2024-04-02  阅读(29)
原文作者:LoyenWang 原文地址: https://www.cnblogs.com/LoyenWang/p/12164683.html

背景

  • Read the fucking source code! --By 鲁迅
  • A picture is worth a thousand words. --By 高尔基

说明:

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

1. 概述

RMAP反向映射是一种物理地址反向映射虚拟地址的方法。

  • 映射
    页表用于虚拟地址到物理地址映射,其中的PTE页表项记录了映射关系,同时struct page结构体中的mapcount字段保存了有多少PTE页表项映射了该物理页。
  • 反向映射
    当某个物理地址要进行回收或迁移时,此时需要去找到有多少虚拟地址射在该物理地址,并断开映射处理。在没有反向映射的机制时,需要去遍历进程的页表,这个效率显然是很低下的。反向映射可以找到虚拟地址空间VMA,并仅从VMA使用的用户页表中取消映射,可以快速解决这个问题。

202404022326203301.png

反向映射的典型应用场景:

  1. kswapd进行页面回收时,需要断开所有映射了该匿名页面的PTE表项;
  2. 页面迁移时,需要断开所有映射了该匿名页面的PTE表项;

2. 数据结构

反向映射有三个关键的结构体:

  1. struct vm_area_struct,简称VMA;
    VMA我们在之前的文章中介绍过,用于描述进程地址空间中的一段区域。与反向映射相关的字段如下:
    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,简称AV;
    AV结构用于管理匿名类型VMAs,当有匿名页需要unmap处理时,可以先找到AV,然后再通过AV进行查找处理。结构如下:
    /*
     * 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,简称AVC;
    AVC是连接VMAAV之间的桥梁。
    /*
     * 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
    };

来一张图就清晰明了了:

202404022326205652.png

  • 通过same_vma链表节点,将anon_vma_chain添加到vma->anon_vma_chain链表中;
  • 通过rb红黑树节点,将anon_vma_chain添加到anon_vma->rb_root的红黑树中;

2. 流程分析

先看一下宏观的图:

202404022326208163.png

  • 地址空间VMA可以通过页表完成虚拟地址到物理地址的映射;
  • 页框与page结构对应,page结构中的mapping字段指向anon_vma,从而可以通过RMAP机制去找到与之关联的VMA

2.1 anon_vma_prepare

之前在page fault的文章中,提到过anon_vma_prepare函数,这个函数完成的工作就是为进程地址空间中的VMA准备struct anon_vma结构。

调用例程及函数流程如下图所示:

202404022326210614.png

至于VMA,AV,AVC三者之间的关联关系,在上文的图中已经有所描述。

当创建了与VMA关联的AV后,还有关键的一步需要做完,才能算是真正的把RMAP通路打通,那就是让pageAV关联起来。只有这样才能通过page找到AV,进而找到VMA,从而完成对应的PTE unmap操作。

202404022326212865.png

2.2 子进程创建anon_vma

父进程通过fork()来创建子进程,子进程会复制整个父进程的地址空间及页表。子进程拷贝了父进程的VMA数据结构内容,而子进程创建相应的anon_vma结构,是通过anon_vma_fork()函数来实现的。

anon_vma_fork()效果图如下:

202404022326214976.png

以实际fork()两次为例,发生COW之后,看看三个进程的链接关系,如下图:

202404022326217277.png

2.3 TTU(try to unmap)Rmap Walk

如果有page被映射到多个虚拟地址,可以通过Rmap Walk机制来遍历所有的VMA,并最终调用回调函数来取消映射。

与之相关的结构体为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);
    };

202404022326219798.png

取消映射的入口为try_to_unmap,流程如下图所示:

202404022326222519.png

基本的套路就是围绕着struct rmap_walk_control结构,初始化回调函数,以便在适当的时候能调用到。

关于取消映射try_to_unmap_one的详细细节就不进一步深入了,把握好大体框架即可。

2024040223262250210.png

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