Linux下獲取虛擬地址對應的物理地址的方法

首先摘錄如下URL對pagemap的描述。

https://www.kernel.org/doc/Documentation/vm/pagemap.txt

 * /proc/pid/pagemap.  This file lets a userspace process find out which
   physical frame each virtual page is mapped to.  It contains one 64-bit
   value for each virtual page, containing the following data (from
   fs/proc/task_mmu.c, above pagemap_read):


    * Bits 0-54  page frame number (PFN) if present
    * Bits 0-4   swap type if swapped
    * Bits 5-54  swap offset if swapped
    * Bit  55    pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
    * Bits 56-60 zero
    * Bit  61    page is file-page or shared-anon
    * Bit  62    page swapped
    * Bit  63    page present


   If the page is not present but in swap, then the PFN contains an
   encoding of the swap file number and the page's offset into the
   swap. Unmapped pages return a null PFN. This allows determining
   precisely which pages are mapped (or in swap) and comparing mapped
   pages between processes.

接下來，我們根據上述描述，給出獲取虛擬地址對應的物理地址的代碼

#define    page_map_file     "/proc/self/pagemap"
#define    PFN_MASK          ((((uint64_t)1)<<55)-1)
#define    PFN_PRESENT_FLAG  (((uint64_t)1)<<63)


int mem_addr_vir2phy(unsigned long vir, unsigned long *phy)
{
    int fd;
    int page_size=getpagesize();
    unsigned long vir_page_idx = vir/page_size;
    unsigned long pfn_item_offset = vir_page_idx*sizeof(uint64_t);
    uint64_t pfn_item;
    
    fd = open(page_map_file, O_RDONLY);
    if (fd<0)
    {
        printf("open %s failed", page_map_file);
        return -1;
    }


    if ((off_t)-1 == lseek(fd, pfn_item_offset, SEEK_SET))
    {
        printf("lseek %s failed", page_map_file);
        return -1;
    }


    if (sizeof(uint64_t) != read(fd, &pfn_item, sizeof(uint64_t)))
    {
        printf("read %s failed", page_map_file);
        return -1;
    }


    if (0==(pfn_item & PFN_PRESENT_FLAG))
    {
        printf("page is not present");
        return -1;
    }


    *phy = (pfn_item & PFN_MASK)*page_size + vir % page_size;
    return 0;


}

如果擔心vir地址對應的頁面不在內存中，可以在調用mem_addr_vir2phy之前，先訪問一下此地址。

例如， int  a=*(int *)(void *)vir;

如果擔心Linux的swap功能將進程的頁面交換到硬盤上從而導致頁面的物理地址變化，可以關閉swap功能。

下面兩個C庫函數可以阻止Linux將當前進程的部分或全部頁面交換到硬盤上。

       int mlock(const void *addr, size_t len);
       int mlockall(int flags);

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