内存映射函数remap_pfn_range学习——代码分析（3）

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作者

彭东林

QQ 405728433

 

平台

Linux-4.10.17

Qemu-2.8 + vexpress-a9

概述

前面两篇介绍了remap_pfn_range的使用，下面学习一下该函数的实现。

 

正文

前提：下面的分析基于2级页表

 

remap_pfn_range的实现在mm/memory.c。

 int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
             unsigned long pfn, unsigned long size, pgprot_t prot)
 {
     pgd_t *pgd;
     unsigned long next;
     unsigned long end = addr + PAGE_ALIGN(size);
     struct mm_struct *mm = vma->vm_mm;
     unsigned long remap_pfn = pfn;
     int err;

     /*
      * Physically remapped pages are special. Tell the
      * rest of the world about it:
      *   VM_IO tells people not to look at these pages
      *    (accesses can have side effects).
      *   VM_PFNMAP tells the core MM that the base pages are just
      *    raw PFN mappings, and do not have a "struct page" associated
      *    with them.
      *   VM_DONTEXPAND
      *      Disable vma merging and expanding with mremap().
      *   VM_DONTDUMP
      *      Omit vma from core dump, even when VM_IO turned off.
      *
      * There's a horrible special case to handle copy-on-write
      * behaviour that some programs depend on. We mark the "original"
      * un-COW'ed pages by matching them up with "vma->vm_pgoff".
      * See vm_normal_page() for details.
      */
     vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;

     BUG_ON(addr >= end);
     pfn -= addr >> PAGE_SHIFT;
     pgd = pgd_offset(mm, addr);
     flush_cache_range(vma, addr, end);
     do {
         next = pgd_addr_end(addr, end);
         err = remap_pud_range(mm, pgd, addr, next,
                 pfn + (addr >> PAGE_SHIFT), prot);
         if (err)
             break;
     } while (pgd++, addr = next, addr != end);

     return err;
 }

第2行，pfn是将要被映射的物理页帧号，size表示需要映射的尺寸

第6行，计算本次映射的结尾虚拟地址

第32行的pfn-=addr>>PAGE_SHIFT，和第38行的pfn+(addr>>PAGE_SHIFT)是为了循环处理上的便利

第33行，计算addr在第1级页表中对应的页表项的地址，pgd_offset宏展开后是：mm->pgd + (addr >>21)

第34行，刷新cache

第36行，pgd_addr_end(addr, end)计算下一个将要被映射的虚拟地址，如果addr到end可以被一个pgd映射的话，那么返回end的值

第37行的remap_pud_range的定义如下:

 static inline int remap_pud_range(struct mm_struct *mm, pgd_t *pgd,
             unsigned long addr, unsigned long end,
             unsigned long pfn, pgprot_t prot)
 {
     pud_t *pud;
     unsigned long next;

     pfn -= addr >> PAGE_SHIFT;
     pud = pud_alloc(mm, pgd, addr);
     if (!pud)
         return -ENOMEM;
     do {
         next = pud_addr_end(addr, end);
         if (remap_pmd_range(mm, pud, addr, next,
                 pfn + (addr >> PAGE_SHIFT), prot))
             return -ENOMEM;
     } while (pud++, addr = next, addr != end);
     return ;
 }

第9行，对于2级页表，pud_alloc(mm, pgd, addr)返回的是pgd的值

第13行，对于2级页表，pud_addr_end(addr, end)返回end的值

第14行，函数remap_pmd_range定义如下：

 static inline int remap_pmd_range(struct mm_struct *mm, pud_t *pud,
             unsigned long addr, unsigned long end,
             unsigned long pfn, pgprot_t prot)
 {
     pmd_t *pmd;
     unsigned long next;

     pfn -= addr >> PAGE_SHIFT;
     pmd = pmd_alloc(mm, pud, addr);
     if (!pmd)
         return -ENOMEM;
     VM_BUG_ON(pmd_trans_huge(*pmd));
     do {
         next = pmd_addr_end(addr, end);
         if (remap_pte_range(mm, pmd, addr, next,
                 pfn + (addr >> PAGE_SHIFT), prot))
             return -ENOMEM;
     } while (pmd++, addr = next, addr != end);
     return ;
 }

第9行，对于2级页表，pmd_alloc(mm, pud, addr)返回的是pud的值，其实也就是pgd的值

第14行，对于2级页表，pmd_addr_end(addr, end)返回end的值

第15行，函数remap_pte_range定义如下：

 static int remap_pte_range(struct mm_struct *mm, pmd_t *pmd,
             unsigned long addr, unsigned long end,
             unsigned long pfn, pgprot_t prot)
 {
     pte_t *pte;
     spinlock_t *ptl;

     pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
     if (!pte)
         return -ENOMEM;
     arch_enter_lazy_mmu_mode();
     do {
         BUG_ON(!pte_none(*pte));
         set_pte_at(mm, addr, pte, pte_mkspecial(pfn_pte(pfn, prot)));
         pfn++;
     } while (pte++, addr += PAGE_SIZE, addr != end);
     arch_leave_lazy_mmu_mode();
     pte_unmap_unlock(pte - , ptl);
     return ;
 }

第8行，pte_alloc_map_lock的定义如下：

#define pte_alloc_map_lock(mm, pmd, address, ptlp)    \
    (pte_alloc(mm, pmd, address) ?            \
         NULL : pte_offset_map_lock(mm, pmd, address, ptlp))

pte_alloc首先检查*pmd是否为空，如果为空的话，表示第2级页表还尚未分配，那么调用__pte_alloc分配一个页（其实是调用alloc_pages分配了一个page，也就是4KB），并将起始地址存放的*pmd中，其实就是*pgd。如果不出意外的话，pte_alloc返回0，这样pte_offset_map_lock就会被调用，返回address在第2级页表中的表项的地址

 

第14行，调用pte_mkspecial构造第2级页表项的内容，函数set_pte_at用于将表项内容设置到pte指向的第2级页表项中

第15行，计算下一个将要被映射的物理页帧号

第16行，计算第2级页表项中下一个将要被填充的表项的地址

 

 

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