arm L_PTE_XXX entry addition for Debugging purpose

From: bill4carson@gmail.com (bill4carson)
To: kernelnewbies@lists.kernelnewbies.org
Subject: arm L_PTE_XXX entry addition for Debugging purpose
Date: Wed, 01 Aug 2012 10:08:34 +0800	[thread overview]
Message-ID: <50188FA2.1090102@gmail.com> (raw)
In-Reply-To: <CABFWu7aMMUaVEoPhK4JWzBhi50SOX+Tw5H+5bS1gsukMPZVqag@mail.gmail.com>

On 2012?07?31? 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@gmail.com
> <mailto:bill4carson@gmail.com>> wrote:
>
>
>
>     On 2012?07?30? 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
>
>
>
>         _________________________________________________
>         Kernelnewbies mailing list
>         Kernelnewbies at kernelnewbies.__org
>         <mailto:Kernelnewbies@kernelnewbies.org>
>         http://lists.kernelnewbies.__org/mailman/listinfo/__kernelnewbies <http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies>
>
>
>     --
>     Love each day!
>
>     --bill
>
>

-- 
Love each day!

--bill