0. 引言

操作系统中与匿名页相对的是文件页，文件页的反向映射对比匿名页的反向映射更为简单。如果还不清楚匿名页反向映射逻辑的，请移步 匿名页反向映射

1. 文件页反向映射数据结构

struct file： 用户进程每open()一次文件，则会生成一个file对象，file->f_mapping指向该管理该文件页面对象

struct file {
	union {
		struct llist_node	fu_llist;
		struct rcu_head 	fu_rcuhead;
	} f_u;
	struct path		f_path;
	struct inode		*f_inode;	/* cached value */
	const struct file_operations	*f_op;

	/*
	 * Protects f_ep_links, f_flags.
	 * Must not be taken from IRQ context.
	 */
	spinlock_t		f_lock;
	enum rw_hint		f_write_hint;
	atomic_long_t		f_count;
	unsigned int 		f_flags;
	fmode_t			f_mode;
	struct mutex		f_pos_lock;
	loff_t			f_pos;
	struct fown_struct	f_owner;
	const struct cred	*f_cred;
	struct file_ra_state	f_ra;

	u64			f_version;
#ifdef CONFIG_SECURITY
	void			*f_security;
#endif
	/* needed for tty driver, and maybe others */
	void			*private_data;

#ifdef CONFIG_EPOLL
	/* Used by fs/eventpoll.c to link all the hooks to this file */
	struct list_head	f_ep_links;
	struct list_head	f_tfile_llink;
#endif /* #ifdef CONFIG_EPOLL */
	struct address_space	*f_mapping; // 文件页缓冲区，文件页都放在该对象中进行管理
	errseq_t		f_wb_err;
	errseq_t		f_sb_err; /* for syncfs */
} __randomize_layout
  __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */

struct address_space： 用来管理一个文件所有内容页面，一个真实的文件只对应一个address_space结构，其中address_space->i_pages是保存页面的结构，address_space->i_mmap是一个红黑树，红黑树上挂载着映射该文件的所有vma对象。

/**
 * struct address_space - Contents of a cacheable, mappable object.
 * @host: Owner, either the inode or the block_device.
 * @i_pages: Cached pages.
 * @gfp_mask: Memory allocation flags to use for allocating pages.
 * @i_mmap_writable: Number of VM_SHARED mappings.
 * @nr_thps: Number of THPs in the pagecache (non-shmem only).
 * @i_mmap: Tree of private and shared mappings.
 * @i_mmap_rwsem: Protects @i_mmap and @i_mmap_writable.
 * @nrpages: Number of page entries, protected by the i_pages lock.
 * @nrexceptional: Shadow or DAX entries, protected by the i_pages lock.
 * @writeback_index: Writeback starts here.
 * @a_ops: Methods.
 * @flags: Error bits and flags (AS_*).
 * @wb_err: The most recent error which has occurred.
 * @private_lock: For use by the owner of the address_space.
 * @private_list: For use by the owner of the address_space.
 * @private_data: For use by the owner of the address_space.
 */
struct address_space {
	struct inode		*host;
	struct xarray		i_pages;
	gfp_t			gfp_mask;
	atomic_t		i_mmap_writable;
#ifdef CONFIG_READ_ONLY_THP_FOR_FS
	/* number of thp, only for non-shmem files */
	atomic_t		nr_thps;
#endif
	struct rb_root_cached	i_mmap;
	struct rw_semaphore	i_mmap_rwsem;
	unsigned long		nrpages;
	unsigned long		nrexceptional;
	pgoff_t			writeback_index;
	const struct address_space_operations *a_ops;
	unsigned long		flags;
	errseq_t		wb_err;
	spinlock_t		private_lock;
	struct list_head	private_list;
	void			*private_data;
} __attribute__((aligned(sizeof(long)))) __randomize_layout;

struct vm_area_struct： 用来描述用户进程的一段虚拟地址空间，其中vma->vm_start表示映射的起始虚拟地址，vma->vm_end表示映射的结束虚拟地址，vma->vm_file指向映射的文件对象，vma->vm_pgoff表示从文件哪里进行映射（以页面为单位）。

