Linux内核中的内存空间与mmap

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为了学习Linux内核的内存管理机制，结合看源码和看书，对Linux内核的内存机制和mmap的过程进行了粗略的学习，本篇博文是对学习到的一些内容的简单记录，暂不涉及slab分配器等内容，由于本人水平有限、源码阅读经验不足，如有错误的地方恳请指正！

本文中的Linux内核源代码为Linux-4.19.65 Linux-5.0.5

参考资料 《Linux Device Drivers》第三版第15章

《深入理解Linux内核》第二章、第八章

https://blog.csdn.net/gatieme/article/details/52384075

https://blog.csdn.net/gatieme/article/details/52384636

物理内存的组织

页框与struct page

每一个页框（物理页），会有一个struct page来描述。而为了节省内存，它内部使用了很多union。

一些比较重要的字段：

参考自：https://blog.csdn.net/gatieme/article/details/52384636

字段名 含义
flags 用来存放页的状态，这些状态定义在linux/page-flags.h中
lru 链表头，用于在各种链表上维护该页, 以便于按页将不同类别分组, 主要有3个用途: 伙伴算法, slab分配器, 被用户态使用或被当做页缓存使用
_refcount 引用计数，表示内核中引用该page的次数, 如果要操作该page, 引用计数会+1, 操作完成-1. 当该值为0时, 表示没有引用该page的位置，所以该page可以被解除映射
_mapcount 映射计数，也就是说该page同时被多少个进程共享
virtual 对于如果物理内存可以直接映射内核的系统, 我们可以之间映射出虚拟地址与物理地址的管理, 但是对于需要使用高端内存区域的页, 即无法直接映射到内核的虚拟地址空间, 因此需要用virtual保存该页的虚拟地址

flags字段一些可能的取值：

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// /include/linux/mm_types.h

struct page {
	unsigned long flags;		/* Atomic flags, some possibly
					 * updated asynchronously */
           //存放页的状态
	/*
	 * Five words (20/40 bytes) are available in this union.
	 * WARNING: bit 0 of the first word is used for PageTail(). That
	 * means the other users of this union MUST NOT use the bit to
	 * avoid collision and false-positive PageTail().
	 */
	union {
		struct {	/* Page cache and anonymous pages */
			/**
			 * @lru: Pageout list, eg. active_list protected by
			 * zone_lru_lock.  Sometimes used as a generic list
			 * by the page owner.
			 */
			struct list_head lru;
			/* See page-flags.h for PAGE_MAPPING_FLAGS */
			struct address_space *mapping;
			pgoff_t index;		/* Our offset within mapping. */
			/**
			 * @private: Mapping-private opaque data.
			 * Usually used for buffer_heads if PagePrivate.
			 * Used for swp_entry_t if PageSwapCache.
			 * Indicates order in the buddy system if PageBuddy.
			 */
			unsigned long private;
		};
		struct {	/* slab, slob and slub */
			union {
				struct list_head slab_list;	/* uses lru */
				struct {	/* Partial pages */
					struct page *next;
#ifdef CONFIG_64BIT
					int pages;	/* Nr of pages left */
					int pobjects;	/* Approximate count */
#else
					short int pages;
					short int pobjects;
#endif
				};
			};
			struct kmem_cache *slab_cache; /* not slob */
			/* Double-word boundary */
			void *freelist;		/* first free object */
			union {
				void *s_mem;	/* slab: first object */
				unsigned long counters;		/* SLUB */
				struct {			/* SLUB */
					unsigned inuse:16;
					unsigned objects:15;
					unsigned frozen:1;
				};
			};
		};
		struct {	/* Tail pages of compound page */
			unsigned long compound_head;	/* Bit zero is set */

