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    /*
     *  linux/mm/page_alloc.c
     *
     *  Manages the free list, the system allocates free pages here.
     *  Note that kmalloc() lives in slab.c
     *
     *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
     *  Swap reorganised 29.12.95, Stephen Tweedie
     *  Support of BIGMEM added by Gerhard Wichert, Siemens AG, July 1999
     *  Reshaped it to be a zoned allocator, Ingo Molnar, Red Hat, 1999
     *  Discontiguous memory support, Kanoj Sarcar, SGI, Nov 1999
     *  Zone balancing, Kanoj Sarcar, SGI, Jan 2000
     *  Per cpu hot/cold page lists, bulk allocation, Martin J. Bligh, Sept 2002
     *          (lots of bits borrowed from Ingo Molnar & Andrew Morton)
     */
    
    #include <linux/stddef.h>
    #include <linux/mm.h>
    #include <linux/swap.h>
    #include <linux/interrupt.h>
    #include <linux/pagemap.h>
    #include <linux/jiffies.h>
    #include <linux/bootmem.h>
    #include <linux/memblock.h>
    #include <linux/compiler.h>
    #include <linux/kernel.h>
    #include <linux/kmemcheck.h>
    #include <linux/kasan.h>
    #include <linux/module.h>
    #include <linux/suspend.h>
    #include <linux/pagevec.h>
    #include <linux/blkdev.h>
    #include <linux/slab.h>
    #include <linux/ratelimit.h>
    #include <linux/oom.h>
    #include <linux/notifier.h>
    #include <linux/topology.h>
    #include <linux/sysctl.h>
    #include <linux/cpu.h>
    #include <linux/cpuset.h>
    #include <linux/memory_hotplug.h>
    #include <linux/nodemask.h>
    #include <linux/vmalloc.h>
    #include <linux/vmstat.h>
    #include <linux/mempolicy.h>
    #include <linux/memremap.h>
    #include <linux/stop_machine.h>
    #include <linux/sort.h>
    #include <linux/pfn.h>
    #include <linux/backing-dev.h>
    #include <linux/fault-inject.h>
    #include <linux/page-isolation.h>
    #include <linux/page_ext.h>
    #include <linux/debugobjects.h>
    #include <linux/kmemleak.h>
    #include <linux/compaction.h>
    #include <trace/events/kmem.h>
    #include <linux/prefetch.h>
    #include <linux/mm_inline.h>
    #include <linux/migrate.h>
    #include <linux/page_ext.h>
    #include <linux/hugetlb.h>
    #include <linux/sched/rt.h>
    #include <linux/page_owner.h>
    #include <linux/kthread.h>
    #include <linux/memcontrol.h>
    
    #include <asm/sections.h>
    #include <asm/tlbflush.h>
    #include <asm/div64.h>
    #include "internal.h"
    
    /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
    static DEFINE_MUTEX(pcp_batch_high_lock);
    #define MIN_PERCPU_PAGELIST_FRACTION	(8)
    
    #ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
    DEFINE_PER_CPU(int, numa_node);
    EXPORT_PER_CPU_SYMBOL(numa_node);
    #endif
    
    #ifdef CONFIG_HAVE_MEMORYLESS_NODES
    /*
     * N.B., Do NOT reference the '_numa_mem_' per cpu variable directly.
     * It will not be defined when CONFIG_HAVE_MEMORYLESS_NODES is not defined.
     * Use the accessor functions set_numa_mem(), numa_mem_id() and cpu_to_mem()
     * defined in <linux/topology.h>.
     */
    DEFINE_PER_CPU(int, _numa_mem_);		/* Kernel "local memory" node */
    EXPORT_PER_CPU_SYMBOL(_numa_mem_);
    int _node_numa_mem_[MAX_NUMNODES];
    #endif
    
    #ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
    volatile unsigned long latent_entropy __latent_entropy;
    EXPORT_SYMBOL(latent_entropy);
    #endif
    
    /*
     * Array of node states.
     */
    nodemask_t node_states[NR_NODE_STATES] __read_mostly = {
    	[N_POSSIBLE] = NODE_MASK_ALL,
    	[N_ONLINE] = { { [0] = 1UL } },
    #ifndef CONFIG_NUMA
    	[N_NORMAL_MEMORY] = { { [0] = 1UL } },
    #ifdef CONFIG_HIGHMEM
    	[N_HIGH_MEMORY] = { { [0] = 1UL } },
    #endif
    #ifdef CONFIG_MOVABLE_NODE
    	[N_MEMORY] = { { [0] = 1UL } },
    #endif
    	[N_CPU] = { { [0] = 1UL } },
    #endif	/* NUMA */
    };
    EXPORT_SYMBOL(node_states);
    
