few doubts in mm

few doubts in mm

[Vishwanath]

Hi all
I am new to this mailing list and also Memory management in linux.
I have a doubt that how exactly the convertion of virtual address to
physical addr
happens. As for i have read in Linux Kernel Internals, it says there is
some thing
called page directory, page middle dir and page table, The virtual addr
is divided into
4 parts(len not mentioned), The first part is index to page dir.

What exactly the page dir entry contains an address or a number index to
page middle dir, if index,
then where the base addr of page dir is stored, and base addr of page
middle dir, and page tabe are
stored.

How exactly this happens, The book also says that x86 supports only 2
level convertion.
How do i find out what my machine supports, i have linux kernel code of
v2.4.x

Please do help me out.

Thanx in advance
Vishy





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Re: few doubts in mm

[Ravi K]

Hi,

There is some good documentation on the Linux MM site
(http://linux-mm.org) which might help you.  Here are
some details that I know.

The virtual-to-physical conversion on x386 happens
like this:

- 10 (most significant) bits are used to index into
the page global directory (PGD).
- The PGD entry points to a page containing 1024 page
table entries (PTE).
- The next 10 bits of the address are used to get the
correct PTE.
- PTE points to a page of size 4096 bytes.
- The last 12 bits are used as offset within the page.

That is: 
10 bits for PGD + 10 bits for PTE + 12 bits offset

Note:
If the PSE (Page Size Extension) flag is set in the
PGD entry, the second level indexing (PGD->PTE) does
not happen. Instead, the PGD entry points to a 4MB
page and the last 22 bits are directly used as offset
within the page.
 
 Intel provides a 'Physical Address Extension'
(PAE)mode on some processors (Pentium Pro and above).
In this mode, the address size is 36-bits allowing for
up to 64GB of memory can be addressed.  Three-level
page table is used in case of PAE mode.
 
If PAE is enabled when building the Linux kernel,
virtual-to-physical conversion happens like this:
 
- 2 bits are used to index into the page global
directory.
- The PGD entry points a page containing 512 page
middle directory (PMD) entries.
- The next 9 bits of the address are used to get the
correct PMD.
- PMD entry points to a page containing 512 PTEs.
- The next 9 bits of the address are used to get the
correct PTE.
- The last 12 bits are used as offset within the page.
 
So: 2 bits for PGD + 9 bits for PMD + 9 bits for PTE +
12 bits offset
 
Note:
 Even though the address size is 36-bits, the virtual
address space is still limited to 4GB (32-bits).
 PGD, PMD and PTEs are 64-bit wide in PAE mode.
 In PAE mode, the PSE flag will be set in the PMD.

Hope this helps
Ravi.

PS: I feel your question would be more appropriate on
the kernel newbies list (kernelnewbies@nl.linux.org)


--- Vishwanath <vishwanath@inablers.net> wrote:
> Hi all
> I am new to this mailing list and also Memory
> management in linux.
> I have a doubt that how exactly the convertion of
> virtual address to
> physical addr
> happens. As for i have read in Linux Kernel
> Internals, it says there is
> some thing
> called page directory, page middle dir and page
> table, The virtual addr
> is divided into
> 4 parts(len not mentioned), The first part is index
> to page dir.
> 
> What exactly the page dir entry contains an address
> or a number index to
> page middle dir, if index,
> then where the base addr of page dir is stored, and
> base addr of page
> middle dir, and page tabe are
> stored.
> 
> How exactly this happens, The book also says that
> x86 supports only 2
> level convertion.
> How do i find out what my machine supports, i have
> linux kernel code of
> v2.4.x
> 
> Please do help me out.
> 
> Thanx in advance
> Vishy

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