【原創】（十五）Linux內存管理之RMAP

背景

• Read the fucking source code! --By 魯迅
• A picture is worth a thousand words. --By 高爾基

說明：

1. Kernel版本：4.14
2. ARM64處理器，Contex-A53，雙核
3. 使用工具：Source Insight 3.5， Visio

1. 概述

RMAP反向映射是一種物理地址反向映射虛擬地址的方法。

• 映射
  頁表用於虛擬地址到物理地址映射，其中的PTE頁表項記錄了映射關係，同時struct page結構體中的mapcount字段保存了有多少PTE頁表項映射了該物理頁。

• 反向映射
  當某個物理地址要進行回收或遷移時，此時需要去找到有多少虛擬地址射在該物理地址，並斷開映射處理。在沒有反向映射的機制時，需要去遍歷進程的頁表，這個效率顯然是很低下的。反向映射可以找到虛擬地址空間VMA，並僅從VMA使用的用戶頁表中取消映射，可以快速解決這個問題。

反向映射的典型應用場景：

1. kswapd進行頁面回收時，需要斷開所有映射了該匿名頁面的PTE表項；
2. 頁面遷移時，需要斷開所有映射了該匿名頁面的PTE表項；

2. 數據結構

反向映射有三個關鍵的結構體：

1. struct vm_area_struct，簡稱VMA;
   VMA我們在之前的文章中介紹過，用於描述進程地址空間中的一段區域。與反向映射相關的字段如下：

struct vm_area_struct {
...
/*
     * A file's MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
     * list, after a COW of one of the file pages.  A MAP_SHARED vma
     * can only be in the i_mmap tree.  An anonymous MAP_PRIVATE, stack
     * or brk vma (with NULL file) can only be in an anon_vma list.
     */
    struct list_head anon_vma_chain; /* Serialized by mmap_sem &
                      * page_table_lock */
    struct anon_vma *anon_vma;  /* Serialized by page_table_lock */
...
}

1. struct anon_vma，簡稱AV;
   AV結構用於管理匿名類型VMAs，當有匿名頁需要unmap處理時，可以先找到AV，然後再通過AV進行查找處理。結構如下：

/*
 * The anon_vma heads a list of private "related" vmas, to scan if
 * an anonymous page pointing to this anon_vma needs to be unmapped:
 * the vmas on the list will be related by forking, or by splitting.
 *
 * Since vmas come and go as they are split and merged (particularly
 * in mprotect), the mapping field of an anonymous page cannot point
 * directly to a vma: instead it points to an anon_vma, on whose list
 * the related vmas can be easily linked or unlinked.
 *
 * After unlinking the last vma on the list, we must garbage collect
 * the anon_vma object itself: we're guaranteed no page can be
 * pointing to this anon_vma once its vma list is empty.
 */
struct anon_vma {
    struct anon_vma *root;      /* Root of this anon_vma tree */
    struct rw_semaphore rwsem;  /* W: modification, R: walking the list */
    /*
     * The refcount is taken on an anon_vma when there is no
     * guarantee that the vma of page tables will exist for
     * the duration of the operation. A caller that takes
     * the reference is responsible for clearing up the
     * anon_vma if they are the last user on release
     */
    atomic_t refcount;

    /*
     * Count of child anon_vmas and VMAs which points to this anon_vma.
     *
     * This counter is used for making decision about reusing anon_vma
     * instead of forking new one. See comments in function anon_vma_clone.
     */
    unsigned degree;

    struct anon_vma *parent;    /* Parent of this anon_vma */

    /*
     * NOTE: the LSB of the rb_root.rb_node is set by
     * mm_take_all_locks() _after_ taking the above lock. So the
     * rb_root must only be read/written after taking the above lock
     * to be sure to see a valid next pointer. The LSB bit itself
     * is serialized by a system wide lock only visible to
     * mm_take_all_locks() (mm_all_locks_mutex).
     */

    /* Interval tree of private "related" vmas */
    struct rb_root_cached rb_root;
};