    /* Protect totalram_pages and zone->managed_pages */
    static DEFINE_SPINLOCK(managed_page_count_lock);
    
    unsigned long totalram_pages __read_mostly;
    unsigned long totalreserve_pages __read_mostly;
    unsigned long totalcma_pages __read_mostly;
    
    int percpu_pagelist_fraction;
    gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
    
    /*
     * A cached value of the page's pageblock's migratetype, used when the page is
     * put on a pcplist. Used to avoid the pageblock migratetype lookup when
     * freeing from pcplists in most cases, at the cost of possibly becoming stale.
     * Also the migratetype set in the page does not necessarily match the pcplist
     * index, e.g. page might have MIGRATE_CMA set but be on a pcplist with any
     * other index - this ensures that it will be put on the correct CMA freelist.
     */
    static inline int get_pcppage_migratetype(struct page *page)
    {
    	return page->index;
    }
    
    static inline void set_pcppage_migratetype(struct page *page, int migratetype)
    {
    	page->index = migratetype;
    }
    
    #ifdef CONFIG_PM_SLEEP
    /*
     * The following functions are used by the suspend/hibernate code to temporarily
     * change gfp_allowed_mask in order to avoid using I/O during memory allocations
     * while devices are suspended.  To avoid races with the suspend/hibernate code,
     * they should always be called with pm_mutex held (gfp_allowed_mask also should
     * only be modified with pm_mutex held, unless the suspend/hibernate code is
     * guaranteed not to run in parallel with that modification).
     */
    
    static gfp_t saved_gfp_mask;
    
    void pm_restore_gfp_mask(void)
    {
    	WARN_ON(!mutex_is_locked(&pm_mutex));
    	if (saved_gfp_mask) {
    		gfp_allowed_mask = saved_gfp_mask;
    		saved_gfp_mask = 0;
    	}
    }
    
    void pm_restrict_gfp_mask(void)
    {
    	WARN_ON(!mutex_is_locked(&pm_mutex));
    	WARN_ON(saved_gfp_mask);
    	saved_gfp_mask = gfp_allowed_mask;
    	gfp_allowed_mask &= ~(__GFP_IO | __GFP_FS);
    }
    
    bool pm_suspended_storage(void)
    {
    	if ((gfp_allowed_mask & (__GFP_IO | __GFP_FS)) == (__GFP_IO | __GFP_FS))
    		return false;
    	return true;
    }
    #endif /* CONFIG_PM_SLEEP */
    
    #ifdef CONFIG_HUGETLB_PAGE_SIZE_VARIABLE
    unsigned int pageblock_order __read_mostly;
    #endif
    
    static void __free_pages_ok(struct page *page, unsigned int order);
    
    /*
     * results with 256, 32 in the lowmem_reserve sysctl:
     *	1G machine -> (16M dma, 800M-16M normal, 1G-800M high)
     *	1G machine -> (16M dma, 784M normal, 224M high)
     *	NORMAL allocation will leave 784M/256 of ram reserved in the ZONE_DMA
     *	HIGHMEM allocation will leave 224M/32 of ram reserved in ZONE_NORMAL
     *	HIGHMEM allocation will leave (224M+784M)/256 of ram reserved in ZONE_DMA
     *
     * TBD: should special case ZONE_DMA32 machines here - in those we normally
     * don't need any ZONE_NORMAL reservation
     */
    int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES-1] = {
    #ifdef CONFIG_ZONE_DMA
    	 256,
    #endif
    #ifdef CONFIG_ZONE_DMA32
    	 256,
    #endif
    #ifdef CONFIG_HIGHMEM
    	 32,
    #endif
    	 32,
    };
    
