【Linux内存源码分析】构建内存管理框架(5)

前面已经分析了内存管理框架的构建实现过程,有部分内容未完全呈现出来,这里主要做个补充。

如下图,这是前面已经看到过的linux物理内存管理框架的层次关系。

现着重分析一下各个管理结构体的成员功能作用。

【file:/include/linux/mmzone.h】
typedef struct pglist_data {
	struct zone node_zones[MAX_NR_ZONES];
	struct zonelist node_zonelists[MAX_ZONELISTS];
	int nr_zones;
#ifdef CONFIG_FLAT_NODE_MEM_MAP	/* means !SPARSEMEM */
	struct page *node_mem_map;
#ifdef CONFIG_MEMCG
	struct page_cgroup *node_page_cgroup;
#endif
#endif
#ifndef CONFIG_NO_BOOTMEM
	struct bootmem_data *bdata;
#endif
#ifdef CONFIG_MEMORY_HOTPLUG
	/*
	 * Must be held any time you expect node_start_pfn, node_present_pages
	 * or node_spanned_pages stay constant.  Holding this will also
	 * guarantee that any pfn_valid() stays that way.
	 *
	 * pgdat_resize_lock() and pgdat_resize_unlock() are provided to
	 * manipulate node_size_lock without checking for CONFIG_MEMORY_HOTPLUG.
	 *
	 * Nests above zone->lock and zone->span_seqlock
	 */
	spinlock_t node_size_lock;
#endif
	unsigned long node_start_pfn;
	unsigned long node_present_pages; /* total number of physical pages */
	unsigned long node_spanned_pages; /* total size of physical page
					     range, including holes */
	int node_id;
	nodemask_t reclaim_nodes;	/* Nodes allowed to reclaim from */
	wait_queue_head_t kswapd_wait;
	wait_queue_head_t pfmemalloc_wait;
	struct task_struct *kswapd;	/* Protected by lock_memory_hotplug() */
	int kswapd_max_order;
	enum zone_type classzone_idx;
#ifdef CONFIG_NUMA_BALANCING
	/* Lock serializing the migrate rate limiting window */
	spinlock_t numabalancing_migrate_lock;

	/* Rate limiting time interval */
	unsigned long numabalancing_migrate_next_window;

	/* Number of pages migrated during the rate limiting time interval */
	unsigned long numabalancing_migrate_nr_pages;
#endif
} pg_data_t;

truct zone
node_zones[MAX_NR_ZONES];

——存放该pg_data_t里面的zone

struct zonelist
node_zonelists[MAX_ZONELISTS];

——其用于管理备用节点及内存域的列表,该列表表示内存分配策略。该链表将node_zones串联起来,其串联zone的顺序就是各区的内存申请顺序,例如normal->dma->highmem,申请时也将会是先从normal区中申请,如果申请不到,再依序到从dma区、highmem区去申请;

int nr_zones;

——用于记录zone的个数;

struct page *node_mem_map;

——其指向一个page结构的数组,数组中的每个成员为该节点中的一个物理页面,于是整个数组就对应了该节点中所有的物理页面;

struct page_cgroup
*node_page_cgroup;

——用于管理page_cgroup,原来的page_cgrouppage页面管理结构的一个成员,现在移到这里了,它将会在初始化时所有的page_cgroup都将申请下来;

struct bootmem_data *bdata;

——该数据指向bootmem_node_data,可以通过system.map查到。原是用于bootmem内存分配器的信息存储,当前改用memblock算法,则不存在该成员;

unsigned long node_start_pfn;

——指向当前pg_data_t结构管理的物理起始页面;

unsigned long
node_present_pages;

——记录物理页面数总量,除开内存空洞的物理页面数;

unsigned long
node_spanned_pages;

——最大和最小页面号的差值,包括内存空洞的总的物理页面大小;

int node_id;

——pg_data_t对应的索引号,非NUMA架构下该值为0

nodemask_t reclaim_nodes;

——用于记录可回收的内存管理节点node信息;

wait_queue_head_t
kswapd_wait;

——kswapd是页面交换守护线程,该线程会阻塞在这个等待队列,当满足条件后,调用wake_up_interruptible()唤醒该队列进行相关操作;

wait_queue_head_t
pfmemalloc_wait;

