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瀹屽杽浜哛EADME.md...
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@@ -70,27 +70,236 @@ Linux把物理内存划分为三个层次来管理
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#2 页帧
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-------
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内核把物理页作为内存管理的基本单位. 尽管处理器的最小可寻址单位通常是字, 但是, 内存管理单元MMU通常以页为单位进行处理. 因此，从虚拟内存的上来看，页就是最小单位. 内核用[struct page(include/linux/mm_types.h?v=4.7, line 45)](http://lxr.free-electrons.com/source/include/linux/mm_types.h?v4.7#L45)结构表示系统中的每个物理页.
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出于节省内存的考虑，struct page中使用了大量的联合体union.
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内核把物理页作为内存管理的基本单位. 尽管处理器的最小可寻址单位通常是字, 但是, 内存管理单元MMU通常以页为单位进行处理. 因此，从虚拟内存的上来看，页就是最小单位.
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##2.1 struct page结构
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-------
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内核用[struct page(include/linux/mm_types.h?v=4.7, line 45)](http://lxr.free-electrons.com/source/include/linux/mm_types.h?v4.7#L45)结构表示系统中的每个物理页.
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出于节省内存的考虑，struct page中使用了大量的联合体union.
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```cpp
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/*
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* Each physical page in the system has a struct page associated with
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* it to keep track of whatever it is we are using the page for at the
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* moment. Note that we have no way to track which tasks are using
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* a page, though if it is a pagecache page, rmap structures can tell us
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* who is mapping it.
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*
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* The objects in struct page are organized in double word blocks in
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* order to allows us to use atomic double word operations on portions
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* of struct page. That is currently only used by slub but the arrangement
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* allows the use of atomic double word operations on the flags/mapping
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* and lru list pointers also.
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*/
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struct page {
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/* First double word block */
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unsigned long flags; /* Atomic flags, some possibly updated asynchronously
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描述page的状态和其他信息 */
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union
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{
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struct address_space *mapping; /* If low bit clear, points to
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* inode address_space, or NULL.
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* If page mapped as anonymous
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* memory, low bit is set, and
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* it points to anon_vma object:
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* see PAGE_MAPPING_ANON below.
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*/
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void *s_mem; /* slab first object */
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atomic_t compound_mapcount; /* first tail page */
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/* page_deferred_list().next -- second tail page */
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};
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/* Second double word */
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struct {
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union {
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pgoff_t index; /* Our offset within mapping.
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在映射的虚拟空间（vma_area）内的偏移；
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一个文件可能只映射一部分，假设映射了1M的空间，
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index指的是在1M空间内的偏移，而不是在整个文件内的偏移。 */
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void *freelist; /* sl[aou]b first free object */
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/* page_deferred_list().prev -- second tail page */
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};
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union {
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#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
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defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
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/* Used for cmpxchg_double in slub */
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unsigned long counters;
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#else
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/*
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* Keep _refcount separate from slub cmpxchg_double
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* data. As the rest of the double word is protected by
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* slab_lock but _refcount is not.
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*/
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unsigned counters;
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#endif
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struct {
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union {
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/*
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* Count of ptes mapped in mms, to show
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* when page is mapped & limit reverse
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* map searches.
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* 页映射计数器
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*/
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atomic_t _mapcount;
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struct { /* SLUB */
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unsigned inuse:16;
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unsigned objects:15;
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unsigned frozen:1;
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};
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int units; /* SLOB */
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};
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/*
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* Usage count, *USE WRAPPER FUNCTION*
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* when manual accounting. See page_ref.h
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* 页引用计数器
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*/
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atomic_t _refcount;
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};
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unsigned int active; /* SLAB */
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};
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};
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/*
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* Third double word block
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*
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* WARNING: bit 0 of the first word encode PageTail(). That means
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* the rest users of the storage space MUST NOT use the bit to
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* avoid collision and false-positive PageTail().
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*/
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union {
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struct list_head lru; /* Pageout list, eg. active_list
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* protected by zone->lru_lock !
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* Can be used as a generic list
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* by the page owner.