    EXPORT_SYMBOL(totalram_pages);
    
    static char * const zone_names[MAX_NR_ZONES] = {
    #ifdef CONFIG_ZONE_DMA
    	 "DMA",
    #endif
    #ifdef CONFIG_ZONE_DMA32
    	 "DMA32",
    #endif
    	 "Normal",
    #ifdef CONFIG_HIGHMEM
    	 "HighMem",
    #endif
    	 "Movable",
    #ifdef CONFIG_ZONE_DEVICE
    	 "Device",
    #endif
    };
    
    char * const migratetype_names[MIGRATE_TYPES] = {
    	"Unmovable",
    	"Movable",
    	"Reclaimable",
    	"HighAtomic",
    #ifdef CONFIG_CMA
    	"CMA",
    #endif
    #ifdef CONFIG_MEMORY_ISOLATION
    	"Isolate",
    #endif
    };
    
    compound_page_dtor * const compound_page_dtors[] = {
    	NULL,
    	free_compound_page,
    #ifdef CONFIG_HUGETLB_PAGE
    	free_huge_page,
    #endif
    #ifdef CONFIG_TRANSPARENT_HUGEPAGE
    	free_transhuge_page,
    #endif
    };
    
    int min_free_kbytes = 1024;
    int user_min_free_kbytes = -1;
    int watermark_scale_factor = 10;
    
    static unsigned long __meminitdata nr_kernel_pages;
    static unsigned long __meminitdata nr_all_pages;
    static unsigned long __meminitdata dma_reserve;
    
    #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
    static unsigned long __meminitdata arch_zone_lowest_possible_pfn[MAX_NR_ZONES];
    static unsigned long __meminitdata arch_zone_highest_possible_pfn[MAX_NR_ZONES];
    static unsigned long __initdata required_kernelcore;
    static unsigned long __initdata required_movablecore;
    static unsigned long __meminitdata zone_movable_pfn[MAX_NUMNODES];
    static bool mirrored_kernelcore;
    
    /* movable_zone is the "real" zone pages in ZONE_MOVABLE are taken from */
    int movable_zone;
    EXPORT_SYMBOL(movable_zone);
    #endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */
    
    #if MAX_NUMNODES > 1
    int nr_node_ids __read_mostly = MAX_NUMNODES;
    int nr_online_nodes __read_mostly = 1;
    EXPORT_SYMBOL(nr_node_ids);
    EXPORT_SYMBOL(nr_online_nodes);
    #endif
    
    int page_group_by_mobility_disabled __read_mostly;
    
    #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
    static inline void reset_deferred_meminit(pg_data_t *pgdat)
    {
    	unsigned long max_initialise;
    	unsigned long reserved_lowmem;
    
    	/*
    	 * Initialise at least 2G of a node but also take into account that
    	 * two large system hashes that can take up 1GB for 0.25TB/node.
    	 */
    	max_initialise = max(2UL << (30 - PAGE_SHIFT),
    		(pgdat->node_spanned_pages >> 8));
    
    	/*
    	 * Compensate the all the memblock reservations (e.g. crash kernel)
    	 * from the initial estimation to make sure we will initialize enough
    	 * memory to boot.
    	 */
    	reserved_lowmem = memblock_reserved_memory_within(pgdat->node_start_pfn,
    			pgdat->node_start_pfn + max_initialise);
    	max_initialise += reserved_lowmem;
    
    	pgdat->static_init_size = min(max_initialise, pgdat->node_spanned_pages);
    	pgdat->first_deferred_pfn = ULONG_MAX;
    }
    
    /* Returns true if the struct page for the pfn is uninitialised */
    static inline bool __meminit early_page_uninitialised(unsigned long pfn)
    {
    	int nid = early_pfn_to_nid(pfn);
    
    	if (node_online(nid) && pfn >= NODE_DATA(nid)->first_deferred_pfn)
    		return true;
    
    	return false;
    }
    
    /*
     * Returns false when the remaining initialisation should be deferred until
     * later in the boot cycle when it can be parallelised.
     */
    static inline bool update_defer_init(pg_data_t *pgdat,
    				unsigned long pfn, unsigned long zone_end,
    				unsigned long *nr_initialised)
    {
    	/* Always populate low zones for address-contrained allocations */
    	if (zone_end < pgdat_end_pfn(pgdat))
    		return true;
    	(*nr_initialised)++;
    	if ((*nr_initialised > pgdat->static_init_size) &&
    	    (pfn & (PAGES_PER_SECTION - 1)) == 0) {
    		pgdat->first_deferred_pfn = pfn;
    		return false;
    	}
    
    	return true;
    }
    #else
    static inline void reset_deferred_meminit(pg_data_t *pgdat)
    {
    }
    
    static inline bool early_page_uninitialised(unsigned long pfn)
    {
    	return false;
    }
    
    static inline bool update_defer_init(pg_data_t *pgdat,
    				unsigned long pfn, unsigned long zone_end,
    				unsigned long *nr_initialised)
    {
    	return true;
    }
    #endif
    