——用于减缓内存直接回收;

struct task_struct *kswapd;

——指向kswapd守护线程的任务指针;

int kswapd_max_order;

——用于表示kswapd守护线程每次回收的页面个数;

enum zone_type classzone_idx;

——该成员与kswapd有关;

【file:/include/linux/mmzone.h】
struct zone {
	/* Fields commonly accessed by the page allocator */

	/* zone watermarks, access with *_wmark_pages(zone) macros */
	unsigned long watermark[NR_WMARK];

	/*
	 * When free pages are below this point, additional steps are taken
	 * when reading the number of free pages to avoid per-cpu counter
	 * drift allowing watermarks to be breached
	 */
	unsigned long percpu_drift_mark;

	/*
	 * We don't know if the memory that we're going to allocate will be freeable
	 * or/and it will be released eventually, so to avoid totally wasting several
	 * GB of ram we must reserve some of the lower zone memory (otherwise we risk
	 * to run OOM on the lower zones despite there's tons of freeable ram
	 * on the higher zones). This array is recalculated at runtime if the
	 * sysctl_lowmem_reserve_ratio sysctl changes.
	 */
	unsigned long		lowmem_reserve[MAX_NR_ZONES];

	/*
	 * This is a per-zone reserve of pages that should not be
	 * considered dirtyable memory.
	 */
	unsigned long		dirty_balance_reserve;

#ifdef CONFIG_NUMA
	int node;
	/*
	 * zone reclaim becomes active if more unmapped pages exist.
	 */
	unsigned long		min_unmapped_pages;
	unsigned long		min_slab_pages;
#endif
	struct per_cpu_pageset __percpu *pageset;
	/*
	 * free areas of different sizes
	 */
	spinlock_t		lock;
#if defined CONFIG_COMPACTION || defined CONFIG_CMA
	/* Set to true when the PG_migrate_skip bits should be cleared */
	bool			compact_blockskip_flush;

	/* pfns where compaction scanners should start */
	unsigned long		compact_cached_free_pfn;
	unsigned long		compact_cached_migrate_pfn;
#endif
#ifdef CONFIG_MEMORY_HOTPLUG
	/* see spanned/present_pages for more description */
	seqlock_t		span_seqlock;
#endif
	struct free_area	free_area[MAX_ORDER];

#ifndef CONFIG_SPARSEMEM
	/*
	 * Flags for a pageblock_nr_pages block. See pageblock-flags.h.
	 * In SPARSEMEM, this map is stored in struct mem_section
	 */
	unsigned long		*pageblock_flags;
#endif /* CONFIG_SPARSEMEM */

#ifdef CONFIG_COMPACTION
	/*
	 * On compaction failure, 1<<compact_defer_shift compactions
	 * are skipped before trying again. The number attempted since
	 * last failure is tracked with compact_considered.
	 */
	unsigned int		compact_considered;
	unsigned int		compact_defer_shift;
	int			compact_order_failed;
#endif

	ZONE_PADDING(_pad1_)

	/* Fields commonly accessed by the page reclaim scanner */
	spinlock_t		lru_lock;
	struct lruvec		lruvec;

	unsigned long		pages_scanned;	   /* since last reclaim */
	unsigned long		flags;		   /* zone flags, see below */

	/* Zone statistics */
	atomic_long_t		vm_stat[NR_VM_ZONE_STAT_ITEMS];

	/*
	 * The target ratio of ACTIVE_ANON to INACTIVE_ANON pages on
	 * this zone's LRU.  Maintained by the pageout code.
	 */
	unsigned int inactive_ratio;