			/* First tail page only */
			unsigned char compound_dtor;
			unsigned char compound_order;
			atomic_t compound_mapcount;
		};
		struct {	/* Second tail page of compound page */
			unsigned long _compound_pad_1;	/* compound_head */
			unsigned long _compound_pad_2;
			struct list_head deferred_list;
		};
		struct {	/* Page table pages */
			unsigned long _pt_pad_1;	/* compound_head */
			pgtable_t pmd_huge_pte; /* protected by page->ptl */
			unsigned long _pt_pad_2;	/* mapping */
			union {
				struct mm_struct *pt_mm; /* x86 pgds only */
				atomic_t pt_frag_refcount; /* powerpc */
			};
#if ALLOC_SPLIT_PTLOCKS
			spinlock_t *ptl;
#else
			spinlock_t ptl;
#endif
		};
		struct {	/* ZONE_DEVICE pages */
			/** @pgmap: Points to the hosting device page map. */
			struct dev_pagemap *pgmap;
			unsigned long hmm_data;
			unsigned long _zd_pad_1;	/* uses mapping */
		};

		/** @rcu_head: You can use this to free a page by RCU. */
		struct rcu_head rcu_head;
	};

	union {		/* This union is 4 bytes in size. */
		/*
		 * If the page can be mapped to userspace, encodes the number
		 * of times this page is referenced by a page table.
		 */
		atomic_t _mapcount;

		/*
		 * If the page is neither PageSlab nor mappable to userspace,
		 * the value stored here may help determine what this page
		 * is used for.  See page-flags.h for a list of page types
		 * which are currently stored here.
		 */
		unsigned int page_type;

		unsigned int active;		/* SLAB */
		int units;			/* SLOB */
	};

	/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
	atomic_t _refcount;

#ifdef CONFIG_MEMCG
	struct mem_cgroup *mem_cgroup;
#endif

	/*
	 * On machines where all RAM is mapped into kernel address space,
	 * we can simply calculate the virtual address. On machines with
	 * highmem some memory is mapped into kernel virtual memory
	 * dynamically, so we need a place to store that address.
	 * Note that this field could be 16 bits on x86 ... ;)
	 *
	 * Architectures with slow multiplication can define
	 * WANT_PAGE_VIRTUAL in asm/page.h
	 */
#if defined(WANT_PAGE_VIRTUAL)
	void *virtual;			/* Kernel virtual address (NULL if
					   not kmapped, ie. highmem) */
#endif /* WANT_PAGE_VIRTUAL */

#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
	int _last_cpupid;
#endif
} _struct_page_alignment;

NUMA与UMA模型

我们习惯上认为计算机内存是一种均匀、共享的资源，我们期望不管内存单元处于何处，也不管CPU处于何处，CPU对内存单元的访问都需要相同的时间。可惜这种假设在某些体系结构上是不成立的，例如多处理器的Alpha或MIPS。

内存节点node

因此，linux支持非一致内存访问NUMA（None-Uniform Memory Access）模型，在这种模型中，给定CPU对不同内存单元的访问时间可能不一样。因此，系统的物理内存被划分为几个节点(node)。在一个单独的节点内，任一给定的CPU访问节点中page所需要的时间都是相同的，不同CPU所需的时间就可能会不同。对于每个CPU而言，都试图把耗时访问时间减到最少，这就需要小心地选择CPU最常引用的内核数据结构的存放位置。

NUMA主要应用于服务器架构中，而在均匀内存访问UMA模型中，就只有一个node。

每个节点中地内存又可以被分为几个管理区（Zone）

内存管理区zone

在一个理想的计算机体系结构中，一个页框就是一个内存存储单元，可以用于任何用途，任何任何种类的数据页都可以装载进任何页框，没有啥限制。

然而，实际地计算机体系结构有硬件的制约，这限制了页框可以使用的方式，尤其是，Linux内核必须处理80x86体系结构地两种硬件约束：

  • ISA总线的DMA（直接内存访问）处理器只能对RAM地前16MB寻址
  • 在具有大容量RAM的32位计算机中，CPU不能直接访问所有物理内存，因为线性地址空间不够（32位只有4G）

