On 2012年08月02日 17:03, Dhyan wrote:
> Dear Bill,
>
> What i found from experimenting on arm Linux kernel is ,after every
> access if i clear the _PG_ACCESS bit of the pte (using
> /proc/<pid>/clear_refs),the next write also will come to kernel
> (handle_pte_fault).But I dont know whether my clearing action causing
> any bad impact on any other system.
>
592 static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
593 unsigned long end, struct mm_walk *walk)
594 {
595 struct vm_area_struct *vma = walk->private;
596 pte_t *pte, ptent;
597 spinlock_t *ptl;
598 struct page *page;
599
600 split_huge_page_pmd(walk->mm, pmd);
601 if (pmd_trans_unstable(pmd))
602 return 0;
603
604 pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
605 for (; addr != end; pte++, addr += PAGE_SIZE) {
606 ptent = *pte;
607 if (!pte_present(ptent))
608 continue;
609
610 page = vm_normal_page(vma, addr, ptent);
611 if (!page)
612 continue;
613
614 /* Clear accessed and referenced bits. */
615 ptep_test_and_clear_young(vma, addr, pte);
^^^^^^^^^^^^
For arm, it clear all the corresponding pte entries.
that's why you got second page fault.
Again, I don't understand your original purpose.
> --
> Thanks
> Dhyan
>
> On Thu, Aug 2, 2012 at 2:12 PM, bill4carson <bill4carson@xxxxxxxxx
> <mailto:bill4carson@xxxxxxxxx>> wrote:
>
>
>
> On 2012年08月01日 12:53, Dhyan wrote:
>
> Dear Bill,
>
> Thank you for spending your valuable time for understanding and
> answering my queries !!!
>
> I was trying to apply some garbage collection algorithm on this
> dumped
> pages,thats why i want only the written pages.
>
> Sorry to ask,but is there any other good way to find the written
> pages
> of a user process?
>
>
> Only the first wirte trigger page fault, which setup the page table by
> grabbing a physical page to backup virtual address page. After this,
> all write into that page goes like wind without any kernel interference
> anymore.
>
>
> But my hunch tell me what you want is to track every user space write
> operation.
>
>
>
> --
> Thanks
> Dhyan
>
> On Wed, Aug 1, 2012 at 7:38 AM, bill4carson
> <bill4carson@xxxxxxxxx <mailto:bill4carson@xxxxxxxxx>
> <mailto:bill4carson@xxxxxxxxx <mailto:bill4carson@xxxxxxxxx>>__>
> wrote:
>
>
>
> On 2012年07月31日 17:20, Dhyan wrote:
>
> Thank You Bill !!!
>
> I dont know my approach is correct or not,But the
> actual purpose
> was to
> dump only written pages of a user process using a kernel
> module.I have
> a kernel thread which will dump user process memory in
> specific
> interval.So i thought of updating this flag
> (L_PTE_DEBUG) from
> handle_pte_fault and clear from my clear thread so that
> i can
> dump only
> the written pages after my last dump.
>
>
> Yes, you can do that, only if your accounting memory
> doesn't get swapped
> out.
>
> If I understand correctly, when writing a page, you mark
> corresponding
> linux pte entry with L_PTE_DEBUG. Then your kernel module
> periodically
> loops all linux pte table, find pte entry marked with
> L_PTE_DEBUG.....
>
> I don't think it's wise to do so, you have 768 1st level
> pgd entries
> for user space, followed by 256 pte entries with each pgd
> entry.
> it's much slower to find out the right one.
>
> moreover, you probably need to remap those L_PTE_DEBUG
> physical pages
> into your own current process address space. IMHO, I don't
> follow
> such idea could be feasible.
>
>
>
> if you have some suggestions could you please share
> wth me?
>
>
> I understand how you plan to do this, could I ask why you
> need to dump
> the written pages?
>
>
> --
> Thanks
> Dhyan
>
> On Tue, Jul 31, 2012 at 12:43 PM, bill4carson
> <bill4carson@xxxxxxxxx <mailto:bill4carson@xxxxxxxxx>
> <mailto:bill4carson@xxxxxxxxx <mailto:bill4carson@xxxxxxxxx>>
> <mailto:bill4carson@xxxxxxxxx <mailto:bill4carson@xxxxxxxxx>
> <mailto:bill4carson@xxxxxxxxx <mailto:bill4carson@xxxxxxxxx>>__>__>
>
> wrote:
>
>
>
> On 2012年07月30日 17:39, Dhyan wrote:
>
> Dear All,
>
> From linux(2.6.35) arm page table
> architecture i can
> see we
> have one
> hardware page table and there is
> corresponding Linux
> page table
> Entry
> (L_PTE_*).The "Linux" PTE definitions are as
> like below
> from
> arch/arm/include/asm/pgtable.______h.
>
>
>
> |#define L_PTE_PRESENT (1<< 0)
> #define L_PTE_FILE (1<< 1)
> #define L_PTE_YOUNG (1<< 1)
> #define L_PTE_BUFFERABLE(1<< 2)
> #define L_PTE_CACHEABLE (1<< 3)
> #define L_PTE_USER (1<< 4)
> #define L_PTE_WRITE (1<< 5)
> #define L_PTE_EXEC (1<< 6)
> #define L_PTE_DIRTY (1<< 7)
> #define L_PTE_COHERENT (1<< 9)
> #define L_PTE_SHARED (1<< 10)
> |
>
> So is it possible to add one more #|define
> L_PTE_DEBUG
> (1 <<
> 11)| for my
>
> debugging purpose (basically to trap all the
> write to
> that page
> and set
> this bit when write happens and clear it off
> in another
> thread
> )? Or
> is there any limitation like we can use only
> L_PTE till
> 10th bit ?
>
>
> No such limitation on bit 11, so you can use define
> L_PTE_DEBUG (1
> << 11)
> However I don't follow why you want to do so?
>
>
> So could you please help
>
> --
>
> Thanks & Regards
>
> Dhayn
>
>
>
>
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>
> --
> Love each day!
>
> --bill
>
>
>
> --
> Love each day!
>
> --bill
>
>
>
> --
> Love each day!
>
> --bill
>
>
--
Love each day!
--bill
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