/*
 * This struct describes a virtual memory area. There is one of these
 * per VM-area/task. A VM area is any part of the process virtual memory
 * space that has a special rule for the page-fault handlers (ie a shared
 * library, the executable area etc).
 */
struct vm_area_struct {
	/* The first cache line has the info for VMA tree walking. */

	unsigned long vm_start;		/* Our start address within vm_mm. */ // 映射的起始虚拟地址
	unsigned long vm_end;		/* The first byte after our end address
					   within vm_mm. */ // 映射的结束虚拟地址

	/* linked list of VM areas per task, sorted by address */
	struct vm_area_struct *vm_next, *vm_prev;

	struct rb_node vm_rb;

	/*
	 * Largest free memory gap in bytes to the left of this VMA.
	 * Either between this VMA and vma->vm_prev, or between one of the
	 * VMAs below us in the VMA rbtree and its ->vm_prev. This helps
	 * get_unmapped_area find a free area of the right size.
	 */
	unsigned long rb_subtree_gap;

	/* Second cache line starts here. */

	struct mm_struct *vm_mm;	/* The address space we belong to. */

	/*
	 * Access permissions of this VMA.
	 * See vmf_insert_mixed_prot() for discussion.
	 */
	pgprot_t vm_page_prot;
	unsigned long vm_flags;		/* Flags, see mm.h. */

	/*
	 * For areas with an address space and backing store,
	 * linkage into the address_space->i_mmap interval tree.
	 */
	struct {
		struct rb_node rb;
		unsigned long rb_subtree_last;
	} shared;

	/*
	 * 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_lock &
					  * page_table_lock */
	struct anon_vma *anon_vma;	/* Serialized by page_table_lock */

	/* Function pointers to deal with this struct. */
	const struct vm_operations_struct *vm_ops;

	/* Information about our backing store: */
	unsigned long vm_pgoff;		/* Offset (within vm_file) in PAGE_SIZE
					   units */ // 该vma从文件哪里开始映射，以页面为单位
	struct file * vm_file;		/* File we map to (can be NULL). */ // 映射的文件对象
	void * vm_private_data;		/* was vm_pte (shared mem) */

#ifdef CONFIG_SWAP
	atomic_long_t swap_readahead_info;
#endif
#ifndef CONFIG_MMU
	struct vm_region *vm_region;	/* NOMMU mapping region */
#endif
#ifdef CONFIG_NUMA
	struct mempolicy *vm_policy;	/* NUMA policy for the VMA */
#endif
	struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
} __randomize_layout;

2. 数据结构关系图

3. 建立文件反向映射流程

我们都是通过一个path去打开一个文件，然后再通过mmap()去将文件映射到进程的虚拟地址空间，后续通过访存这块虚拟地址空间来达到读写文件的目的。

int fd = open("/path/to/your/file", O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
void *ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
// do something
munmap(ptr, size);
close(fd);

现在让我们从mmap()映射文件描述符这里看起。
arm64体系结构下，且内核是linux-5.10版本：

SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
		unsigned long, prot, unsigned long, flags,
		unsigned long, fd, unsigned long, off)
{
	if (offset_in_page(off) != 0)
		return -EINVAL;

	return ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
}

unsigned long ksys_mmap_pgoff(unsigned long addr, unsigned long len,
			      unsigned long prot, unsigned long flags,
			      unsigned long fd, unsigned long pgoff)
{
	struct file *file = NULL;
	unsigned long retval;

	if (!(flags & MAP_ANONYMOUS)) { // 由于我们传递的flags是不包含MAP_ANONYMOUS，所以会走到这个分支里
		audit_mmap_fd(fd, flags);
		file = fget(fd); // 根据fd获取struct file对象，并增加引用计数
		if (!file) // 如果该fd没有对应的struct file对象，则返回-EBADF错误
			return -EBADF;
		if (is_file_hugepages(file)) {
			len = ALIGN(len, huge_page_size(hstate_file(file)));
		} else if (unlikely(flags & MAP_HUGETLB)) {
			retval = -EINVAL;
			goto out_fput;
		}
	} else if (flags & MAP_HUGETLB) {
		struct user_struct *user = NULL;
		struct hstate *hs;