又或者在64位机器中，32位的DMA设备只能寻址4GB以内的内存。

内核中定义了以下一些Zone：

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// /include/linux/mmzone.h

enum zone_type {
#ifdef CONFIG_ZONE_DMA
	/*
	 * ZONE_DMA is used when there are devices that are not able
	 * to do DMA to all of addressable memory (ZONE_NORMAL). Then we
	 * carve out the portion of memory that is needed for these devices.
	 * The range is arch specific.
	 *
	 * Some examples
	 *
	 * Architecture		Limit
	 * ---------------------------
	 * parisc, ia64, sparc	<4G
	 * s390, powerpc	<2G
	 * arm			Various
	 * alpha		Unlimited or 0-16MB.
	 *
	 * i386, x86_64 and multiple other arches
	 * 			<16M.
	 */
	ZONE_DMA,
#endif
#ifdef CONFIG_ZONE_DMA32
	/*
	 * x86_64 needs two ZONE_DMAs because it supports devices that are
	 * only able to do DMA to the lower 16M but also 32 bit devices that
	 * can only do DMA areas below 4G.
	 */
	ZONE_DMA32,
#endif
	/*
	 * Normal addressable memory is in ZONE_NORMAL. DMA operations can be
	 * performed on pages in ZONE_NORMAL if the DMA devices support
	 * transfers to all addressable memory.
	 */
	ZONE_NORMAL,
#ifdef CONFIG_HIGHMEM
	/*
	 * A memory area that is only addressable by the kernel through
	 * mapping portions into its own address space. This is for example
	 * used by i386 to allow the kernel to address the memory beyond
	 * 900MB. The kernel will set up special mappings (page
	 * table entries on i386) for each page that the kernel needs to
	 * access.
	 */
	ZONE_HIGHMEM,
#endif
	ZONE_MOVABLE,
#ifdef CONFIG_ZONE_DEVICE
	ZONE_DEVICE,
#endif
	__MAX_NR_ZONES

};

ZONE_DMA和ZONE_DMA32都是划分出来供DMA设备使用的，ZONE_NORMAL区则是可以直接线性映射到线性地址空间中的第4个G中（即进程地址空间的内核区），因此内核可以直接访问，而ZONE_HIGHMEM包含的内存页不能直接由内核访问的（超出了线性地址空间，>4G?）。在64位体系结构上ZONE_HIGHMEM总是空的。

进程地址空间

虚拟内存区 VMA

一个vma表示进程的虚拟地址空间中的一个同类区域，它们拥有同样的权限标志，和同样的映射文件或交换空间。可以模糊地将其理解为“段”的概念，但是将其描述为“拥有自身属性的内存对象”可能更为贴切。每一个VMA会有一个vm_area_struct结构体来描述。

vm_area_struct

每个VMA都会有一个对应的vm_area_struct结构体，包含这个VMA的起始、结束地址（虚拟地址），映射的文件指针（可以为空），权限、属性（vm_flags），所属的虚拟地址空间（struct mm_struct *vm_mm），对这块VMA进行操作的函数指针（const struct vm_operations_struct *vm_ops;）等等，vm_ops的存在说明VMA其实是一个内核对象。

另外，可以看到，VMAs是通过双向链表来组织的。

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// /inlcude/linux/mm_types.h
/*
 * This struct defines a memory VMM 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. */
	unsigned long vm_end;		/* The first byte after our end address
					   within vm_mm. */

	/* 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. */
	pgprot_t vm_page_prot;		/* Access permissions of this VMA. */
	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_sem &
					  * 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 */
	struct file * vm_file;		/* File we map to (can be NULL). */
	void * vm_private_data;		/* was vm_pte (shared mem) */

	atomic_long_t swap_readahead_info;
#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;

vm_operations_struct

一组函数指针，每个VMA都有自己的这个结构体指针。

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/*  /include/linux/mm.h  */

/*
 * These are the virtual MM functions - opening of an area, closing and
 * unmapping it (needed to keep files on disk up-to-date etc), pointer
 * to the functions called when a no-page or a wp-page exception occurs.
 */
struct vm_operations_struct {
	void (*open)(struct vm_area_struct * area);
	void (*close)(struct vm_area_struct * area);
	int (*split)(struct vm_area_struct * area, unsigned long addr);
	int (*mremap)(struct vm_area_struct * area);
	vm_fault_t (*fault)(struct vm_fault *vmf);
	vm_fault_t (*huge_fault)(struct vm_fault *vmf,
			enum page_entry_size pe_size);
	void (*map_pages)(struct vm_fault *vmf,
			pgoff_t start_pgoff, pgoff_t end_pgoff);
	unsigned long (*pagesize)(struct vm_area_struct * area);