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*/
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struct dev_pagemap *pgmap; /* ZONE_DEVICE pages are never on an
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* lru or handled by a slab
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* allocator, this points to the
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* hosting device page map.
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*/
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struct { /* slub per cpu partial pages */
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struct page *next; /* Next partial slab */
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#ifdef CONFIG_64BIT
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int pages; /* Nr of partial slabs left */
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int pobjects; /* Approximate # of objects */
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#else
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short int pages;
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short int pobjects;
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#endif
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};
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struct rcu_head rcu_head; /* Used by SLAB
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* when destroying via RCU
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*/
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/* Tail pages of compound page */
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struct {
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unsigned long compound_head; /* If bit zero is set */
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/* First tail page only */
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#ifdef CONFIG_64BIT
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/*
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* On 64 bit system we have enough space in struct page
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* to encode compound_dtor and compound_order with
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* unsigned int. It can help compiler generate better or
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* smaller code on some archtectures.
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*/
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unsigned int compound_dtor;
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unsigned int compound_order;
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#else
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unsigned short int compound_dtor;
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unsigned short int compound_order;
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#endif
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};
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#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
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struct {
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unsigned long __pad; /* do not overlay pmd_huge_pte
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* with compound_head to avoid
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* possible bit 0 collision.
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*/
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pgtable_t pmd_huge_pte; /* protected by page->ptl */
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};
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#endif
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};
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/* Remainder is not double word aligned */
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union {
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unsigned long private; /* Mapping-private opaque data:
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* usually used for buffer_heads
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* if PagePrivate set; used for
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* swp_entry_t if PageSwapCache;
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* indicates order in the buddy
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* system if PG_buddy is set.
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* 私有数据指针，由应用场景确定其具体的含义
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*/
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#if USE_SPLIT_PTE_PTLOCKS
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#if ALLOC_SPLIT_PTLOCKS
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spinlock_t *ptl;
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#else
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spinlock_t ptl;
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#endif
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#endif
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struct kmem_cache *slab_cache; /* SL[AU]B: Pointer to slab */
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};
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#ifdef CONFIG_MEMCG
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struct mem_cgroup *mem_cgroup;
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#endif
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/*
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* On machines where all RAM is mapped into kernel address space,
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* we can simply calculate the virtual address. On machines with
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* highmem some memory is mapped into kernel virtual memory
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* dynamically, so we need a place to store that address.
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* Note that this field could be 16 bits on x86 ... ;)
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*
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* Architectures with slow multiplication can define
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* WANT_PAGE_VIRTUAL in asm/page.h
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*/
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#if defined(WANT_PAGE_VIRTUAL)
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void *virtual; /* Kernel virtual address (NULL if
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not kmapped, ie. highmem) */
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#endif /* WANT_PAGE_VIRTUAL */
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#ifdef CONFIG_KMEMCHECK
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/*
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* kmemcheck wants to track the status of each byte in a page; this
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* is a pointer to such a status block. NULL if not tracked.
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*/
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void *shadow;
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#endif
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#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
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int _last_cpupid;
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#endif
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}
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/*
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* The struct page can be forced to be double word aligned so that atomic ops
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* on double words work. The SLUB allocator can make use of such a feature.
