1. migrate_pages系统调用

Linux提供了migrate_pages系统调用，从old_nodes中获取原内存节点，从new_nodes中获取目的内存节点；然后将当前进程的mm_struct作为参数，调用do_migrate_pages进行迁移操作。

SYSCALL_DEFINE4(migrate_pages, pid_t, pid, unsigned long, maxnode,        const unsigned long __user *, old_nodes,        const unsigned long __user *, new_nodes){    const struct cred *cred = current_cred(), *tcred;    struct mm_struct *mm = NULL;    struct task_struct *task;    nodemask_t task_nodes;    int err;    nodemask_t *old;    nodemask_t *new;    NODEMASK_SCRATCH(scratch);    if (!scratch)        return -ENOMEM;    old = &scratch->mask1;    new = &scratch->mask2;    err = get_nodes(old, old_nodes, maxnode);    if (err)        goto out;    err = get_nodes(new, new_nodes, maxnode);    if (err)        goto out;...    mm = get_task_mm(task);    put_task_struct(task);    if (!mm) {        err = -EINVAL;        goto out;    }    err = do_migrate_pages(mm, old, new,        capable(CAP_SYS_NICE) ? MPOL_MF_MOVE_ALL : MPOL_MF_MOVE);...}

do_migrate_pages最终将迁移工作交给migrate_pages来处理：do_migrate_pages-->migrate_to_node-->migrate_pages。

2. migrate_pages

-------------------------------------页面迁移核心函数    unmap_and_move        get_new_page------------------------------分配新页面        --------------------------迁移页面到新页面                            ----------------------找到页面对应的地址空间                ----------------------将旧页面的相关信息迁移到新页面                    migrate_page_copy                -------------利用方向映射找到映射旧页面的每个PTE                    remove_migration_pte----------处理其中一个虚拟地址

from表示将要迁移的页面链表，get_new_page是内存函数指针，put_new_page是迁移失败时释放目标页面的函数指针，private是传递给get_new_page的参数，mode是迁移模式，reason表示迁移原因。

int migrate_pages(struct list_head *from, new_page_t get_new_page,        free_page_t put_new_page, unsigned long private,        enum migrate_mode mode, int reason){    int retry = 1;    int nr_failed = 0;    int nr_succeeded = 0;    int pass = 0;    struct page *page;    struct page *page2;    int swapwrite = current->flags & PF_SWAPWRITE;    int rc;    if (!swapwrite)        current->flags |= PF_SWAPWRITE;    for(pass = 0; pass < 10 && retry; pass++) {--------------------------尝试10次，从from摘取一个页面，调用unmap_and_move()函数进行页迁移，返回MIGRATEPAGE_SUCCESS表示页迁移成功。        retry = 0;        list_for_each_entry_safe(page, page2, from, lru) {            cond_resched();            if (PageHuge(page))                rc = unmap_and_move_huge_page(get_new_page,                        put_new_page, private, page,                        pass > 2, mode);            else                rc = unmap_and_move(get_new_page, put_new_page,                        private, page, pass > 2, mode);            switch(rc) {            case -ENOMEM:                goto out;            case -EAGAIN:                retry++;                break;            case MIGRATEPAGE_SUCCESS:                nr_succeeded++;                break;            default:                /*                 * Permanent failure (-EBUSY, -ENOSYS, etc.):                 * unlike -EAGAIN case, the failed page is                 * removed from migration page list and not                 * retried in the next outer loop.                 */                nr_failed++;                break;            }        }    }    rc = nr_failed + retry;out:    if (nr_succeeded)        count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);    if (nr_failed)        count_vm_events(PGMIGRATE_FAIL, nr_failed);    trace_mm_migrate_pages(nr_succeeded, nr_failed, mode, reason);    if (!swapwrite)        current->flags &= ~PF_SWAPWRITE;    return rc;}