	/* notification that a previously read-only page is about to become
	 * writable, if an error is returned it will cause a SIGBUS */
	vm_fault_t (*page_mkwrite)(struct vm_fault *vmf);

	/* same as page_mkwrite when using VM_PFNMAP|VM_MIXEDMAP */
	vm_fault_t (*pfn_mkwrite)(struct vm_fault *vmf);

	/* called by access_process_vm when get_user_pages() fails, typically
	 * for use by special VMAs that can switch between memory and hardware
	 */
	int (*access)(struct vm_area_struct *vma, unsigned long addr,
		      void *buf, int len, int write);

	/* Called by the /proc/PID/maps code to ask the vma whether it
	 * has a special name.  Returning non-NULL will also cause this
	 * vma to be dumped unconditionally. */
	const char *(*name)(struct vm_area_struct *vma);

#ifdef CONFIG_NUMA
	/*
	 * set_policy() op must add a reference to any non-NULL @new mempolicy
	 * to hold the policy upon return.  Caller should pass NULL @new to
	 * remove a policy and fall back to surrounding context--i.e. do not
	 * install a MPOL_DEFAULT policy, nor the task or system default
	 * mempolicy.
	 */
	int (*set_policy)(struct vm_area_struct *vma, struct mempolicy *new);

	/*
	 * get_policy() op must add reference [mpol_get()] to any policy at
	 * (vma,addr) marked as MPOL_SHARED.  The shared policy infrastructure
	 * in mm/mempolicy.c will do this automatically.
	 * get_policy() must NOT add a ref if the policy at (vma,addr) is not
	 * marked as MPOL_SHARED. vma policies are protected by the mmap_sem.
	 * If no [shared/vma] mempolicy exists at the addr, get_policy() op
	 * must return NULL--i.e., do not "fallback" to task or system default
	 * policy.
	 */
	struct mempolicy *(*get_policy)(struct vm_area_struct *vma,
					unsigned long addr);
#endif
	/*
	 * Called by vm_normal_page() for special PTEs to find the
	 * page for @addr.  This is useful if the default behavior
	 * (using pte_page()) would not find the correct page.
	 */
	struct page *(*find_special_page)(struct vm_area_struct *vma,
					  unsigned long addr);
};

内存映射与struct mm_struct

每个进程都有一个mm_struct结构体，用于描述进程虚拟地址空间的内存映射信息，包含了这个进程的VMAs列表，还有其他一些内存管理相关的数据结构，如信号量和自旋锁。

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// /inlcude/linux/mm_types.h
struct mm_struct {
	struct {
		struct vm_area_struct *mmap;		/* list of VMAs */
		struct rb_root mm_rb;
		u64 vmacache_seqnum;                   /* per-thread vmacache */
#ifdef CONFIG_MMU
		unsigned long (*get_unmapped_area) (struct file *filp,
				unsigned long addr, unsigned long len,
				unsigned long pgoff, unsigned long flags);
#endif
		unsigned long mmap_base;	/* base of mmap area */
		unsigned long mmap_legacy_base;	/* base of mmap area in bottom-up allocations */
#ifdef CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES
		/* Base adresses for compatible mmap() */
		unsigned long mmap_compat_base;
		unsigned long mmap_compat_legacy_base;
#endif
		unsigned long task_size;	/* size of task vm space */
		unsigned long highest_vm_end;	/* highest vma end address */
		pgd_t * pgd;

		/**
		 * @mm_users: The number of users including userspace.
		 *
		 * Use mmget()/mmget_not_zero()/mmput() to modify. When this
		 * drops to 0 (i.e. when the task exits and there are no other
		 * temporary reference holders), we also release a reference on
		 * @mm_count (which may then free the &struct mm_struct if
		 * @mm_count also drops to 0).
		 */
		atomic_t mm_users;

		/**
		 * @mm_count: The number of references to &struct mm_struct
		 * (@mm_users count as 1).
		 *
		 * Use mmgrab()/mmdrop() to modify. When this drops to 0, the
		 * &struct mm_struct is freed.
		 */
		atomic_t mm_count;