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*/
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#ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE
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__aligned(2 * sizeof(unsigned long))
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#endif
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;
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```
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| 字段 | 描述 |
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|:---:|:----:|
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| flag | 用来存放页的状态，每一位代表一种状态，所以至少可以同时表示出32中不同的状态,这些状态定义在linux/page-flags.h中 |
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| mapping | 指向与该页相关的address_space对象
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| virtual | 页的虚拟地址，它就是页在虚拟内存中的地址。要理解的一点是page结构与物理页相关，而并非与虚拟页相关。因此，该结构对页的描述是短暂的。内核仅仅用这个结构来描述当前时刻在相关的物理页中存放的东西。这种数据结构的目的在于描述物理内存本身，而不是描述包含在其中的数据 |
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| virtual | 对于如果物理内存可以直接映射内核的系统, 我们可以之间映射出虚拟地址与物理地址的管理, 但是对于需要使用高端内存区域的页, 即无法直接映射到内核的虚拟地址空间, 因此需要用virtual保存该页的虚拟地址 |
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| _refcount | 引用计数，表示内核中引用该page的次数, 如果要操作该page, 引用计数会+1, 操作完成-1. 当该值为0时, 表示没有引用该page的位置，所以该page可以被解除映射，这往往在内存回收时是有用的 |
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| _mapcount | 被页表映射的次数，也就是说该page同时被多少个进程共享。初始值为-1，如果只被一个进程的页表映射了，该值为0. 如果该page处于伙伴系统中，该值为PAGE_BUDDY_MAPCOUNT_VALUE（-128），内核通过判断该值是否为PAGE_BUDDY_MAPCOUNT_VALUE来确定该page是否属于伙伴系统 |
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| index | 在映射的虚拟空间（vma_area）内的偏移；一个文件可能只映射一部分，假设映射了1M的空间，index指的是在1M空间内的偏移，而不是在整个文件内的偏移 |
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| private | 私有数据指针，由应用场景确定其具体的含义 |
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| lru | LRU算法的链表头，主要有3个用途: 伙伴算法, slab分配器, 被用户态使用或被当做页缓存使用 |
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| lru |链表头，用于在各种链表上维护该页, 以便于按页将不同类别分组, 主要有3个用途: 伙伴算法, slab分配器, 被用户态使用或被当做页缓存使用 |
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| mapping | 指向与该页相关的address_space对象 |
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| index | 页帧在映射内部的偏移量
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>注意区分_count和_mapcount，_mapcount表示的是映射次数，而_count表示的是使用次数；被映射了不一定在使用，但要使用必须先映射。
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@@ -98,14 +307,72 @@ Linux把物理内存划分为三个层次来管理
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#2.2 页面的状态flags
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##2.2 mapping & index
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-------
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page->flags描述page的状态和其他信息
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mapping指定了页帧所在的地址空间, index是页帧在映射内部的偏移量. 地址空间是一个非常一般的概念. 例如, 可以用在向内存读取文件时. 地址空间用于将文件的内容与装载数据的内存区关联起来. mapping不仅能够保存一个指针, 而且还能包含一些额外的信息, 用于判断页是否属于未关联到地址空间的某个匿名内存区.
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1. 如果mapping = 0，说明该page属于交换高速缓存页（swap cache）；当需要使用地址空间时会指定交换分区的地址空间swapper_space。
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2. 如果mapping != 0，第0位bit[0] = 0，说明该page属于页缓存或文件映射，mapping指向文件的地址空间address_space。
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3. 如果mapping != 0，第0位bit[0] != 0，说明该page为匿名映射，mapping指向struct anon_vma对象。
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通过mapping恢复anon_vma的方法：anon_vma = (struct anon_vma *)(mapping - PAGE_MAPPING_ANON)。
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pgoff_t index是该页描述结构在地址空间radix树page_tree中的对象索引号即页号, 表示该页在vm_file中的偏移页数, 其类型pgoff_t被定义为unsigned long即一个机器字长.
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```cpp
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/*
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* The type of an index into the pagecache.
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*/
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#define pgoff_t unsigned long
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```
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##2.3 private私有数据指针
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-------
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private私有数据指针, 由应用场景确定其具体的含义：
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1. 如果设置了PG_private标志，则private字段指向struct buffer_head
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2. 如果设置了PG_compound，则指向struct page
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3. 如果设置了PG_swapcache标志，private存储了该page在交换分区中对应的位置信息swp_entry_t。
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4. 如果_mapcount = PAGE_BUDDY_MAPCOUNT_VALUE，说明该page位于伙伴系统，private存储该伙伴的阶
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##2.4 lru链表头
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-------
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最近、最久未使用struct slab结构指针变量
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lru：链表头，主要有3个用途：
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1. 则page处于伙伴系统中时，用于链接相同阶的伙伴（只使用伙伴中的第一个page的lru即可达到目的）。
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2. 设置PG_slab, 则page属于slab，page->lru.next指向page驻留的的缓存的管理结构，page->lru.prec指向保存该page的slab的管理结构。
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3. page被用户态使用或被当做页缓存使用时，用于将该page连入zone中相应的lru链表，供内存回收时使用。
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#3 体系结构无关的页面的状态flags
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-------
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页的不同属性通过一系列页标志描述, 存储在struct page的flag成员中的各个比特位.