1. struct anon_vma_chain，簡稱AVC;
   AVC是連接VMA和AV之間的橋樑。

/*
 * The copy-on-write semantics of fork mean that an anon_vma
 * can become associated with multiple processes. Furthermore,
 * each child process will have its own anon_vma, where new
 * pages for that process are instantiated.
 *
 * This structure allows us to find the anon_vmas associated
 * with a VMA, or the VMAs associated with an anon_vma.
 * The "same_vma" list contains the anon_vma_chains linking
 * all the anon_vmas associated with this VMA.
 * The "rb" field indexes on an interval tree the anon_vma_chains
 * which link all the VMAs associated with this anon_vma.
 */
struct anon_vma_chain {
    struct vm_area_struct *vma;
    struct anon_vma *anon_vma;
    struct list_head same_vma;   /* locked by mmap_sem & page_table_lock */
    struct rb_node rb;          /* locked by anon_vma->rwsem */
    unsigned long rb_subtree_last;
#ifdef CONFIG_DEBUG_VM_RB
    unsigned long cached_vma_start, cached_vma_last;
#endif
};

來一張圖就清晰明瞭了：

• 通過same_vma鏈表節點，將anon_vma_chain添加到vma->anon_vma_chain鏈表中；
• 通過rb紅黑樹節點，將anon_vma_chain添加到anon_vma->rb_root的紅黑樹中；

2. 流程分析

先看一下宏觀的圖：

• 地址空間VMA可以通過頁表完成虛擬地址到物理地址的映射；
• 頁框與page結構對應，page結構中的mapping字段指向anon_vma，從而可以通過RMAP機制去找到與之關聯的VMA；

2.1 anon_vma_prepare

之前在page fault的文章中，提到過anon_vma_prepare函數，這個函數完成的工作就是爲進程地址空間中的VMA準備struct anon_vma結構。

調用例程及函數流程如下圖所示：

至於VMA，AV，AVC三者之間的關聯關係，在上文的圖中已經有所描述。

當創建了與VMA關聯的AV後，還有關鍵的一步需要做完，才能算是真正的把RMAP通路打通，那就是讓page與AV關聯起來。只有這樣才能通過page找到AV，進而找到VMA，從而完成對應的PTE unmap操作。

2.2 子進程創建anon_vma

父進程通過fork()來創建子進程，子進程會複製整個父進程的地址空間及頁表。子進程拷貝了父進程的VMA數據結構內容，而子進程創建相應的anon_vma結構，是通過anon_vma_fork()函數來實現的。

anon_vma_fork()效果圖如下：

以實際fork()兩次爲例，發生COW之後，看看三個進程的鏈接關係，如下圖：

2.3 TTU（try to unmap）和Rmap Walk

如果有page被映射到多個虛擬地址，可以通過Rmap Walk機制來遍歷所有的VMA，並最終調用回調函數來取消映射。

與之相關的結構體爲struct rmap_walk_control，如下：

/*
 * rmap_walk_control: To control rmap traversing for specific needs
 *
 * arg: passed to rmap_one() and invalid_vma()
 * rmap_one: executed on each vma where page is mapped
 * done: for checking traversing termination condition
 * anon_lock: for getting anon_lock by optimized way rather than default
 * invalid_vma: for skipping uninterested vma
 */
struct rmap_walk_control {
    void *arg;
    /*
     * Return false if page table scanning in rmap_walk should be stopped.
     * Otherwise, return true.
     */
    bool (*rmap_one)(struct page *page, struct vm_area_struct *vma,
                    unsigned long addr, void *arg);
    int (*done)(struct page *page);
    struct anon_vma *(*anon_lock)(struct page *page);
    bool (*invalid_vma)(struct vm_area_struct *vma, void *arg);
};

取消映射的入口爲try_to_unmap，流程如下圖所示：

基本的套路就是圍繞着struct rmap_walk_control結構，初始化回調函數，以便在適當的時候能調用到。

關於取消映射try_to_unmap_one的詳細細節就不進一步深入了，把握好大體框架即可。