On Oct 17, 2012 2:22 PM, "Kshemendra KP" [off-list ref] wrote:

On Wed, Oct 17, 2012 at 2:04 PM, Chetan Nanda [off-list ref]

wrote:

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On Tue, Oct 16, 2012 at 5:47 PM, Kshemendra KP [off-list ref]

wrote:

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   On x86 kernel is normally split into 3GB (user) : 1 GB (Kernel)

spaces. Kernel can only directly

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   manipulate 1 GB (around 889 MB) from the PAGE_OFFSET (0xC0000000).

The user space

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   memofy below PAGE_OFFSET kernel can't directly access, it considers

this memory as

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   high memory. Kernel needs to kmap()/kmap_atomic() map user page and

access that region.

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As per my understanding, if kernel code is running under a process

context then it can access lower 3GB address space, provided address in
0-3GB is in process address space.  And its not a high memory.

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High memory is a virtual address space with 1GB kernel space to map RAM

pages beyond 896MB.

One thing not clear to me is, during boot,  kernel is copied to 1st MB of

the RAM in x86 architecture as this  architecture

has ISA mem map hole (640k -to 1MB). From 1st BM till 896MB is occupied

by the kernel. Then user space will be

made available beyond 896 MB.  With this reasoning, it is not clear to

me, whether user space which is present

beyond 896 MB is in high memory.

On x86 kernel will be loaded in RAM from 1st MB. But kernel code and data
will not go upto 896MB.
Kernel code + data will be loaded in ram from 1MB - end_data (IIRC). and
address space between end_data to 896Mb is used for dynamic allocation
(kmalloc ) etc...

I would recomend you to read 2nd chapter of understanding linux kernel.
This chapter contain detailed info on memory addressing used by kernel on
x86

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   If the memory is above 4 GB with page extension, it is clear that

kernel can't access it as

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   kernel uses "void *" and/or  "unsigned long" to hold the address.

But it is not clear for me

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   why kernel's can't directly access memory below PAGE_OFFSET ( 0-3GB)

directly.

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Kernel can access 0-3GB memory, take an example of application sending

data to kernel via system call e.g. write.

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in Write system call pointer to the buffer will be in 0-3Gb address

space of calling application. And kernel will access this address to write
data to drivers/etc ..

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Regards

Kshemendra

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Chetan Nanda
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