On Fri, Oct 3, 2014 at 1:04 AM, mind entropy <mindentropy@xxxxxxxxx> wrote:
> Hi,
>
> I am experimenting with page table walking. I run a user space
> program and mmap a memory region and pass the address to the module.
>
> In the kernel module I get the current->mm to get the current task
> memory descriptor and use it to walk the pages. Once I get the pte
> address I reboot and dump the memory at that point using u-boot. (md.b
> <addr> 1)
> but I do not find the values present there. ( I have filled the region
> with incremental value starting from 0 to 255).
>
> My processor is a Samsung S3C2440 ARM920T (mini2440 development
> board). The physical address starts at 0x30000000.
>
> -------------------------------------------------------------------
> Sample code of the page walk: (This is just for illustration and
> experimenting and does
> not contain error checks etc.).
>
> void my_follow_page(struct mm_struct *mm,
> unsigned long addr_res)
> {
> pgd_t *pgd;
> pmd_t *pmd;
> pud_t *pud;
> pte_t *ptep, pte;
> struct page *page;
>
>
> down_read(&(mm->mmap_sem));
>
> pgd = pgd_offset(mm,addr_res);
>
> if(pgd_none(*pgd) || pgd_bad(*pgd)) {
> printk(KERN_ALERT "pgd bad\n");
> return;
> } else {
> printk(KERN_ALERT "pgd 0x%lx\n",(unsigned long)pgd);
> }
>
> pud = pud_offset(pgd,addr_res);
>
> if(pud_none(*pud) || pud_bad(*pud)) {
> printk(KERN_ALERT "pud bad\n");
> return;
> } else {
> printk(KERN_ALERT "pud 0x%lx\n",(unsigned long)pud);
> }
>
> pmd = pmd_offset(pud,addr_res);
>
> if(pmd_none(*pmd) || pmd_bad(*pmd)) {
> printk(KERN_ALERT "pmd bad\n");
> return;
> } else {
> printk(KERN_ALERT "pmd 0x%lx\n",(unsigned long)pmd);
> }
>
>
> ptep = pte_offset_map(pmd,addr_res);
> if(!ptep) {
> printk(KERN_ALERT "ptep bad\n");
> } else {
> printk(KERN_ALERT "ptep 0x%lx\n",(unsigned long)ptep);
> }
>
> pte = *ptep;
>
>
> if(pte_present(pte)) {
> printk(KERN_ALERT "pte : 0x%lx\n",(unsigned long)pte);
> page = pte_page(pte);
> } else {
> printk(KERN_ALERT "pte not present\n");
> }
>
> printk(KERN_ALERT "pte with offset 0x%lx offset : 0x%lx\n",
> pte+((addr_res) & ((1<<PAGE_SHIFT)-1)),
> addr_res & ((1<<PAGE_SHIFT)-1));
>
> up_read(&(mm->mmap_sem));
> }
>
> -------------------------------------------------------------------
>
> Sample output:
>
> [ 86.447788] Current task pid: 2384
> [ 86.447885] mm: 0xc3b39a80, active_mm 0xc3b39a80
> [ 86.451594] Page global directory : 0xc3af4000
> [ 86.456127] mmap base : 0xb6f60000
> [ 86.459175] vm_start : 0x8000, vm_end : 0x9000
> [ 86.463684] vm_start : 0x10000, vm_end : 0x11000
> [ 86.468202] vm_start : 0x100000, vm_end : 0x101000
> [ 86.472884] vm_start : 0xb6dfc000, vm_end : 0xb6f25000
> [ 86.477975] vm_start : 0xb6f25000, vm_end : 0xb6f2c000
> [ 86.483000] vm_start : 0xb6f2c000, vm_end : 0xb6f2e000
> [ 86.488070] vm_start : 0xb6f2e000, vm_end : 0xb6f2f000
> [ 86.493080] vm_start : 0xb6f2f000, vm_end : 0xb6f32000
> [ 86.498089] vm_start : 0xb6f3a000, vm_end : 0xb6f57000
> [ 86.503120] vm_start : 0xb6f5a000, vm_end : 0xb6f5d000
> [ 86.508149] vm_start : 0xb6f5d000, vm_end : 0xb6f5e000
> [ 86.513187] vm_start : 0xb6f5e000, vm_end : 0xb6f5f000
> [ 86.518225] vm_start : 0xb6f5f000, vm_end : 0xb6f60000
> [ 86.523251] vm_start : 0xbe934000, vm_end : 0xbe956000
> [ 86.528299] kval : 3020100
> [ 86.530870] addr: 100000
> [ 86.533446] pgd 0xc3af4000
> [ 86.535992] pud 0xc3af4000
> [ 86.538720] pmd 0xc3af4000
> [ 86.541281] ptep 0xc3b0e400
> [ 86.544071] pte : 0x3278214f
> [ 86.546831] pte with offset 0x3278214f offset : 0x0
>
>
> The problem is that when I do a md.b 0x3278214f 100 in u-boot to do a
> memory dump I see the values in it but I don't see the exact value
> filled in that memory location. What am I doing wrong?
>
> Thanks,
> Gautam.
Got it working. Thanks to riel on #mm the pte which I am using
contains other bits not related to the physical address. I have to do
a pte_pfn(pte) to get the page frame number. The pfn to physical addr
can be got by (pfn<<PAGE_SHIFT) + (addr_res & ~PAGE_MASK). The
addr_res & ~PAGE_MASK gives the offset in the physical frame.
So the final code looks as
--------------------------------------------------------
printk(KERN_ALERT "pfn from pte : 0x%lx\n",pfn = pte_pfn(pte));
printk(KERN_ALERT "pfn to addr : 0x%lx, addr_res :
0x%lx\n",(pfn<<PAGE_SHIFT),
(unsigned long)PAGE_MASK);
printk(KERN_ALERT "phys_addr : 0x%lx\n",(pfn<<PAGE_SHIFT) +
(addr_res & ~PAGE_MASK));
--------------------------------------------------------
Thanks,
Gautam.
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