    /* Return a pointer to the bitmap storing bits affecting a block of pages */
    static inline unsigned long *get_pageblock_bitmap(struct page *page,
    							unsigned long pfn)
    {
    #ifdef CONFIG_SPARSEMEM
    	return __pfn_to_section(pfn)->pageblock_flags;
    #else
    	return page_zone(page)->pageblock_flags;
    #endif /* CONFIG_SPARSEMEM */
    }
    
    static inline int pfn_to_bitidx(struct page *page, unsigned long pfn)
    {
    #ifdef CONFIG_SPARSEMEM
    	pfn &= (PAGES_PER_SECTION-1);
    	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
    #else
    	pfn = pfn - round_down(page_zone(page)->zone_start_pfn, pageblock_nr_pages);
    	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
    #endif /* CONFIG_SPARSEMEM */
    }
    
    /**
     * get_pfnblock_flags_mask - Return the requested group of flags for the pageblock_nr_pages block of pages
     * @page: The page within the block of interest
     * @pfn: The target page frame number
     * @end_bitidx: The last bit of interest to retrieve
     * @mask: mask of bits that the caller is interested in
     *
     * Return: pageblock_bits flags
     */
    static __always_inline unsigned long __get_pfnblock_flags_mask(struct page *page,
    					unsigned long pfn,
    					unsigned long end_bitidx,
    					unsigned long mask)
    {
    	unsigned long *bitmap;
    	unsigned long bitidx, word_bitidx;
    	unsigned long word;
    
    	bitmap = get_pageblock_bitmap(page, pfn);
    	bitidx = pfn_to_bitidx(page, pfn);
    	word_bitidx = bitidx / BITS_PER_LONG;
    	bitidx &= (BITS_PER_LONG-1);
    
    	word = bitmap[word_bitidx];
    	bitidx += end_bitidx;
    	return (word >> (BITS_PER_LONG - bitidx - 1)) & mask;
    }
    
    unsigned long get_pfnblock_flags_mask(struct page *page, unsigned long pfn,
    					unsigned long end_bitidx,
    					unsigned long mask)
    {
    	return __get_pfnblock_flags_mask(page, pfn, end_bitidx, mask);
    }
    
    static __always_inline int get_pfnblock_migratetype(struct page *page, unsigned long pfn)
    {
    	return __get_pfnblock_flags_mask(page, pfn, PB_migrate_end, MIGRATETYPE_MASK);
    }
    
    /**
     * set_pfnblock_flags_mask - Set the requested group of flags for a pageblock_nr_pages block of pages
     * @page: The page within the block of interest
     * @flags: The flags to set
     * @pfn: The target page frame number
     * @end_bitidx: The last bit of interest
     * @mask: mask of bits that the caller is interested in
     */
    void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
    					unsigned long pfn,
    					unsigned long end_bitidx,
    					unsigned long mask)
    {
    	unsigned long *bitmap;
    	unsigned long bitidx, word_bitidx;
    	unsigned long old_word, word;
    
    	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
    
    	bitmap = get_pageblock_bitmap(page, pfn);
    	bitidx = pfn_to_bitidx(page, pfn);
    	word_bitidx = bitidx / BITS_PER_LONG;
    	bitidx &= (BITS_PER_LONG-1);
    
    	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
    
    	bitidx += end_bitidx;
    	mask <<= (BITS_PER_LONG - bitidx - 1);
    	flags <<= (BITS_PER_LONG - bitidx - 1);
    
    	word = READ_ONCE(bitmap[word_bitidx]);
    	for (;;) {
    		old_word = cmpxchg(&bitmap[word_bitidx], word, (word & ~mask) | flags);
    		if (word == old_word)
    			break;
    		word = old_word;
    	}
    }
    
    void set_pageblock_migratetype(struct page *page, int migratetype)
    {
    	if (unlikely(page_group_by_mobility_disabled &&
    		     migratetype < MIGRATE_PCPTYPES))
    		migratetype = MIGRATE_UNMOVABLE;
    
    	set_pageblock_flags_group(page, (unsigned long)migratetype,
    					PB_migrate, PB_migrate_end);
    }
    