#ifdef CONFIG_MMU
		atomic_long_t pgtables_bytes;	/* PTE page table pages */
#endif
		int map_count;			/* number of VMAs */

		spinlock_t page_table_lock; /* Protects page tables and some
					     * counters
					     */
		struct rw_semaphore mmap_sem;

		struct list_head mmlist; /* List of maybe swapped mm's.	These
					  * are globally strung together off
					  * init_mm.mmlist, and are protected
					  * by mmlist_lock
					  */


		unsigned long hiwater_rss; /* High-watermark of RSS usage */
		unsigned long hiwater_vm;  /* High-water virtual memory usage */

		unsigned long total_vm;	   /* Total pages mapped */
		unsigned long locked_vm;   /* Pages that have PG_mlocked set */
		unsigned long pinned_vm;   /* Refcount permanently increased */
		unsigned long data_vm;	   /* VM_WRITE & ~VM_SHARED & ~VM_STACK */
		unsigned long exec_vm;	   /* VM_EXEC & ~VM_WRITE & ~VM_STACK */
		unsigned long stack_vm;	   /* VM_STACK */
		unsigned long def_flags;

		spinlock_t arg_lock; /* protect the below fields */
		unsigned long start_code, end_code, start_data, end_data;
		unsigned long start_brk, brk, start_stack;
		unsigned long arg_start, arg_end, env_start, env_end;

		unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */

		/*
		 * Special counters, in some configurations protected by the
		 * page_table_lock, in other configurations by being atomic.
		 */
		struct mm_rss_stat rss_stat;

		struct linux_binfmt *binfmt;

		/* Architecture-specific MM context */
		mm_context_t context;

		unsigned long flags; /* Must use atomic bitops to access */

		struct core_state *core_state; /* coredumping support */
#ifdef CONFIG_MEMBARRIER
		atomic_t membarrier_state;
#endif
#ifdef CONFIG_AIO
		spinlock_t			ioctx_lock;
		struct kioctx_table __rcu	*ioctx_table;
#endif
#ifdef CONFIG_MEMCG
		/*
		 * "owner" points to a task that is regarded as the canonical
		 * user/owner of this mm. All of the following must be true in
		 * order for it to be changed:
		 *
		 * current == mm->owner
		 * current->mm != mm
		 * new_owner->mm == mm
		 * new_owner->alloc_lock is held
		 */
		struct task_struct __rcu *owner;
#endif
		struct user_namespace *user_ns;

		/* store ref to file /proc/<pid>/exe symlink points to */
		struct file __rcu *exe_file;
#ifdef CONFIG_MMU_NOTIFIER
		struct mmu_notifier_mm *mmu_notifier_mm;
#endif
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
		pgtable_t pmd_huge_pte; /* protected by page_table_lock */
#endif
#ifdef CONFIG_NUMA_BALANCING
		/*
		 * numa_next_scan is the next time that the PTEs will be marked
		 * pte_numa. NUMA hinting faults will gather statistics and
		 * migrate pages to new nodes if necessary.
		 */
		unsigned long numa_next_scan;

		/* Restart point for scanning and setting pte_numa */
		unsigned long numa_scan_offset;

		/* numa_scan_seq prevents two threads setting pte_numa */
		int numa_scan_seq;
#endif
		/*
		 * An operation with batched TLB flushing is going on. Anything
		 * that can move process memory needs to flush the TLB when
		 * moving a PROT_NONE or PROT_NUMA mapped page.
		 */
		atomic_t tlb_flush_pending;
#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
		/* See flush_tlb_batched_pending() */
		bool tlb_flush_batched;
#endif
		struct uprobes_state uprobes_state;
#ifdef CONFIG_HUGETLB_PAGE
		atomic_long_t hugetlb_usage;
#endif
		struct work_struct async_put_work;

#if IS_ENABLED(CONFIG_HMM)
		/* HMM needs to track a few things per mm */
		struct hmm *hmm;
#endif
	} __randomize_layout;