newpage是由get_new_page分配的页面，__unmap_and_move尝试迁移页面page到新分配的页面newpage。

__unmap_new_page被unmap_new_page调用，其中force在尝试次数超过2时，就被置1。

static int __unmap_and_move(struct page *page, struct page *newpage,                int force, enum migrate_mode mode){    int rc = -EAGAIN;    int page_was_mapped = 0;    struct anon_vma *anon_vma = NULL;    if (!trylock_page(page)) {------------------------------------尝试给页面枷锁，返回false表示已经有别的进程给page枷锁，返回true表示当前进程可以成功获取锁。        if (!force || mode == MIGRATE_ASYNC)----------------------加锁失败，且强制迁移或异步模式，则忽略这个页面。            goto out;        /*         * It's not safe for direct compaction to call lock_page.         * For example, during page readahead pages are added locked         * to the LRU. Later, when the IO completes the pages are         * marked uptodate and unlocked. However, the queueing         * could be merging multiple pages for one bio (e.g.         * mpage_readpages). If an allocation happens for the         * second or third page, the process can end up locking         * the same page twice and deadlocking. Rather than         * trying to be clever about what pages can be locked,         * avoid the use of lock_page for direct compaction         * altogether.         */        if (current->flags & PF_MEMALLOC)-------------------------可能在直接内存压缩路径上，睡眠等待页面锁是不安全的，忽略此页面。            goto out;        lock_page(page);    }    if (PageWriteback(page)) {        /*         * Only in the case of a full synchronous migration is it         * necessary to wait for PageWriteback. In the async case,         * the retry loop is too short and in the sync-light case,         * the overhead of stalling is too much         */        if (mode != MIGRATE_SYNC) {            rc = -EBUSY;            goto out_unlock;        }        if (!force)            goto out_unlock;        wait_on_page_writeback(page);    }    /*     * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,     * we cannot notice that anon_vma is freed while we migrates a page.     * This get_anon_vma() delays freeing anon_vma pointer until the end     * of migration. File cache pages are no problem because of page_lock()     * File Caches may use write_page() or lock_page() in migration, then,     * just care Anon page here.     */    if (PageAnon(page) && !PageKsm(page)) {        /*         * Only page_lock_anon_vma_read() understands the subtleties of         * getting a hold on an anon_vma from outside one of its mms.         */        anon_vma = page_get_anon_vma(page);        if (anon_vma) {            /*             * Anon page             */        } else if (PageSwapCache(page)) {            /*             * We cannot be sure that the anon_vma of an unmapped             * swapcache page is safe to use because we don't             * know in advance if the VMA that this page belonged             * to still exists. If the VMA and others sharing the             * data have been freed, then the anon_vma could             * already be invalid.             *             * To avoid this possibility, swapcache pages get             * migrated but are not remapped when migration             * completes             */        } else {            goto out_unlock;        }    }    if (unlikely(isolated_balloon_page(page))) {        /*         * A ballooned page does not need any special attention from         * physical to virtual reverse mapping procedures.         * Skip any attempt to unmap PTEs or to remap swap cache,         * in order to avoid burning cycles at rmap level, and perform         * the page migration right away (proteced by page lock).         */        rc = balloon_page_migrate(newpage, page, mode);        goto out_unlock;    }    /*     * Corner case handling:     * 1. When a new swap-cache page is read into, it is added to the LRU     * and treated as swapcache but it has no rmap yet.     * Calling try_to_unmap() against a page->mapping==NULL page will     * trigger a BUG.  So handle it here.     * 2. An orphaned page (see truncate_complete_page) might have     * fs-private metadata. The page can be picked up due to memory     * offlining.  Everywhere else except page reclaim, the page is     * invisible to the vm, so the page can not be migrated.  So try to     * free the metadata, so the page can be freed.     */    if (!page->mapping) {        VM_BUG_ON_PAGE(PageAnon(page), page);        if (page_has_private(page)) {            try_to_free_buffers(page);            goto out_unlock;        }        goto skip_unmap;    }    /* Establish migration ptes or remove ptes */    if (page_mapped(page)) {----------------------------------------------有pte映射的页面，调用try_to_unmap()解除页面所有映射        try_to_unmap(page,            TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);        page_was_mapped = 1;    }skip_unmap:    if (!page_mapped(page))-----------------------------------------------已经解除完所有映射的页面，将页面迁移到新分配的页面newpage。        rc =(newpage, page, page_was_mapped, mode);    if (rc && page_was_mapped)--------------------------------------------rc不为0表示迁移页面失败，调用remove_migration_ptes()删掉迁移的pte。        remove_migration_ptes(page, page);    /* Drop an anon_vma reference if we took one */    if (anon_vma)        put_anon_vma(anon_vma);out_unlock:    unlock_page(page);out:    return rc;}