    #ifdef CONFIG_DEBUG_VM
    static int page_outside_zone_boundaries(struct zone *zone, struct page *page)
    {
    	int ret = 0;
    	unsigned seq;
    	unsigned long pfn = page_to_pfn(page);
    	unsigned long sp, start_pfn;
    
    	do {
    		seq = zone_span_seqbegin(zone);
    		start_pfn = zone->zone_start_pfn;
    		sp = zone->spanned_pages;
    		if (!zone_spans_pfn(zone, pfn))
    			ret = 1;
    	} while (zone_span_seqretry(zone, seq));
    
    	if (ret)
    		pr_err("page 0x%lx outside node %d zone %s [ 0x%lx - 0x%lx ]\n",
    			pfn, zone_to_nid(zone), zone->name,
    			start_pfn, start_pfn + sp);
    
    	return ret;
    }
    
    static int page_is_consistent(struct zone *zone, struct page *page)
    {
    	if (!pfn_valid_within(page_to_pfn(page)))
    		return 0;
    	if (zone != page_zone(page))
    		return 0;
    
    	return 1;
    }
    /*
     * Temporary debugging check for pages not lying within a given zone.
     */
    static int bad_range(struct zone *zone, struct page *page)
    {
    	if (page_outside_zone_boundaries(zone, page))
    		return 1;
    	if (!page_is_consistent(zone, page))
    		return 1;
    
    	return 0;
    }
    #else
    static inline int bad_range(struct zone *zone, struct page *page)
    {
    	return 0;
    }
    #endif
    
    static void bad_page(struct page *page, const char *reason,
    		unsigned long bad_flags)
    {
    	static unsigned long resume;
    	static unsigned long nr_shown;
    	static unsigned long nr_unshown;
    
    	/*
    	 * Allow a burst of 60 reports, then keep quiet for that minute;
    	 * or allow a steady drip of one report per second.
    	 */
    	if (nr_shown == 60) {
    		if (time_before(jiffies, resume)) {
    			nr_unshown++;
    			goto out;
    		}
    		if (nr_unshown) {
    			pr_alert(
    			      "BUG: Bad page state: %lu messages suppressed\n",
    				nr_unshown);
    			nr_unshown = 0;
    		}
    		nr_shown = 0;
    	}
    	if (nr_shown++ == 0)
    		resume = jiffies + 60 * HZ;
    
    	pr_alert("BUG: Bad page state in process %s  pfn:%05lx\n",
    		current->comm, page_to_pfn(page));
    	__dump_page(page, reason);
    	bad_flags &= page->flags;
    	if (bad_flags)
    		pr_alert("bad because of flags: %#lx(%pGp)\n",
    						bad_flags, &bad_flags);
    	dump_page_owner(page);
    
    	print_modules();
    	dump_stack();
    out:
    	/* Leave bad fields for debug, except PageBuddy could make trouble */
    	page_mapcount_reset(page); /* remove PageBuddy */
    	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
    }
    
    /*
     * Higher-order pages are called "compound pages".  They are structured thusly:
     *
     * The first PAGE_SIZE page is called the "head page" and have PG_head set.
     *
     * The remaining PAGE_SIZE pages are called "tail pages". PageTail() is encoded
     * in bit 0 of page->compound_head. The rest of bits is pointer to head page.
     *
     * The first tail page's ->compound_dtor holds the offset in array of compound
     * page destructors. See compound_page_dtors.
     *
     * The first tail page's ->compound_order holds the order of allocation.
     * This usage means that zero-order pages may not be compound.
     */
    
    void free_compound_page(struct page *page)
    {
    	__free_pages_ok(page, compound_order(page));
    }
    
    void prep_compound_page(struct page *page, unsigned int order)
    {
    	int i;
    	int nr_pages = 1 << order;
    
    	set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
    	set_compound_order(page, order);
    	__SetPageHead(page);
    	for (i = 1; i < nr_pages; i++) {
    		struct page *p = page + i;
    		set_page_count(p, 0);
    		p->mapping = TAIL_MAPPING;
    		set_compound_head(p, page);
    	}
    	atomic_set(compound_mapcount_ptr(page), -1);
    }
    
