of memory resulting in a group of pages sitting at the start of
your bitmap unhinted.
In my patches I resolved this by looping through all of the zones,
however your approach is missing the necessary pieces to make that
safe as you could end up in a soft lockup with the scanning thread
spinning on a noisy system.
> + zone_idx = zone_idx(page_zone(page));
> + if (atomic_read(&free_area[zone_idx].free_pages) >=
> + page_hitning_conf->max_pages) {
> + int cpu = smp_processor_id();
> +
> + queue_work_on(cpu, system_wq, &hinting_work);
> + }
> +}