user space vs kernel space address space

Anders Darander anders.darander at
Tue Dec 31 07:51:02 EST 2013

On 31 Dec 2013 13:39, "Pritam Bankar" <pritambankar1988 at> wrote:
>>> On Sun, Dec 29, 2013 at 7:04 PM, Miles MH Chen <orca.chen at>
>>>> Hi Pritam,
>>>> 1) Yes, all 512RAM will be direct mapped to kernel address space IF
the kernel have a 896MB direct mapping area.
>>>> Actually you can change the range of kernel direct mapping by the
vmalloc=<size> in boot command line.
>>>> In 32-bit and 3:1 split configuration, kernel direct mapping area +
vmalloc area is roughly 1G.
>>>> 2) User space and kernel space can have different virtual addresses
mapping to the same physical frame at the same time.
>             How above scenario of user space and kernel space having
mapping to the same physical frame               is handled. This would be
like sharing kernel data with user which is not allowed.

One trivial example would be a driver that meets the userspace application
to mmap a kernel buffer. In that case, there will exist both a kernelspace
and a userspace mapping to the same physical frame(s).

It'll let you share data between the two in a rather efficient way. Of
course, you need to handle any necessary syncing yourself, so that you're
not reading inconsistent data.

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