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_cgroup是page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_MIN、WMARK_LOW、WMARK_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zone的lruvec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;