		hs = hstate_sizelog((flags >> MAP_HUGE_SHIFT) & MAP_HUGE_MASK);
		if (!hs)
			return -EINVAL;

		len = ALIGN(len, huge_page_size(hs));
		/*
		 * VM_NORESERVE is used because the reservations will be
		 * taken when vm_ops->mmap() is called
		 * A dummy user value is used because we are not locking
		 * memory so no accounting is necessary
		 */
		file = hugetlb_file_setup(HUGETLB_ANON_FILE, len,
				VM_NORESERVE,
				&user, HUGETLB_ANONHUGE_INODE,
				(flags >> MAP_HUGE_SHIFT) & MAP_HUGE_MASK);
		if (IS_ERR(file))
			return PTR_ERR(file);
	}

	flags &= ~(MAP_EXECUTABLE | MAP_DENYWRITE);

	// 进入到vm_mmap_pgoff处理
	retval = vm_mmap_pgoff(file, addr, len, prot, flags, pgoff);
out_fput:
	if (file)
		fput(file);
	return retval;
}

unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
	unsigned long len, unsigned long prot,
	unsigned long flag, unsigned long pgoff)
{
	unsigned long ret;
	struct mm_struct *mm = current->mm;
	unsigned long populate;
	LIST_HEAD(uf);

	ret = security_mmap_file(file, prot, flag);
	if (!ret) {
		if (mmap_write_lock_killable(mm))
			return -EINTR;
		// 进入do_mmap处理
		ret = do_mmap(file, addr, len, prot, flag, pgoff, &populate,
			      &uf);
		mmap_write_unlock(mm);
		userfaultfd_unmap_complete(mm, &uf);
		if (populate)
			mm_populate(ret, populate);
	}
	return ret;
}

接下来我们进入到映射主要处理函数do_mmap()。

/*
 * The caller must write-lock current->mm->mmap_lock.
 */
unsigned long do_mmap(struct file *file, unsigned long addr,
			unsigned long len, unsigned long prot,
			unsigned long flags, unsigned long pgoff,
			unsigned long *populate, struct list_head *uf)
{
	struct mm_struct *mm = current->mm;
	vm_flags_t vm_flags;
	int pkey = 0;

	*populate = 0;

	if (!len) // 如果要映射的空间大小为0，则返回-EINVAL
		return -EINVAL;

	/*
	 * Does the application expect PROT_READ to imply PROT_EXEC?
	 *
	 * (the exception is when the underlying filesystem is noexec
	 *  mounted, in which case we dont add PROT_EXEC.)
	 */
	if ((prot & PROT_READ) && (current->personality & READ_IMPLIES_EXEC))
		if (!(file && path_noexec(&file->f_path)))
			prot |= PROT_EXEC;

	/* force arch specific MAP_FIXED handling in get_unmapped_area */
	if (flags & MAP_FIXED_NOREPLACE)
		flags |= MAP_FIXED;

	if (!(flags & MAP_FIXED))
		addr = round_hint_to_min(addr);

	/* Careful about overflows.. */
	len = PAGE_ALIGN(len); // 按页粒度对齐要映射的空间大小
	if (!len)
		return -ENOMEM;

	/* offset overflow? */
	if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
		return -EOVERFLOW;

	/* Too many mappings? */
	if (mm->map_count > sysctl_max_map_count)
		return -ENOMEM;

	/* Obtain the address to map to. we verify (or select) it and ensure
	 * that it represents a valid section of the address space.
	 */
	addr = get_unmapped_area(file, addr, len, pgoff, flags); // 找到一块未映射的虚拟地址
	if (IS_ERR_VALUE(addr))
		return addr;

	if (flags & MAP_FIXED_NOREPLACE) {
		struct vm_area_struct *vma = find_vma(mm, addr);

		if (vma && vma->vm_start < addr + len)
			return -EEXIST;
	}

	if (prot == PROT_EXEC) {
		pkey = execute_only_pkey(mm);
		if (pkey < 0)
			pkey = 0;
	}

