so after the setting of user space virtual address the physical memory can
be directly addreseed by just doing a page table translation in user space
and kernel doesn't need to be involved.As it has already created the page
table and returned the corresponding page.Am i right?

Right. Basically this translation is done by CPU while executing any
portion of user program code segment instruction which tries to access
this mapped memory region (vma). Note that this is user vma getting
accessed in user mode, so access is allowed at RPL=3, with smooth
access :). However if page is not mapped, it will result in page fault
exception which traps into kernel and calls nopage method of vma. So,
kernel is involved only for establishing the mapping on demand.

You can say it is equivalent to accessing any other user data segment
in application space, e.g. accessing heap (malloced area). If page is
not already mapped into the heap, it traps into kernel with page fault
and page fault handler of heap (nopage method of vma) will allocate a
page-frame and assign it to vma. So, you can see the equivalence
between two. Only difference comes from kernel handlers for first time
mapping page-frames (RAM or device memory).

Rajat

On Sat, Feb 19, 2011 at 3:02 PM, anish singh
[off-list ref] wrote:

On Sat, Feb 19, 2011 at 2:44 PM, Rajat Sharma [off-list ref] wrote:

quoted

quoted

So, in the start, the page might have kernel mode address, but in the
end it has user mode address. But kernel is still the one that tracks
all the page...be it anonymous or non anonymous ones.

Not really. In this particular case of .nopage (page fault handler) of
vma, we already have a user space virtual address for the faulty page,
we just need it to map it with a physical page frame (RAM or device
memory). Just allocate a physical page frame without any address,
caller for .nopage will take care of properly setting its user space
virtual address (page->va). So, no association with kernel space
virtual address is required, e.g. it can be high memory page too
without any kernel mapping.

so after the setting of user space virtual address the physical memory can
be directly addreseed by just doing a page table translation in user space
and kernel doesn't need to be involved.As it has already created the page
table and returned the corresponding page.Am i right?

quoted

Rajat

On Sat, Feb 19, 2011 at 2:10 PM, Mulyadi Santosa
[off-list ref] wrote:

quoted

Hi :)

I'll try to help...

On Sat, Feb 19, 2011 at 13:10, anish singh [off-list ref]
wrote:

quoted

As i understood?whenver a user space program is run it is represented
in
kernel using VMA which is managed by struct mm_struct
and whenever the program is trying to read/write to a memory location
in
user space it will be directed to physical address using PAGE TABLE
translation done by struct mm_struct(done in kernel space).Am i right?

i think not "done" but mm_struct points to PGD that represent's the
whole process address space. Using information provided there, MMU
does translation.

quoted

Suppose?a simple driver wants?the?user to directly access its device
memory
then?we use mmap.This mmap?associates a set of user space virtual
address
with?device driver memory and it is done by creating kernel page tables
for
the user space virtual addresses.Is the page table translation done
everytime whenever user space?does read/write to the device memory??

if it's recently translated, quite likely it is already cached in TLB
(translation look aside buffer)

quoted

In .nopage function?call we return the page associated?with the
physical
address which the user wants to associate with user space?virtual
address.Is
the page?address?returned by the nopage function same as?seen by the
user or
will it be converted to user space virtual address(range between 0-2
GB)?

AFAIK, nopage is one of the functions that handle minor page fault, no?

Anyway, memory allocation, usually start by kernel allocates a page.
Then it is "handed" to user space memory allocator.

So, in the start, the page might have kernel mode address, but in the
end it has user mode address. But kernel is still the one that tracks
all the page...be it anonymous or non anonymous ones.

Does it help? :)



--
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

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