	ZONE_PADDING(_pad2_)
	/* Rarely used or read-mostly fields */

	/*
	 * wait_table		-- the array holding the hash table
	 * wait_table_hash_nr_entries	-- the size of the hash table array
	 * wait_table_bits	-- wait_table_size == (1 << wait_table_bits)
	 *
	 * The purpose of all these is to keep track of the people
	 * waiting for a page to become available and make them
	 * runnable again when possible. The trouble is that this
	 * consumes a lot of space, especially when so few things
	 * wait on pages at a given time. So instead of using
	 * per-page waitqueues, we use a waitqueue hash table.
	 *
	 * The bucket discipline is to sleep on the same queue when
	 * colliding and wake all in that wait queue when removing.
	 * When something wakes, it must check to be sure its page is
	 * truly available, a la thundering herd. The cost of a
	 * collision is great, but given the expected load of the
	 * table, they should be so rare as to be outweighed by the
	 * benefits from the saved space.
	 *
	 * __wait_on_page_locked() and unlock_page() in mm/filemap.c, are the
	 * primary users of these fields, and in mm/page_alloc.c
	 * free_area_init_core() performs the initialization of them.
	 */
	wait_queue_head_t	* wait_table;
	unsigned long		wait_table_hash_nr_entries;
	unsigned long		wait_table_bits;

	/*
	 * Discontig memory support fields.
	 */
	struct pglist_data	*zone_pgdat;
	/* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */
	unsigned long		zone_start_pfn;

	/*
	 * spanned_pages is the total pages spanned by the zone, including
	 * holes, which is calculated as:
	 * 	spanned_pages = zone_end_pfn - zone_start_pfn;
	 *
	 * present_pages is physical pages existing within the zone, which
	 * is calculated as:
	 *	present_pages = spanned_pages - absent_pages(pages in holes);
	 *
	 * managed_pages is present pages managed by the buddy system, which
	 * is calculated as (reserved_pages includes pages allocated by the
	 * bootmem allocator):
	 *	managed_pages = present_pages - reserved_pages;
	 *
	 * So present_pages may be used by memory hotplug or memory power
	 * management logic to figure out unmanaged pages by checking
	 * (present_pages - managed_pages). And managed_pages should be used
	 * by page allocator and vm scanner to calculate all kinds of watermarks
	 * and thresholds.
	 *
	 * Locking rules:
	 *
	 * zone_start_pfn and spanned_pages are protected by span_seqlock.
	 * It is a seqlock because it has to be read outside of zone->lock,
	 * and it is done in the main allocator path.  But, it is written
	 * quite infrequently.
	 *
	 * The span_seq lock is declared along with zone->lock because it is
	 * frequently read in proximity to zone->lock.  It's good to
	 * give them a chance of being in the same cacheline.
	 *
	 * Write access to present_pages at runtime should be protected by
	 * lock_memory_hotplug()/unlock_memory_hotplug().  Any reader who can't
	 * tolerant drift of present_pages should hold memory hotplug lock to
	 * get a stable value.
	 *
	 * Read access to managed_pages should be safe because it's unsigned
	 * long. Write access to zone->managed_pages and totalram_pages are
	 * protected by managed_page_count_lock at runtime. Idealy only
	 * adjust_managed_page_count() should be used instead of directly
	 * touching zone->managed_pages and totalram_pages.
	 */
	unsigned long		spanned_pages;
	unsigned long		present_pages;
	unsigned long		managed_pages;

	/*
	 * Number of MIGRATE_RESEVE page block. To maintain for just
	 * optimization. Protected by zone->lock.
	 */
	int			nr_migrate_reserve_block;

	/*
	 * rarely used fields:
	 */
	const char		*name;
} ____cacheline_internodealigned_in_smp;

unsigned long
watermark[NR_WMARK];

——该数组有三个值WMARK_MINWMARK_LOWWMARK_HIGH,如命名所标识,min最小,low居中,high最大。内存分配过程中,当空闲页面达到low时,内存分配器会唤醒kswapd守护进程来回收物理页面;当空闲页面达到min时,内存分配器就会唤醒kswapd以同步方式回收;如果kswapd被唤醒后,空闲页面达到high时,则会使kswapd再次休眠;

unsigned long
percpu_drift_mark;

——当空闲页面低于该值,将会引发附加操作的执行,用于避免前面的watermark被冲破;

unsigned long
lowmem_reserve[MAX_NR_ZONES];

——记录每个管理区中必须保留的物理页面数,以用于紧急状况下的内存分配;

unsigned long
dirty_balance_reserve;

——用于表示不会被内存分配器分配出去的空闲页面部分的近似值;

struct per_cpu_pageset
__percpu *pageset;