    #ifdef CONFIG_DEBUG_PAGEALLOC
    unsigned int _debug_guardpage_minorder;
    bool _debug_pagealloc_enabled __read_mostly
    			= IS_ENABLED(CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT);
    EXPORT_SYMBOL(_debug_pagealloc_enabled);
    bool _debug_guardpage_enabled __read_mostly;
    
    static int __init early_debug_pagealloc(char *buf)
    {
    	if (!buf)
    		return -EINVAL;
    	return kstrtobool(buf, &_debug_pagealloc_enabled);
    }
    early_param("debug_pagealloc", early_debug_pagealloc);
    
    static bool need_debug_guardpage(void)
    {
    	/* If we don't use debug_pagealloc, we don't need guard page */
    	if (!debug_pagealloc_enabled())
    		return false;
    
    	if (!debug_guardpage_minorder())
    		return false;
    
    	return true;
    }
    
    static void init_debug_guardpage(void)
    {
    	if (!debug_pagealloc_enabled())
    		return;
    
    	if (!debug_guardpage_minorder())
    		return;
    
    	_debug_guardpage_enabled = true;
    }
    
    struct page_ext_operations debug_guardpage_ops = {
    	.need = need_debug_guardpage,
    	.init = init_debug_guardpage,
    };
    
    static int __init debug_guardpage_minorder_setup(char *buf)
    {
    	unsigned long res;
    
    	if (kstrtoul(buf, 10, &res) < 0 ||  res > MAX_ORDER / 2) {
    		pr_err("Bad debug_guardpage_minorder value\n");
    		return 0;
    	}
    	_debug_guardpage_minorder = res;
    	pr_info("Setting debug_guardpage_minorder to %lu\n", res);
    	return 0;
    }
    early_param("debug_guardpage_minorder", debug_guardpage_minorder_setup);
    
    static inline bool set_page_guard(struct zone *zone, struct page *page,
    				unsigned int order, int migratetype)
    {
    	struct page_ext *page_ext;
    
    	if (!debug_guardpage_enabled())
    		return false;
    
    	if (order >= debug_guardpage_minorder())
    		return false;
    
    	page_ext = lookup_page_ext(page);
    	if (unlikely(!page_ext))
    		return false;
    
    	__set_bit(PAGE_EXT_DEBUG_GUARD, &page_ext->flags);
    
    	INIT_LIST_HEAD(&page->lru);
    	set_page_private(page, order);
    	/* Guard pages are not available for any usage */
    	__mod_zone_freepage_state(zone, -(1 << order), migratetype);
    
    	return true;
    }
    
    static inline void clear_page_guard(struct zone *zone, struct page *page,
    				unsigned int order, int migratetype)
    {
    	struct page_ext *page_ext;
    
    	if (!debug_guardpage_enabled())
    		return;
    
    	page_ext = lookup_page_ext(page);
    	if (unlikely(!page_ext))
    		return;
    
    	__clear_bit(PAGE_EXT_DEBUG_GUARD, &page_ext->flags);
    
    	set_page_private(page, 0);
    	if (!is_migrate_isolate(migratetype))
    		__mod_zone_freepage_state(zone, (1 << order), migratetype);
    }
    #else
    struct page_ext_operations debug_guardpage_ops;
    static inline bool set_page_guard(struct zone *zone, struct page *page,
    			unsigned int order, int migratetype) { return false; }
    static inline void clear_page_guard(struct zone *zone, struct page *page,
    				unsigned int order, int migratetype) {}
    #endif
    
    static inline void set_page_order(struct page *page, unsigned int order)
    {
    	set_page_private(page, order);
    	__SetPageBuddy(page);
    }
    
    static inline void rmv_page_order(struct page *page)
    {
    	__ClearPageBuddy(page);
    	set_page_private(page, 0);
    }
    
    /*
     * This function checks whether a page is free && is the buddy
     * we can do coalesce a page and its buddy if
     * (a) the buddy is not in a hole &&
     * (b) the buddy is in the buddy system &&
     * (c) a page and its buddy have the same order &&
     * (d) a page and its buddy are in the same zone.
     *
     * For recording whether a page is in the buddy system, we set ->_mapcount
     * PAGE_BUDDY_MAPCOUNT_VALUE.
     * Setting, clearing, and testing _mapcount PAGE_BUDDY_MAPCOUNT_VALUE is
     * serialized by zone->lock.
     *
     * For recording page's order, we use page_private(page).
     */
    static inline int page_is_buddy(struct page *page, struct page *buddy,
    							unsigned int order)
    {
    	if (!pfn_valid_within(page_to_pfn(buddy)))
    		return 0;
    
    	if (page_is_guard(buddy) && page_order(buddy) == order) {
    		if (page_zone_id(page) != page_zone_id(buddy))
    			return 0;
    
