linux page table

linux page table

[Xin Tong]

When a process is created in Linux, corresponding page table is
implemented. In the current x86 linux, the page table is a multi-level
page table and CR3 points to the first level of the page table.  I
have 2 questions.

1. is the value in CR3 virtual address or physical address ?
2. can the address of the first level of the page table during a
process's lifetime change ?
3. can two different processes have their CR3 being the same value
even though they have different first level page tables ?

Thanks

Re: linux page table

[Shentino]

On Sat, Sep 1, 2012 at 12:30 PM, Xin Tong <xerox.time.tech@gmail.com> wrote:
> When a process is created in Linux, corresponding page table is
> implemented. In the current x86 linux, the page table is a multi-level
> page table and CR3 points to the first level of the page table.  I
> have 2 questions.
>
> 1. is the value in CR3 virtual address or physical address ?

It's a physical address.  It points the CPU to it in physical memory.

More generally, all addresses in page tables, directories, etc are
physical addresses.

> 2. can the address of the first level of the page table during a
> process's lifetime change ?

This I don't know.

> 3. can two different processes have their CR3 being the same value
> even though they have different first level page tables ?

I'm not sure about this, but I think CR3 is actually bound to the
mm_struct and not the process.

Think about separate processes with the same address space, such as
multithreaded processes.

>
> Thanks
> --
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Re: linux page table

[Xin Tong]

On Sat, Sep 1, 2012 at 1:01 PM, Shentino <shentino@gmail.com> wrote:
> On Sat, Sep 1, 2012 at 12:30 PM, Xin Tong <xerox.time.tech@gmail.com> wrote:
>> When a process is created in Linux, corresponding page table is
>> implemented. In the current x86 linux, the page table is a multi-level
>> page table and CR3 points to the first level of the page table.  I
>> have 2 questions.
>>
>> 1. is the value in CR3 virtual address or physical address ?
>
> It's a physical address.  It points the CPU to it in physical memory.
>
> More generally, all addresses in page tables, directories, etc are
> physical addresses.
>
>> 2. can the address of the first level of the page table during a
>> process's lifetime change ?
>
> This I don't know.
>
>> 3. can two different processes have their CR3 being the same value
>> even though they have different first level page tables ?
>
> I'm not sure about this, but I think CR3 is actually bound to the
> mm_struct and not the process.
>
> Think about separate processes with the same address space, such as
> multithreaded processes.

Do not all the threads share the same address space in a multithreaded program ?

Xin

>
>>
>> Thanks
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at  http://www.tux.org/lkml/

Re: linux page table

[Jiri Kosina]

On Sat, 1 Sep 2012, Xin Tong wrote:

> When a process is created in Linux, corresponding page table is
> implemented. In the current x86 linux, the page table is a multi-level
> page table and CR3 points to the first level of the page table.  I
> have 2 questions.
> 
> 1. is the value in CR3 virtual address or physical address ?

Physical, otherwise you will have chicken-egg problem.

> 2. can the address of the first level of the page table during a
> process's lifetime change ?

In theory it would be possible to implement. But I don't see a scenario 
when it might be useful.

> 3. can two different processes have their CR3 being the same value
> even though they have different first level page tables ?

Yes, if they are created by clone(CLONE_VM). In such case they share the 
same mm_struct, and therefore mm_struct->pgd (which is exactly what is 
loaded into cr3 in switch_mm()) is the same.

LKML is however very inappropriate list for such questions. Please ask on 
kernelnewbies list next time.

-- 
Jiri Kosina
SUSE Labs

Re: linux page table

[Xin Tong]

On Sun, Sep 2, 2012 at 1:10 AM, Jiri Kosina <jkosina@suse.cz> wrote:
> On Sat, 1 Sep 2012, Xin Tong wrote:
>
>> When a process is created in Linux, corresponding page table is
>> implemented. In the current x86 linux, the page table is a multi-level
>> page table and CR3 points to the first level of the page table.  I
>> have 2 questions.
>>
>> 1. is the value in CR3 virtual address or physical address ?
>
> Physical, otherwise you will have chicken-egg problem.
>
>> 2. can the address of the first level of the page table during a
>> process's lifetime change ?
>
> In theory it would be possible to implement. But I don't see a scenario
> when it might be useful.
>
>> 3. can two different processes have their CR3 being the same value
>> even though they have different first level page tables ?
>
> Yes, if they are created by clone(CLONE_VM). In such case they share the
> same mm_struct, and therefore mm_struct->pgd (which is exactly what is
> loaded into cr3 in switch_mm()) is the same.
>

Is this the COW mechanism in linux. what if the cloned process need to
have set of its own pages later. do the CR3s for the 2 processes
become different at that point ?

> LKML is however very inappropriate list for such questions. Please ask on
> kernelnewbies list next time.
>

Thank you for letting me know. Will do next time.
> --
> Jiri Kosina
> SUSE Labs
>

Re: linux page table

[Jiri Kosina]

On Sun, 2 Sep 2012, Xin Tong wrote:

> >> 3. can two different processes have their CR3 being the same value
> >> even though they have different first level page tables ?
> >
> > Yes, if they are created by clone(CLONE_VM). In such case they share the
> > same mm_struct, and therefore mm_struct->pgd (which is exactly what is
> > loaded into cr3 in switch_mm()) is the same.
> >
> 
> Is this the COW mechanism in linux. what if the cloned process need to
> have set of its own pages later. do the CR3s for the 2 processes
> become different at that point ?

That is a different story. COW is applied on fork() (i.e. spawning new 
process), not on clone(CLONE_VM) (i.e. spawning new thread).

-- 
Jiri Kosina
SUSE Labs

Re: linux page table

[Cong Wang]

On 09/03/2012 02:26 AM, Jiri Kosina wrote:
> On Sun, 2 Sep 2012, Xin Tong wrote:
>
>>>> 3. can two different processes have their CR3 being the same value
>>>> even though they have different first level page tables ?
>>>
>>> Yes, if they are created by clone(CLONE_VM). In such case they share the
>>> same mm_struct, and therefore mm_struct->pgd (which is exactly what is
>>> loaded into cr3 in switch_mm()) is the same.
>>>
>>
>> Is this the COW mechanism in linux. what if the cloned process need to
>> have set of its own pages later. do the CR3s for the 2 processes
>> become different at that point ?
>
> That is a different story. COW is applied on fork() (i.e. spawning new
> process), not on clone(CLONE_VM) (i.e. spawning new thread).
>

Yeah, and unshare(2) does not implement the flag that reverses the 
effects of CLONE_VM.