linux内存管理（八）- 反向映射RMAP

这里有一篇博客讲的不错。linux内存管理笔记(三十八）----反向映射_linux 反向映射-CSDN博客

页表是把虚拟地址映射到物理页面，但是如何根据一个物理页找到所有映射它的pte呢？答案是用反向映射Reverse Mapping（RMAP)。这在页面回收中很有用。回收页面需要将到物理页的映射断开（改一下pte），前提是找到所有映射的pte，那就必须在page结构中留下线索，这就是mapping字段。

在讲page结构时我们提到过mapping可以指向匿名页的anon_vma结构，这个结构是反向映射的关键之一。

struct anon_vma {
    struct anon_vma *root;        /* Root of this anon_vma tree */
    struct rw_semaphore rwsem;    /* W: modification, R: walking the list */
    atomic_t refcount;
    unsigned long num_children;
    /* Count of VMAs whose ->anon_vma pointer points to this object. */
    unsigned long num_active_vmas;
    struct anon_vma *parent;    /* Parent of this anon_vma */
    struct rb_root_cached rb_root;  //avc结构会链接到这棵rbtree上
};

除了anon_vma（AV）还有一个AVC（anon_vma_chain)结构。

/*
 * 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_lock & 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
};

看注释可知AVC可以通过rb_node链接到anon_vma的rbtree上，这个rbtree会链接所有与AV相关的VMA。也可以通过same_vma链表链接所有与该VMA相关的AV。AVC是anon_vma和vma的枢纽，可以让三者相互找到另外两方。

上图是一个简单的情形，描述三者之间的关系，这对理解后面反向映射的应用很重要。

反向映射的应用。

使用反向映射最常见的是在回收页面，try_to_unmap是其重要函数。

void try_to_unmap(struct folio *folio, enum ttu_flags flags)
{
    struct rmap_walk_control rwc = {
        .rmap_one = try_to_unmap_one,
        .arg = (void *)flags,
        .done = folio_not_mapped,
        .anon_lock = folio_lock_anon_vma_read,
    };

    if (flags & TTU_RMAP_LOCKED)
        rmap_walk_locked(folio, &rwc);
    else
        rmap_walk(folio, &rwc);
}

rmap_walk

void rmap_walk(struct folio *folio, struct rmap_walk_control *rwc)
{
    if (unlikely(folio_test_ksm(folio)))
        rmap_walk_ksm(folio, rwc);
    else if (folio_test_anon(folio))
        rmap_walk_anon(folio, rwc, false);
    else
        rmap_walk_file(folio, rwc, false);
}

只看rmap_walk_anon

static void rmap_walk_anon(struct folio *folio,
        struct rmap_walk_control *rwc, bool locked)
{
    struct anon_vma *anon_vma;
    pgoff_t pgoff_start, pgoff_end;
    struct anon_vma_chain *avc;

    if (locked) {
        anon_vma = folio_anon_vma(folio);
        /* anon_vma disappear under us? */
        VM_BUG_ON_FOLIO(!anon_vma, folio);
    } else {
        //从folio中获取anon_vma
        anon_vma = rmap_walk_anon_lock(folio, rwc);
    }
    if (!anon_vma)
        return;
    //page->index是page在file或内存区域中的偏移，单位是page
    pgoff_start = folio_pgoff(folio);
    //得到复合页的末尾index
    pgoff_end = pgoff_start + folio_nr_pages(folio) - 1;
    //遍历avc
    anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root,
            pgoff_start, pgoff_end) {
        struct vm_area_struct *vma = avc->vma;
        //得到page的虚拟地址
        unsigned long address = vma_address(&folio->page, vma);

        VM_BUG_ON_VMA(address == -EFAULT, vma);
        cond_resched();

        if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
            continue;
        //使用回调函数rmap_one断开虚拟地址对应的pte
        if (!rwc->rmap_one(folio, vma, address, rwc->arg))
            break;
        if (rwc->done && rwc->done(folio))
            break;
    }

    if (!locked)
        anon_vma_unlock_read(anon_vma);
}

rmap_walk_anon遍历anon_vma对应的rbtree，找到所有映射该页的vma，然后断开物理页对应虚拟地址的pte。

这里要理解page->index和vma->pgoff和vma->vm_start的关系。

由上图可以看到page->index指的是在整个文件或者进程地址空间（不一定是vma）上的偏移，要想得到它在当前vma内的偏移公式是vma->vm_start + (page->index - vma->vm_pgoff) << PAGE_SHIFT.