	/* Do simple checking here so the lower-level routines won't have
	 * to. we assume access permissions have been handled by the open
	 * of the memory object, so we don't do any here.
	 */
	vm_flags = calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |
			mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;

	if (flags & MAP_LOCKED)
		if (!can_do_mlock())
			return -EPERM;

	if (mlock_future_check(mm, vm_flags, len))
		return -EAGAIN;

	if (file) {
		struct inode *inode = file_inode(file);
		unsigned long flags_mask;

		if (!file_mmap_ok(file, inode, pgoff, len))
			return -EOVERFLOW;

		flags_mask = LEGACY_MAP_MASK | file->f_op->mmap_supported_flags;

		switch (flags & MAP_TYPE) {
		case MAP_SHARED:
			/*
			 * Force use of MAP_SHARED_VALIDATE with non-legacy
			 * flags. E.g. MAP_SYNC is dangerous to use with
			 * MAP_SHARED as you don't know which consistency model
			 * you will get. We silently ignore unsupported flags
			 * with MAP_SHARED to preserve backward compatibility.
			 */
			flags &= LEGACY_MAP_MASK;
			fallthrough;
		case MAP_SHARED_VALIDATE:
			if (flags & ~flags_mask)
				return -EOPNOTSUPP;
			if (prot & PROT_WRITE) {
				if (!(file->f_mode & FMODE_WRITE))
					return -EACCES;
				if (IS_SWAPFILE(file->f_mapping->host))
					return -ETXTBSY;
			}

			/*
			 * Make sure we don't allow writing to an append-only
			 * file..
			 */
			if (IS_APPEND(inode) && (file->f_mode & FMODE_WRITE))
				return -EACCES;

			/*
			 * Make sure there are no mandatory locks on the file.
			 */
			if (locks_verify_locked(file))
				return -EAGAIN;

			vm_flags |= VM_SHARED | VM_MAYSHARE;
			if (!(file->f_mode & FMODE_WRITE))
				vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
			fallthrough;
		case MAP_PRIVATE:
			if (!(file->f_mode & FMODE_READ))
				return -EACCES;
			if (path_noexec(&file->f_path)) {
				if (vm_flags & VM_EXEC)
					return -EPERM;
				vm_flags &= ~VM_MAYEXEC;
			}

			if (!file->f_op->mmap)
				return -ENODEV;
			if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
				return -EINVAL;
			break;

		default:
			return -EINVAL;
		}
	} else {
		switch (flags & MAP_TYPE) {
		case MAP_SHARED:
			if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
				return -EINVAL;
			/*
			 * Ignore pgoff.
			 */
			pgoff = 0;
			vm_flags |= VM_SHARED | VM_MAYSHARE;
			break;
		case MAP_PRIVATE:
			/*
			 * Set pgoff according to addr for anon_vma.
			 */
			pgoff = addr >> PAGE_SHIFT;
			break;
		default:
			return -EINVAL;
		}
	}

	/*
	 * Set 'VM_NORESERVE' if we should not account for the
	 * memory use of this mapping.
	 */
	if (flags & MAP_NORESERVE) {
		/* We honor MAP_NORESERVE if allowed to overcommit */
		if (sysctl_overcommit_memory != OVERCOMMIT_NEVER)
			vm_flags |= VM_NORESERVE;

		/* hugetlb applies strict overcommit unless MAP_NORESERVE */
		if (file && is_file_hugepages(file))
			vm_flags |= VM_NORESERVE;
	}

	// 进入mmap_region进行下一步处理
	addr = mmap_region(file, addr, len, vm_flags, pgoff, uf);
	if (!IS_ERR_VALUE(addr) &&
	    ((vm_flags & VM_LOCKED) ||
	     (flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE))
		*populate = len;
	return addr;
}

unsigned long mmap_region(struct file *file, unsigned long addr,
		unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
		struct list_head *uf)
{
	struct mm_struct *mm = current->mm;
	struct vm_area_struct *vma, *prev, *merge;
	int error;
	struct rb_node **rb_link, *rb_parent;
	unsigned long charged = 0;