	/*
	 * The mm_cpumask needs to be at the end of mm_struct, because it
	 * is dynamically sized based on nr_cpu_ids.
	 */
	unsigned long cpu_bitmap[];
};

mmap

对于驱动程序来说，内存映射可以提供给用户程序直接访问设备内存的能力。

映射（mmap）一个设备意味着将进程虚拟地址空间中的一块区域（VMA）将设备内存进行直接关联，当用户对这块VMA进行读写操作时，访问的实际上就是其所映射的设备内存。

mmap()方法是file_operations结构的其中一个成员。因此，不同的文件(设备)可以实现自己的mmap()方法。而且mmap系统调用最终会调用到这个file_operations->mmap()方法。

另外，在mmap系统调用中，内核在调用到file_operations->mmap()之前，会先完成大量的工作，因此，file_operations->mmap()的函数原型与系统调用mmap原型是不太一样的：

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//file_operations->mmap:
int (*mmap) (struct file *, struct vm_area_struct *);

//syscall mmap:
mmap (caddr_t addr, size_t len, int prot, int flags, int fd, off_t offset)

当用户进程调用mmap映射一个设备内存到用户地址空间时，系统会建立一个新的VMA来响应这个映射，而设备需要支持mmap（或者实现mmap方法）来帮助完成这个VMA的初始化。从file_operations->mmap()的函数原型也可以看到，只有两个参数，一个struct file *，以及struct vm_area_struct *，从这里可以看出，file_operations->mmap()需要完成的内容，就是VMA的初始化。例如，构建合适的页表，用参数中的struct file *来初始化vm_area_struct->vm_file，又或者在必要时，用一组新的操作替换vma->vm_ops。

页表的构建

而完成页表的构建有两种方式，一种是通过内核接口remap_pfn_range()一次性完成所有工作，或者通过VMA的nopage()方法一次完成一个页，两种方法都有各自的优劣。

remap_pfn_range()

函数原型如下，这里暂时不深入探索它的实现

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/*  /mm/memory.c  */
/**
 * remap_pfn_range - remap kernel memory to userspace
 * @vma: user vma to map to
 * @addr: target user address to start at
 * @pfn: physical address of kernel memory
 * @size: size of map area
 * @prot: page protection flags for this mapping
 *
 *  Note: this is only safe if the mm semaphore is held when called.
 */
int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn, unsigned long size, pgprot_t prot)
{
	...
}
EXPORT_SYMBOL(remap_pfn_range);

我在源码中找到的，使用remap_pfn_range()完成页表构建的一个file_operations->mmap()实现如下，可以看到，用remap_pfn_range()可以很快捷简便的完成VMA的页表构建。

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/*  /drivers/char/mem.c  */
static int mmap_mem(struct file *file, struct vm_area_struct *vma)
{
	size_t size = vma->vm_end - vma->vm_start;
	phys_addr_t offset = (phys_addr_t)vma->vm_pgoff << PAGE_SHIFT;

	/* Does it even fit in phys_addr_t? */
	if (offset >> PAGE_SHIFT != vma->vm_pgoff)
		return -EINVAL;

	/* It's illegal to wrap around the end of the physical address space. */
	if (offset + (phys_addr_t)size - 1 < offset)
		return -EINVAL;

	if (!valid_mmap_phys_addr_range(vma->vm_pgoff, size))
		return -EINVAL;

	if (!private_mapping_ok(vma))
		return -ENOSYS;

	if (!range_is_allowed(vma->vm_pgoff, size))
		return -EPERM;

	if (!phys_mem_access_prot_allowed(file, vma->vm_pgoff, size,
						&vma->vm_page_prot))
		return -EINVAL;

	vma->vm_page_prot = phys_mem_access_prot(file, vma->vm_pgoff,
						 size,
						 vma->vm_page_prot);

	vma->vm_ops = &mmap_mem_ops;

	/* Remap-pfn-range will mark the range VM_IO */
	if (remap_pfn_range(vma,
			    vma->vm_start,
			    vma->vm_pgoff,
			    size,
			    vma->vm_page_prot)) {
		return -EAGAIN;
	}
	return 0;
}

nopage

ldd3中还提到一种建立正确页映射的方式为调用nopage方法，例如，当调用mremap系统调用扩展VMA时，就会调用nopage方法来设置新页。但是我自己去看4.19的源码，没有找到这个函数的定义，所以这个应该是要驱动自己实现的？。书中给出的定义如下：

