Translate Virtual Address To Physical Address

The followings are some ways to translate a virtual address to the corresponding physical address, if exist.

Step 1. Walk the page tables to get pte_t *.

// struct mm_struct *mm : The memory descriptor of the current process.
// unsigned long address: The virtual address to be translated.
pgd_t *pgd = pgd_offset(mm, address);
if (!pgd_none(*pgd) && !pgd_bad(*pgd)) {
    pud_t *pud = pud_offset(pgd, address);
    if (!pud_none(*pud) && !pud_bad(*pud)) {
        pmd_t *pmd = pmd_offset(pud, address);
        if (!pmd_none(*pmd) && !pmd_bad(*pmd)) {
            pte_t *pte = pte_offset_map(pmd, address);
            if (!pte_none(*pte)) {
                // now you get a pointer pointing to a pte entry!
            }
            pte_unmap(pte);
        }
    }
}       

Step 2.

• A) Calculate the bits directly.

// Assume it is on 32-bit machine with 4 KB pages.
unsigned long phys = pte_val(*pte) & 0xfffff000 + address & 0x00000fff;

• B) With the help of a page descriptor.

struct page *pg = pte_page(*pte);
unsigned long phys = page_to_phys(pg);

Userspace:

cat /proc/<pid>/maps to get virtual address intervals.

xxd /proc/<pid>/pagemap to get virtual - physical address mappings. documention

Others:

• virt_to_page(unsigned long address) seems to be a faster way to get struct page*, but you have to make sure the return value is valid. (code)

• virt_to_phys(volatile void *address) is only suitable for addresses directly mapped or mapped by kmalloc. (code)

Reference:

• Understanding the Linux Kernel 3rd (Daniel P. Bovet, Marco Cesati), Sections 2.5, 8.1, 9.2, etc.
• page_to_phys from <asm/io.h>
• How to get the physical address from the logical one in a Linux kernel module? Stack Overflow
• How to find the physical address of a variable from user-space in Linux? Stack Overflow