	/* Check against address space limit. */
	if (!may_expand_vm(mm, vm_flags, len >> PAGE_SHIFT)) {
		unsigned long nr_pages;

		/*
		 * MAP_FIXED may remove pages of mappings that intersects with
		 * requested mapping. Account for the pages it would unmap.
		 */
		nr_pages = count_vma_pages_range(mm, addr, addr + len);

		if (!may_expand_vm(mm, vm_flags,
					(len >> PAGE_SHIFT) - nr_pages))
			return -ENOMEM;
	}

	/* Clear old maps, set up prev, rb_link, rb_parent, and uf */
	if (munmap_vma_range(mm, addr, len, &prev, &rb_link, &rb_parent, uf))
		return -ENOMEM;
	/*
	 * Private writable mapping: check memory availability
	 */
	if (accountable_mapping(file, vm_flags)) {
		charged = len >> PAGE_SHIFT;
		if (security_vm_enough_memory_mm(mm, charged))
			return -ENOMEM;
		vm_flags |= VM_ACCOUNT;
	}

	/*
	 * Can we just expand an old mapping?
	 */
	vma = vma_merge(mm, prev, addr, addr + len, vm_flags,
			NULL, file, pgoff, NULL, NULL_VM_UFFD_CTX);
	if (vma)
		goto out;

	/*
	 * Determine the object being mapped and call the appropriate
	 * specific mapper. the address has already been validated, but
	 * not unmapped, but the maps are removed from the list.
	 */
	vma = vm_area_alloc(mm); // 分配一个新的vma管理结构对象
	if (!vma) {
		error = -ENOMEM;
		goto unacct_error;
	}

	vma->vm_start = addr; // 填充vma信息，vma的起始虚拟地址是addr
	vma->vm_end = addr + len; // vma的结束虚拟地址是addr + len
	vma->vm_flags = vm_flags;
	vma->vm_page_prot = vm_get_page_prot(vm_flags);
	vma->vm_pgoff = pgoff; // vma的vm_pgoff保存的是相对于文件起始地址的偏移量，是以页为单位

	if (file) {
		if (vm_flags & VM_DENYWRITE) {
			error = deny_write_access(file);
			if (error)
				goto free_vma;
		}
		if (vm_flags & VM_SHARED) {
			error = mapping_map_writable(file->f_mapping);
			if (error)
				goto allow_write_and_free_vma;
		}

		/* ->mmap() can change vma->vm_file, but must guarantee that
		 * vma_link() below can deny write-access if VM_DENYWRITE is set
		 * and map writably if VM_SHARED is set. This usually means the
		 * new file must not have been exposed to user-space, yet.
		 */
		vma->vm_file = get_file(file); // 再次增加struct file对象引用计数
		error = call_mmap(file, vma); // call_mmap()是为了填充vma->vm_ops，page fault时使用
		if (error)
			goto unmap_and_free_vma;

		/* Can addr have changed??
		 *
		 * Answer: Yes, several device drivers can do it in their
		 *         f_op->mmap method. -DaveM
		 * Bug: If addr is changed, prev, rb_link, rb_parent should
		 *      be updated for vma_link()
		 */
		WARN_ON_ONCE(addr != vma->vm_start);

		addr = vma->vm_start;

		/* If vm_flags changed after call_mmap(), we should try merge vma again
		 * as we may succeed this time.
		 */
		if (unlikely(vm_flags != vma->vm_flags && prev)) {
			merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
				NULL, vma->vm_file, vma->vm_pgoff, NULL, NULL_VM_UFFD_CTX);
			if (merge) {
				/* ->mmap() can change vma->vm_file and fput the original file. So
				 * fput the vma->vm_file here or we would add an extra fput for file
				 * and cause general protection fault ultimately.
				 */
				fput(vma->vm_file);
				vm_area_free(vma);
				vma = merge;
				/* Update vm_flags to pick up the change. */
				vm_flags = vma->vm_flags;
				goto unmap_writable;
			}
		}

		vm_flags = vma->vm_flags;
	} else if (vm_flags & VM_SHARED) {
		error = shmem_zero_setup(vma);
		if (error)
			goto free_vma;
	} else {
		vma_set_anonymous(vma);
	}