    		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
    
    		return 1;
    	}
    
    	if (PageBuddy(buddy) && page_order(buddy) == order) {
    		/*
    		 * zone check is done late to avoid uselessly
    		 * calculating zone/node ids for pages that could
    		 * never merge.
    		 */
    		if (page_zone_id(page) != page_zone_id(buddy))
    			return 0;
    
    		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
    
    		return 1;
    	}
    	return 0;
    }
    
    /*
     * Freeing function for a buddy system allocator.
     *
     * The concept of a buddy system is to maintain direct-mapped table
     * (containing bit values) for memory blocks of various "orders".
     * The bottom level table contains the map for the smallest allocatable
     * units of memory (here, pages), and each level above it describes
     * pairs of units from the levels below, hence, "buddies".
     * At a high level, all that happens here is marking the table entry
     * at the bottom level available, and propagating the changes upward
     * as necessary, plus some accounting needed to play nicely with other
     * parts of the VM system.
     * At each level, we keep a list of pages, which are heads of continuous
     * free pages of length of (1 << order) and marked with _mapcount
     * PAGE_BUDDY_MAPCOUNT_VALUE. Page's order is recorded in page_private(page)
     * field.
     * So when we are allocating or freeing one, we can derive the state of the
     * other.  That is, if we allocate a small block, and both were
     * free, the remainder of the region must be split into blocks.
     * If a block is freed, and its buddy is also free, then this
     * triggers coalescing into a block of larger size.
     *
     * -- nyc
     */
    
    static inline void __free_one_page(struct page *page,
    		unsigned long pfn,
    		struct zone *zone, unsigned int order,
    		int migratetype)
    {
    	unsigned long page_idx;
    	unsigned long combined_idx;
    	unsigned long uninitialized_var(buddy_idx);
    	struct page *buddy;
    	unsigned int max_order;
    
    	max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
    
    	VM_BUG_ON(!zone_is_initialized(zone));
    	VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
    
    	VM_BUG_ON(migratetype == -1);
    	if (likely(!is_migrate_isolate(migratetype)))
    		__mod_zone_freepage_state(zone, 1 << order, migratetype);
    
    	page_idx = pfn & ((1 << MAX_ORDER) - 1);
    
    	VM_BUG_ON_PAGE(page_idx & ((1 << order) - 1), page);
    	VM_BUG_ON_PAGE(bad_range(zone, page), page);
    
    continue_merging:
    	while (order < max_order - 1) {
    		buddy_idx = __find_buddy_index(page_idx, order);
    		buddy = page + (buddy_idx - page_idx);
    		if (!page_is_buddy(page, buddy, order))
    			goto done_merging;
    		/*
    		 * Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page,
    		 * merge with it and move up one order.
    		 */
    		if (page_is_guard(buddy)) {
    			clear_page_guard(zone, buddy, order, migratetype);
    		} else {
    			list_del(&buddy->lru);
    			zone->free_area[order].nr_free--;
    			rmv_page_order(buddy);
    		}
    		combined_idx = buddy_idx & page_idx;
    		page = page + (combined_idx - page_idx);
    		page_idx = combined_idx;
    		order++;
    	}
    	if (max_order < MAX_ORDER) {
    		/* If we are here, it means order is >= pageblock_order.
    		 * We want to prevent merge between freepages on isolate
    		 * pageblock and normal pageblock. Without this, pageblock
    		 * isolation could cause incorrect freepage or CMA accounting.
    		 *
    		 * We don't want to hit this code for the more frequent
    		 * low-order merging.
    		 */
    		if (unlikely(has_isolate_pageblock(zone))) {
    			int buddy_mt;
    
    			buddy_idx = __find_buddy_index(page_idx, order);
    			buddy = page + (buddy_idx - page_idx);
    			buddy_mt = get_pageblock_migratetype(buddy);
    
    			if (migratetype != buddy_mt
    					&& (is_migrate_isolate(migratetype) ||
    						is_migrate_isolate(buddy_mt)))
    				goto done_merging;
    		}
    		max_order++;
    		goto continue_merging;
    	}
    