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struct page *(*nopage)(struct vm_area_struct *vma,
 unsigned long address, int *type);

当进程试图访问一个还未在内存中的页时，相应的nopage方法就会被调用。

分析内核mmap源码

用户空间的方法xxx，对应系统调用层方法则是sys_xxx，因此mmap系统调用在内核中是sys_mmap()方法来响应的。

sys_mmap实现的系统调用与平台有关，但最终都调用ksys_mmap_pgoff()来实现。另外，mmap传递的offset参数会在调用的过程中转换成页数，ksys_mmap_pgoff()接收到的参数已经是已页为单位的。

例如，x86架构在arch/x86/um/sys_call_table_64.c和arch/x86/um/sys_call_table_64.c中：通过#define sys_mmap old_mmap宏定义语句，用old_mmap方法来响应sys_mmap。

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mm/mmap.c

SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
{
	struct mmap_arg_struct a;

	if (copy_from_user(&a, arg, sizeof(a)))
		return -EFAULT;
	if (offset_in_page(a.offset))
		return -EINVAL;

	return ksys_mmap_pgoff(a.addr, a.len, a.prot, a.flags, a.fd,
			       a.offset >> PAGE_SHIFT);
}
#endif /* __ARCH_WANT_SYS_OLD_MMAP */

可以看到，将offset右移了PAGE_SHIFT(12)位，将其转化为页号。

在arch/ia64/kernel/sys_ia64.c也有对sys_mmap的实现，其实和上面的old_mmap差不多，最后都是将offset右移PAGE_SHIFT后调用ksys_mmap_pgoff()。

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arch/ia64/kernel/sys_ia64.c

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

	addr = ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
	if (!IS_ERR((void *) addr))
		force_successful_syscall_return();
	return addr;
}

ksys_mmap_pgoff()：

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mm/mmap.c

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)) {
		audit_mmap_fd(fd, flags);
		file = fget(fd);
		if (!file)
			return -EBADF;
		if (is_file_hugepages(file))
			len = ALIGN(len, huge_page_size(hstate_file(file)));
		retval = -EINVAL;
		if (unlikely(flags & MAP_HUGETLB && !is_file_hugepages(file)))
			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);

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

ksys_mmap_pgoff()先通过mmap参数里的文件描述符获得目标文件的struct file结构体，做了一些检查后调用vm_mmap_pgoff()

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mm/mmap.c

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 (down_write_killable(&mm->mmap_sem))
			return -EINTR;
		ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
				    &populate, &uf);
		up_write(&mm->mmap_sem);
		userfaultfd_unmap_complete(mm, &uf);
		if (populate)
			mm_populate(ret, populate);
	}
	return ret;
}

vm_mmap_pgoff()最后调用do_mmap_pgoff()，这是一个内联函数，直接封装了do_mmap()

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include/linux/mm.h

static inline unsigned long
do_mmap_pgoff(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)
{
	return do_mmap(file, addr, len, prot, flags, 0, pgoff, populate, uf);
}

do_mmap

重点看do_mmap()函数：

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mm/mmap.c

unsigned long do_mmap(struct file *file, unsigned long addr,
			unsigned long len, unsigned long prot,
			unsigned long flags, vm_flags_t vm_flags,
			unsigned long pgoff, unsigned long *populate,
			struct list_head *uf)
{
	struct mm_struct *mm = current->mm; //获取当前进程的 mm_struct结构体
	int pkey = 0;

	*populate = 0;

	if (!len)
		return -EINVAL; //len参数不可为零

	/*
	 * 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); //页对齐
	if (!len)
		return -ENOMEM;

	/* offset overflow? */
	if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
		return -EOVERFLOW; //检查是否会整数溢出？但是pgoff和len都是无符号整数，应该不会出现这种情况才对啊

	/* Too many mappings? */
	if (mm->map_count > sysctl_max_map_count)
		return -ENOMEM; //mm->map_count有最大值限制