	/* Allow architectures to sanity-check the vm_flags */
	if (!arch_validate_flags(vma->vm_flags)) {
		error = -EINVAL;
		if (file)
			goto unmap_and_free_vma;
		else
			goto free_vma;
	}

	// 建立页面映射
	vma_link(mm, vma, prev, rb_link, rb_parent);
	/* Once vma denies write, undo our temporary denial count */
	if (file) {
unmap_writable:
		if (vm_flags & VM_SHARED)
			mapping_unmap_writable(file->f_mapping);
		if (vm_flags & VM_DENYWRITE)
			allow_write_access(file);
	}
	file = vma->vm_file;
out:
	perf_event_mmap(vma);

	vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
	if (vm_flags & VM_LOCKED) {
		if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
					is_vm_hugetlb_page(vma) ||
					vma == get_gate_vma(current->mm))
			vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
		else
			mm->locked_vm += (len >> PAGE_SHIFT);
	}

	if (file)
		uprobe_mmap(vma);

	/*
	 * New (or expanded) vma always get soft dirty status.
	 * Otherwise user-space soft-dirty page tracker won't
	 * be able to distinguish situation when vma area unmapped,
	 * then new mapped in-place (which must be aimed as
	 * a completely new data area).
	 */
	vma->vm_flags |= VM_SOFTDIRTY;

	vma_set_page_prot(vma);

	return addr;

unmap_and_free_vma:
	vma->vm_file = NULL;
	fput(file);

	/* Undo any partial mapping done by a device driver. */
	unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
	charged = 0;
	if (vm_flags & VM_SHARED)
		mapping_unmap_writable(file->f_mapping);
allow_write_and_free_vma:
	if (vm_flags & VM_DENYWRITE)
		allow_write_access(file);
free_vma:
	vm_area_free(vma);
unacct_error:
	if (charged)
		vm_unacct_memory(charged);
	return error;
}

这里终于到了给文件页建立反向映射，主要处理函数是vma_link()。

static void vma_link(struct mm_struct *mm, struct vm_area_struct *vma,
			struct vm_area_struct *prev, struct rb_node **rb_link,
			struct rb_node *rb_parent)
{
	struct address_space *mapping = NULL;

	if (vma->vm_file) {
		mapping = vma->vm_file->f_mapping;
		i_mmap_lock_write(mapping);
	}

	// 将vma加到mm->mm_rb红黑树中，另外也加到mm->mmap vma链表中
	__vma_link(mm, vma, prev, rb_link, rb_parent);
	// 建立文件页反向映射
	__vma_link_file(vma);

	if (mapping)
		i_mmap_unlock_write(mapping);

	mm->map_count++;
	validate_mm(mm);
}

static void __vma_link_file(struct vm_area_struct *vma)
{
	struct file *file;

	file = vma->vm_file;
	if (file) {
		// 获取文件的address_space
		struct address_space *mapping = file->f_mapping;

		if (vma->vm_flags & VM_DENYWRITE)
			put_write_access(file_inode(file));
		if (vma->vm_flags & VM_SHARED)
			mapping_allow_writable(mapping);

		flush_dcache_mmap_lock(mapping);
		// 将vma加入到address_space->i_mmap红黑树中
		vma_interval_tree_insert(vma, &mapping->i_mmap);
		flush_dcache_mmap_unlock(mapping);
	}
}