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```cpp
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struct page {
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/* First double word block */
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@@ -113,6 +380,9 @@ struct page {
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some possibly updated asynchronously, 描述page的状态和其他信息 */
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```
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这些标识是独立于体系结构的, 因而无法通过特定于CPU或计算机的信息(该信息保存在页表中)
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如下如所示
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@@ -131,18 +401,18 @@ struct page {
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| 页面状态 | 描述 |
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|:-------:|:----:|
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| PG_locked | page被锁定，说明有使用者正在操作该page |
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| PG_error | 状态标志，表示涉及该page的IO操作发生了错误 |
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| PG_locked | 指定了页是否被锁定, 如果该比特未被置位, 说明有使用者正在操作该page, 则内核的其他部分不允许访问该页， 这可以防止内存管理出现竞态条件 |
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| PG_error | 如果涉及该page的I/O操作发生了错误, 则该位被设置 |
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| PG_referenced | 表示page刚刚被访问过 |
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| PG_uptodate | 表示page的数据已经与后备存储器是同步的，是最新的 |
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| PG_dirty | 与后备存储器中的数据相比，该page的内容已经被修改 |
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| PG_lru | 表示该page处于LRU链表上 |
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| PG_uptodate | 表示page的数据已经与后备存储器是同步的, 即页的数据已经从块设备读取，且没有出错,数据是最新的 |
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| PG_dirty | 与后备存储器中的数据相比，该page的内容已经被修改. 出于性能能的考虑，页并不在每次改变后立即回写, 因此内核需要使用该标识来表明页面中的数据已经改变, 应该在稍后刷出 |
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| PG_lru | 表示该page处于LRU链表上， 这有助于实现页面的回收和切换. 内核使用两个最近最少使用(least recently used-LRU)链表来区别活动和不活动页. 如果页在其中一个链表中, 则该位被设置 |
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| PG_active | page处于inactive LRU链表, PG_active和PG_referenced一起控制该page的活跃程度，这在内存回收时将会非常有用 |
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| PG_slab | 该page属于slab分配器 |
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| PG_onwer_priv_1 | |
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| PG_arch_1 | |
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| PG_reserved | 设置该标志，防止该page被交换到swap |
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| PG_private | 如果page中的private成员非空，则需要设置该标志 |
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| PG_private | 如果page中的private成员非空，则需要设置该标志, 用于I/O的页可使用该字段将页细分为多核缓冲区 |
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| PG_private_2 | |
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| PG_writeback | page中的数据正在被回写到后备存储器 |
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| PG_head | |
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||||
@@ -157,100 +427,17 @@ struct page {