	/* 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); //寻找一个合适的线性地址
	if (offset_in_page(addr)) //#define offset_in_page(p)	((unsigned long)(p) & ~PAGE_MASK) // PAGE_MASK=0XFFFFF000
		return addr; //其实在get_unmapped_area中也做了一次这个检查，如果不是页对齐，会返回-EINVAL

	if (flags & MAP_FIXED_NOREPLACE) { // MAP_FIXED_NOREPLACE和MAP_FIXED类似，但是不会抢占
		struct vm_area_struct *vma = find_vma(mm, addr);

		if (vma && vma->vm_start < addr + len)
			return -EEXIST; //如果目标地址已被占用，直接返回-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)) //检查len的合法性
			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;
			/* fall through */
		case MAP_SHARED_VALIDATE:
			if (flags & ~flags_mask)
				return -EOPNOTSUPP;
			if ((prot&PROT_WRITE) && !(file->f_mode&FMODE_WRITE))
				return -EACCES;

			/*
			 * 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);

			/* fall through */
		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;
	}

	addr = mmap_region(file, addr, len, vm_flags, pgoff, uf); //完成vma的初始化、映射
	if (!IS_ERR_VALUE(addr) &&
	    ((vm_flags & VM_LOCKED) ||
	     (flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE))
		*populate = len;
	return addr;
}

do_mmap()的大致逻辑如下：

首先获取了当前进程的mm_struct结构体

将len参数（映射的长度）调整为页对齐

对参数合法性等做检查

调用get_unmapped_area(file, addr, len, pgoff, flags)来寻找并返回一个合适的线性地址，这里优先会调用file->f_op->get_unmapped_area()，如果文件没有定义这个方法，则调用current->mm->get_unmapped_area：

get_unmapped_area返回的地址如果不是页对齐的话会直接返回-EINVAL错误代码。

在get_area()内部，如果参数addr等于0，或者(addr, addr+len)覆盖了已被占用的区域，则会根据当前进程的mm_struct的mm_rb字段指向的进程所有的vm_area_struct组成的红黑树查找一段合适的区域，如果addr不等于0，且(addr, addr+len)未被占用，则直接返回addr。另外，如果flag的MAP_FIXED被置位的话(MAP_FIXED表示严格按照参数传入的addr来映射，而且可以“抢占”)，也会直接将addr返回，不会检查区间是否已被占用，如果该区间已被占用的话，后面会在mmap_region函数中取消该区间的映射。

拿到线性地址后，开始对flag值做计算…….

最后，进入mmap_region()函数来进行映射。

mmap_region

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mm/mmap.c

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;
	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 */
	while (find_vma_links(mm, addr, addr + len, &prev, &rb_link,
			      &rb_parent)) {
		if (do_munmap(mm, addr, len, 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);
	if (!vma) {
		error = -ENOMEM;
		goto unacct_error;
	}

	vma->vm_start = addr;
	vma->vm_end = addr + len;
	vma->vm_flags = vm_flags;
	vma->vm_page_prot = vm_get_page_prot(vm_flags);
	vma->vm_pgoff = pgoff;

	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);
		error = call_mmap(file, vma); //file->f_op->mmap(file, vma)这里是真正完成映射的(建立页表等)
		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;
		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);
	}

	vma_link(mm, vma, prev, rb_link, rb_parent); //将vma插入链表和红黑树
	/* Once vma denies write, undo our temporary denial count */
	if (file) {
		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;
}

mmap_region主要是初始化vm_area_struct对象、完成映射、将对象插入链表和红黑树

完成映射是通过调用file->f_op->mmap(file, vma)来完成的，也就是说，是由设备驱动来决定如何完成映射(虚拟地址到物理地址的映射，即建立页表等)。如果设备有自己的物理内存的话，可以直接调用内核函数完成映射，如果没有的话，可以申请物理内存，然后通过上面提到的页表的构建来完成映射，又或者根本不做映射，只返回没有映射到物理内存的虚拟地址，然后后续真正要访问该地址时就会导致内存访问异常，那时才分配物理地址完成映射。

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