4. 回收文件页时使用反向映射

这里以kswapd进行文件页回收为例，与直接回收的路径下半部分是重合的（从shrink_node()函数开始）

kswapd()
-> balance_pgdat()
--> kswapd_shrink_node()
---> shrink_node()
----> shrink_node_memcgs()
-----> shrink_lruvec()
------> shrink_list()
-------> shrink_inactive_list()
--------> shrink_page_list()
---------> try_to_unmap()

在回收文件页的时候，需要使用到反向映射，所以我们来看一下**try_to_unmap()**对于文件页都做了哪些操作，

/**
 * try_to_unmap - try to remove all page table mappings to a page
 * @page: the page to get unmapped
 * @flags: action and flags
 *
 * Tries to remove all the page table entries which are mapping this
 * page, used in the pageout path.  Caller must hold the page lock.
 *
 * If unmap is successful, return true. Otherwise, false.
 */
bool try_to_unmap(struct page *page, enum ttu_flags flags)
{
	struct rmap_walk_control rwc = {
		.rmap_one = try_to_unmap_one,
		.arg = (void *)flags,
		.done = page_mapcount_is_zero,
		.anon_lock = page_lock_anon_vma_read,
	};

	/*
	 * During exec, a temporary VMA is setup and later moved.
	 * The VMA is moved under the anon_vma lock but not the
	 * page tables leading to a race where migration cannot
	 * find the migration ptes. Rather than increasing the
	 * locking requirements of exec(), migration skips
	 * temporary VMAs until after exec() completes.
	 */
	if ((flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))
	    && !PageKsm(page) && PageAnon(page))
		rwc.invalid_vma = invalid_migration_vma;

	if (flags & TTU_RMAP_LOCKED)
		rmap_walk_locked(page, &rwc);
	else
		// 文件页走这里
		rmap_walk(page, &rwc);

	return !page_mapcount(page) ? true : false;
}

void rmap_walk(struct page *page, struct rmap_walk_control *rwc)
{
	if (unlikely(PageKsm(page)))
		rmap_walk_ksm(page, rwc);
	else if (PageAnon(page))
		rmap_walk_anon(page, rwc, false);
	else
		// 文件页走这里
		rmap_walk_file(page, rwc, false);
}

/*
 * rmap_walk_file - do something to file page using the object-based rmap method
 * @page: the page to be handled
 * @rwc: control variable according to each walk type
 *
 * Find all the mappings of a page using the mapping pointer and the vma chains
 * contained in the address_space struct it points to.
 *
 * When called from try_to_munlock(), the mmap_lock of the mm containing the vma
 * where the page was found will be held for write.  So, we won't recheck
 * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
 * LOCKED.
 */
static void rmap_walk_file(struct page *page, struct rmap_walk_control *rwc,
		bool locked)
{
	struct address_space *mapping = page_mapping(page); // 获取到该page对应的映射
	pgoff_t pgoff_start, pgoff_end;
	struct vm_area_struct *vma;

	/*
	 * The page lock not only makes sure that page->mapping cannot
	 * suddenly be NULLified by truncation, it makes sure that the
	 * structure at mapping cannot be freed and reused yet,
	 * so we can safely take mapping->i_mmap_rwsem.
	 */
	VM_BUG_ON_PAGE(!PageLocked(page), page);

	if (!mapping)
		return;

	pgoff_start = page_to_pgoff(page); // 这点忘记介绍了，可以通过page找到该page是映射到文件的哪个位置，便于找到该page在哪个vma中有映射
	pgoff_end = pgoff_start + thp_nr_pages(page) - 1;
	if (!locked)
		i_mmap_lock_read(mapping);
	vma_interval_tree_foreach(vma, &mapping->i_mmap, // 从address_space->i_mmap红黑树中查找映射了该page到vma
			pgoff_start, pgoff_end) {
		unsigned long address = vma_address(page, vma);

		cond_resched();

		if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
			continue;

		if (!rwc->rmap_one(page, vma, address, rwc->arg))
			goto done;
		if (rwc->done && rwc->done(page))
			goto done;
	}

done:
	if (!locked)
		i_mmap_unlock_read(mapping);
}

至此文件页的反向映射处理和使用流程介绍完毕，感谢各位读者浏览！