    done_merging:
    	set_page_order(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
    	 * that pages are being freed that will coalesce soon. In case,
    	 * that is happening, add the free page to the tail of the list
    	 * so it's less likely to be used soon and more likely to be merged
    	 * as a higher order page
    	 */
    	if ((order < MAX_ORDER-2) && pfn_valid_within(page_to_pfn(buddy))) {
    		struct page *higher_page, *higher_buddy;
    		combined_idx = buddy_idx & page_idx;
    		higher_page = page + (combined_idx - page_idx);
    		buddy_idx = __find_buddy_index(combined_idx, order + 1);
    		higher_buddy = higher_page + (buddy_idx - combined_idx);
    		if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
    			list_add_tail(&page->lru,
    				&zone->free_area[order].free_list[migratetype]);
    			goto out;
    		}
    	}
    
    	list_add(&page->lru, &zone->free_area[order].free_list[migratetype]);
    out:
    	zone->free_area[order].nr_free++;
    }
    
    /*
     * A bad page could be due to a number of fields. Instead of multiple branches,
     * try and check multiple fields with one check. The caller must do a detailed
     * check if necessary.
     */
    static inline bool page_expected_state(struct page *page,
    					unsigned long check_flags)
    {
    	if (unlikely(atomic_read(&page->_mapcount) != -1))
    		return false;
    
    	if (unlikely((unsigned long)page->mapping |
    			page_ref_count(page) |
    #ifdef CONFIG_MEMCG
    			(unsigned long)page->mem_cgroup |
    #endif
    			(page->flags & check_flags)))
    		return false;
    
    	return true;
    }
    
    static void free_pages_check_bad(struct page *page)
    {
    	const char *bad_reason;
    	unsigned long bad_flags;
    
    	bad_reason = NULL;
    	bad_flags = 0;
    
    	if (unlikely(atomic_read(&page->_mapcount) != -1))
    		bad_reason = "nonzero mapcount";
    	if (unlikely(page->mapping != NULL))
    		bad_reason = "non-NULL mapping";
    	if (unlikely(page_ref_count(page) != 0))
    		bad_reason = "nonzero _refcount";
    	if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_FREE)) {
    		bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag(s) set";
    		bad_flags = PAGE_FLAGS_CHECK_AT_FREE;
    	}
    #ifdef CONFIG_MEMCG
    	if (unlikely(page->mem_cgroup))
    		bad_reason = "page still charged to cgroup";
    #endif
    	bad_page(page, bad_reason, bad_flags);
    }
    
    static inline int free_pages_check(struct page *page)
    {
    	if (likely(page_expected_state(page, PAGE_FLAGS_CHECK_AT_FREE)))
    		return 0;
    
    	/* Something has gone sideways, find it */
    	free_pages_check_bad(page);
    	return 1;
    }
    
    static int free_tail_pages_check(struct page *head_page, struct page *page)
    {
    	int ret = 1;
    
    	/*
    	 * We rely page->lru.next never has bit 0 set, unless the page
    	 * is PageTail(). Let's make sure that's true even for poisoned ->lru.
    	 */
    	BUILD_BUG_ON((unsigned long)LIST_POISON1 & 1);
    
    	if (!IS_ENABLED(CONFIG_DEBUG_VM)) {
    		ret = 0;
    		goto out;
    	}
    	switch (page - head_page) {
    	case 1:
    		/* the first tail page: ->mapping is compound_mapcount() */
    		if (unlikely(compound_mapcount(page))) {
    			bad_page(page, "nonzero compound_mapcount", 0);
    			goto out;
    		}
    		break;
    	case 2:
    		/*
    		 * the second tail page: ->mapping is
    		 * page_deferred_list().next -- ignore value.
    		 */
    		break;
    	default:
    		if (page->mapping != TAIL_MAPPING) {
    			bad_page(page, "corrupted mapping in tail page", 0);
    			goto out;
    		}
    		break;
    	}
    	if (unlikely(!PageTail(page))) {
    		bad_page(page, "PageTail not set", 0);
    		goto out;
    	}
    	if (unlikely(compound_head(page) != head_page)) {
    		bad_page(page, "compound_head not consistent", 0);
    		goto out;
    	}
    	ret = 0;
    out:
    	page->mapping = NULL;
    	clear_compound_head(page);