——该数组里面的成员pcp用于实现冷热页面的管理;

spinlock_t lock;

——spinlock锁,用于解决该管理区的并发问题;

struct free_area
free_area[MAX_ORDER];

——主要用于Buddy内存管理算法(伙伴算法),里面有数量为MIGRATE_TYPES个的free_list链表,分别用于管理不同迁移类型的内存页面;

unsigned long
*pageblock_flags;

——与伙伴算法的碎片迁移算法有关;

spinlock_t lru_lock;

——用于保护lruvec结构数据;

struct lruvec lruvec;

——lruvec该数组里面有一个lists是用于lru管理的链表,另外有一个reclaim_stat用于页面回收的状态标示;

unsigned long pages_scanned;

——用于记录上次物理页面回收时,扫描过的页描述符总数;

unsigned long flags;

——用于表示当前内存管理区的状态;

atomic_long_t
vm_stat[NR_VM_ZONE_STAT_ITEMS];

——用于统计该内存管理区中各项状态的数值;

unsigned int inactive_ratio;

——不活跃的页面比例;

wait_queue_head_t *wait_table;

unsigned long
wait_table_hash_nr_entries;

unsigned long
wait_table_bits;

——当多个进程同时访问同一页面时,必然会有进程先行访问操作,此时该页面不可用,因此其他的则需阻塞等待。当页面不可用时,则会将页面进行hash运算加入到该管理区wait_table的哈希表中,当页面可用时将会把里面任务列表中等待的进程进行唤醒。如果存在多个页面有相同的hash值,那么这些等待不同页面的任务仍然会睡眠在同一个hash表节点下,当相同hash值的某个页面可用时,将会唤醒所有进程,当进程在唤醒时需要检查是否是自己所等待的页面。其中wait_table_hash_nr_entries表示该哈希表中等待队列的数量,

struct pglist_data *zone_pgdat;

——指向该内存管理区的pg_data_list

unsigned long zone_start_pfn;

——记录当前内存管理区中最小的物理页面号;

unsigned long spanned_pages;

——记录内存管理区的总页面数,包括内存空洞的页面数,实则上是管理区末尾页面号和起始页面号的差值;

unsigned long present_pages;

——除去内存空洞后的内存管理区实际有效的总页面数;

unsigned long managed_pages;

——用于记录被内存管理算法管理的物理页面数,这是除去了在初始化阶段被申请的页面;

int nr_migrate_reserve_block;

——用于优化的,记录内存迁移保留的页面数;

const char *name;

——用于记录该管理区的名字;

【file:/include/linux/mmzone.h】
/*
 * Each physical page in the system has a struct page associated with
 * it to keep track of whatever it is we are using the page for at the
 * moment. Note that we have no way to track which tasks are using
 * a page, though if it is a pagecache page, rmap structures can tell us
 * who is mapping it.
 *
 * The objects in struct page are organized in double word blocks in
 * order to allows us to use atomic double word operations on portions
 * of struct page. That is currently only used by slub but the arrangement
 * allows the use of atomic double word operations on the flags/mapping
 * and lru list pointers also.
 */
struct page {
	/* First double word block */
	unsigned long flags;		/* Atomic flags, some possibly
					 * updated asynchronously */
	union {
		struct address_space *mapping;	/* If low bit clear, points to
						 * inode address_space, or NULL.
						 * If page mapped as anonymous
						 * memory, low bit is set, and
						 * it points to anon_vma object:
						 * see PAGE_MAPPING_ANON below.
						 */
		void *s_mem;			/* slab first object */
	};