move_to_new_page试讲page内容迁移到newpage，mode是迁移模式异步、或者同步。

static int move_to_new_page(struct page *newpage, struct page *page,                int page_was_mapped, enum migrate_mode mode){    struct address_space *mapping;    int rc;    /*     * Block others from accessing the page when we get around to     * establishing additional references. We are the only one     * holding a reference to the new page at this point.     */    if (!trylock_page(newpage))--------------------------------------------持锁失败，说明可能被其它进程加锁，BUG进行处理。        BUG();    /* Prepare mapping for the new page.*/    newpage->index = page->index;    newpage->mapping = page->mapping;    if (PageSwapBacked(page))        SetPageSwapBacked(newpage);    mapping = page_mapping(page);-------------------------------------------检查当前页面你是否被映射。如果page属于slab或是匿名页面，返回为空。SWAP则返回swap_address_space空间；其余page cache直接返回page->mapping。    if (!mapping)        rc =(mapping, newpage, page, mode);--------------------slab或者匿名页面调用migrate_page()将旧页面相关信息迁移到新页面。    else if (mapping->a_ops->migratepage)        /*         * Most pages have a mapping and most filesystems provide a         * migratepage callback. Anonymous pages are part of swap         * space which also has its own migratepage callback. This         * is the most common path for page migration.         */        rc = mapping->a_ops->migratepage(mapping,                        newpage, page, mode);-------------------------------有mapping的情况，调用migratepage函数进行迁移。    else        rc = fallback_migrate_page(mapping, newpage, page, mode);    if (rc != MIGRATEPAGE_SUCCESS) {        newpage->mapping = NULL;    } else {        mem_cgroup_migrate(page, newpage, false);        if (page_was_mapped)            (page, newpage);        page->mapping = NULL;    }    unlock_page(newpage);    return rc;}

migrate_pte函数进行页面复制工作。

int migrate_page(struct address_space *mapping,        struct page *newpage, struct page *page,        enum migrate_mode mode){    int rc;    BUG_ON(PageWriteback(page));    /* Writeback must be complete */    rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);-------------对于匿名页面来说，什么也不做直接返回。    if (rc != MIGRATEPAGE_SUCCESS)        return rc;    migrate_page_copy(newpage, page);--------------------------------------------------把页面page复制到新页面newpage中。    return MIGRATEPAGE_SUCCESS;}

remove_migration_ptes利用RMAP反向映射系统找到映射旧页面的每个pte，然后和新页面建立新的映射关系。

static void remove_migration_ptes(struct page *old, struct page *new){    struct rmap_walk_control rwc = {        .rmap_one =,        .arg = old,    };    rmap_walk(new, &rwc);}static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,                 unsigned long addr, void *old){    struct mm_struct *mm = vma->vm_mm;    swp_entry_t entry;     pmd_t *pmd;    pte_t *ptep, pte;     spinlock_t *ptl;    if (unlikely(PageHuge(new))) {        ptep = huge_pte_offset(mm, addr);        if (!ptep)            goto out;        ptl = huge_pte_lockptr(hstate_vma(vma), mm, ptep);    } else {        pmd = mm_find_pmd(mm, addr);        if (!pmd)            goto out;        ptep = pte_offset_map(pmd, addr);-------------------------------------通过mm和addr找到对应页表项pte。        /*         * Peek to check is_swap_pte() before taking ptlock?  No, we         * can race mremap's move_ptes(), which skips anon_vma lock.         */        ptl = pte_lockptr(mm, pmd);    }     spin_lock(ptl);    pte = *ptep;    if (!is_swap_pte(pte))        goto unlock;    entry = pte_to_swp_entry(pte);    if (!is_migration_entry(entry) ||        migration_entry_to_page(entry) != old)        goto unlock;    get_page(new);    pte = pte_mkold(mk_pte(new, vma->vm_page_prot));    if (pte_swp_soft_dirty(*ptep))        pte = pte_mksoft_dirty(pte);    /* Recheck VMA as permissions can change since migration started  */    if (is_write_migration_entry(entry))        pte = maybe_mkwrite(pte, vma);#ifdef CONFIG_HUGETLB_PAGE    if (PageHuge(new)) {        pte = pte_mkhuge(pte);        pte = arch_make_huge_pte(pte, vma, new, 0);    }#endif    flush_dcache_page(new);    set_pte_at(mm, addr, ptep, pte);-----------------------------------------把映射的pte页表项内容设置到新页面pte中，重新建立映射关系。    if (PageHuge(new)) {        if (PageAnon(new))            hugepage_add_anon_rmap(new, vma, addr);        else            page_dup_rmap(new);    } else if (PageAnon(new))        page_add_anon_rmap(new, vma, addr);    else        page_add_file_rmap(new);---------------------------------------------把新页面newpage添加到RMAP反向映射系统中。    /* No need to invalidate - it was non-present before */    update_mmu_cache(vma, addr, ptep);---------------------------------------更新相应的cacheunlock:    pte_unmap_unlock(ptep, ptl);out:    return SWAP_AGAIN;}

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