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| PG_young | |
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||||
| PG_idle | |
|
||||
|
||||
内核中提供了一些标准宏，用来检查、操作某些特定的比特位，如：
|
||||
-> PageXXX(page)：检查page是否设置了PG_XXX位
|
||||
-> SetPageXXX(page)：设置page的PG_XXX位
|
||||
-> ClearPageXXX(page)：清除page的PG_XXX位
|
||||
-> TestSetPageXXX(page)：设置page的PG_XXX位，并返回原值
|
||||
-> TestClearPageXXX(page)：清除page的PG_XXX位，并返回原值
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
##2.3 mapping：有三种含义
|
||||
-------
|
||||
|
||||
|
||||
mappind是该页所在地址空间描述结构指针
|
||||
|
||||
|
||||
1. 如果mapping = 0，说明该page属于交换高速缓存页（swap cache）；当需要使用地址空间时会指定交换分区的地址空间swapper_space。
|
||||
|
||||
2. 如果mapping != 0，第0位bit[0] = 0，说明该page属于页缓存或文件映射，mapping指向文件的地址空间address_space。
|
||||
|
||||
3. 如果mapping != 0，第0位bit[0] != 0，说明该page为匿名映射，mapping指向struct anon_vma对象。
|
||||
|
||||
|
||||
通过mapping恢复anon_vma的方法：anon_vma = (struct anon_vma *)(mapping - PAGE_MAPPING_ANON)。
|
||||
|
||||
##2.4 private私有数据指针
|
||||
-------
|
||||
|
||||
private私有数据指针, 由应用场景确定其具体的含义：
|
||||
|
||||
|
||||
1. 如果设置了PG_private标志，则private字段指向struct buffer_head
|
||||
|
||||
2. 如果设置了PG_compound，则指向struct page
|
||||
|
||||
|
||||
3. 如果设置了PG_swapcache标志，private存储了该page在交换分区中对应的位置信息swp_entry_t。
|
||||
|
||||
4. 如果_mapcount = PAGE_BUDDY_MAPCOUNT_VALUE，说明该page位于伙伴系统，private存储该伙伴的阶
|
||||
|
||||
##2.5 lru链表头
|
||||
-------
|
||||
|
||||
最近、最久未使用struct slab结构指针变量
|
||||
|
||||
lru：链表头，主要有3个用途：
|
||||
|
||||
1. 则page处于伙伴系统中时，用于链接相同阶的伙伴（只使用伙伴中的第一个page的lru即可达到目的）。
|
||||
|
||||
2. 设置PG_slab, 则page属于slab，page->lru.next指向page驻留的的缓存的管理结构，page->lru.prec指向保存该page的slab的管理结构。
|
||||
|
||||
3. page被用户态使用或被当做页缓存使用时，用于将该page连入zone中相应的lru链表，供内存回收时使用。
|
||||
|
||||
|
||||
##2.6 index索引
|
||||
-------
|
||||
|
||||
|
||||
pgoff_t index是该页描述结构在地址空间radix树page_tree中的对象索引号即页号, 表示该页在vm_file中的偏移页数
|
||||
```cpp
|
||||
#define unsigned long pgoff_t
|
||||
```
|
||||
|
||||
|
||||
#struct pagevec
|
||||
-------
|
||||
内核中提供了一些标准宏，用来检查、操作某些特定的比特位，如
|
||||
|
||||
```c
|
||||
struct pagevec { //页向量描述结构
|
||||
unsigned long nr; //该页向量中的内存页数
|
||||
unsigned long cold; //冷区标志，0表示热区，非0表示冷区
|
||||
struct page *pages[PAGEVEC_SIZE];
|
||||
//该也想两种的页描述结构指针数组（PAGEVEC_SIZE=14）
|
||||
};
|
||||
```
|
||||
PageXXX(page)：检查page是否设置了PG_XXX位
|
||||
SetPageXXX(page)：设置page的PG_XXX位
|
||||
ClearPageXXX(page)：清除page的PG_XXX位
|
||||
TestSetPageXXX(page)：设置page的PG_XXX位，并返回原值
|
||||
TestClearPageXXX(page)：清除page的PG_XXX位，并返回原值
|
||||
|
||||
|
||||
|
||||
|
||||
#struct page_address_map
|
||||
|
||||
```cpp
|
||||
struct page_address_map { //页地址映射
|
||||
struct page *page; //页的描述结构
|
||||
void *virtual; //页的虚拟地址
|
||||
struct list_head list; //通过list字段链接到页表池全局链表page_address_pool中或page_address_htable[hash_ptr(page,PA_HASH_ORDER)].lh
|
||||
};
|
||||
```
|
||||
|
||||
#struct page_address_slot
|
||||
------
|
||||
```cpp
|
||||
static struct page_address_slot {
|
||||
struct list_head lh; /* List of page_address_maps */
|
||||
spinlock_t lock; /* Protect this bucket's list */
|
||||
} ____cacheline_aligned_in_smp page_address_htable[1<<PA_HASH_ORDER];
|
||||
```
|
||||
Reference in New Issue
Block a user