	/* Second double word */
	struct {
		union {
			pgoff_t index;		/* Our offset within mapping. */
			void *freelist;		/* sl[aou]b first free object */
			bool pfmemalloc;	/* If set by the page allocator,
						 * ALLOC_NO_WATERMARKS was set
						 * and the low watermark was not
						 * met implying that the system
						 * is under some pressure. The
						 * caller should try ensure
						 * this page is only used to
						 * free other pages.
						 */
		};

		union {
#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
	defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
			/* Used for cmpxchg_double in slub */
			unsigned long counters;
#else
			/*
			 * Keep _count separate from slub cmpxchg_double data.
			 * As the rest of the double word is protected by
			 * slab_lock but _count is not.
			 */
			unsigned counters;
#endif

			struct {

				union {
					/*
					 * Count of ptes mapped in
					 * mms, to show when page is
					 * mapped & limit reverse map
					 * searches.
					 *
					 * Used also for tail pages
					 * refcounting instead of
					 * _count. Tail pages cannot
					 * be mapped and keeping the
					 * tail page _count zero at
					 * all times guarantees
					 * get_page_unless_zero() will
					 * never succeed on tail
					 * pages.
					 */
					atomic_t _mapcount;

					struct { /* SLUB */
						unsigned inuse:16;
						unsigned objects:15;
						unsigned frozen:1;
					};
					int units;	/* SLOB */
				};
				atomic_t _count;		/* Usage count, see below. */
			};
			unsigned int active;	/* SLAB */
		};
	};

	/* Third double word block */
	union {
		struct list_head lru;	/* Pageout list, eg. active_list
					 * protected by zone->lru_lock !
					 */
		struct {		/* slub per cpu partial pages */
			struct page *next;	/* Next partial slab */
#ifdef CONFIG_64BIT
			int pages;	/* Nr of partial slabs left */
			int pobjects;	/* Approximate # of objects */
#else
			short int pages;
			short int pobjects;
#endif
		};

		struct list_head list;	/* slobs list of pages */
		struct slab *slab_page; /* slab fields */
		struct rcu_head rcu_head;	/* Used by SLAB
						 * when destroying via RCU
						 */
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
		pgtable_t pmd_huge_pte; /* protected by page->ptl */
#endif
	};

	/* Remainder is not double word aligned */
	union {
		unsigned long private;		/* Mapping-private opaque data:
					 	 * usually used for buffer_heads
						 * if PagePrivate set; used for
						 * swp_entry_t if PageSwapCache;
						 * indicates order in the buddy
						 * system if PG_buddy is set.
						 */
#if USE_SPLIT_PTE_PTLOCKS
#if ALLOC_SPLIT_PTLOCKS
		spinlock_t *ptl;
#else
		spinlock_t ptl;
#endif
#endif
		struct kmem_cache *slab_cache;	/* SL[AU]B: Pointer to slab */
		struct page *first_page;	/* Compound tail pages */
	};

	/*
	 * On machines where all RAM is mapped into kernel address space,
	 * we can simply calculate the virtual address. On machines with
	 * highmem some memory is mapped into kernel virtual memory
	 * dynamically, so we need a place to store that address.
	 * Note that this field could be 16 bits on x86 ... 😉
	 *
	 * Architectures with slow multiplication can define
	 * WANT_PAGE_VIRTUAL in asm/page.h
	 */
#if defined(WANT_PAGE_VIRTUAL)
	void *virtual;			/* Kernel virtual address (NULL if
					   not kmapped, ie. highmem) */
#endif /* WANT_PAGE_VIRTUAL */
#ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
	unsigned long debug_flags;	/* Use atomic bitops on this */
#endif

#ifdef CONFIG_KMEMCHECK
	/*
	 * kmemcheck wants to track the status of each byte in a page; this
	 * is a pointer to such a status block. NULL if not tracked.
	 */
	void *shadow;
#endif

#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
	int _last_cpupid;
#endif
}

(该结构很多union结构,主要是用于各种算法不同数据的空间复用,暂时记录部分常见的数据成员)

unsigned long flags;

——用于记录页框的类型;

struct address_space
*mapping;

——用于区分该页是映射页框还是匿名页框;

atomic_t _mapcount;

——记录了系统中页表有多少项指向该页;

atomic_t _count;

——当前系统对该页面的引用次数;

struct list_head lru;

——当页框处于分配状态时,该成员用于zonelruvec里面的list,当页框未被分配时则用于伙伴算法;

unsigned long private;

——指向私有数据的指针。根据页的用途,可以用不同的方式使用该指针,通常用于与数据缓冲区关联起来;

void *virtual;

——用于高端内存区域的页,即用于无法直接映射的页,该成员用于存储该页的虚拟地址;

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