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

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

quoted

   On x86 kernel is normally split into 3GB (user) : 1 GB (Kernel)
spaces. Kernel can only directly
   manipulate 1 GB (around 889 MB) from the PAGE_OFFSET (0xC0000000). The
user space
   memofy below PAGE_OFFSET kernel can't directly access, it considers
this memory as
   high memory. Kernel needs to kmap()/kmap_atomic() map user page and
access that region.

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.
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.

quoted

   If the memory is above 4 GB with page extension, it is clear that
kernel can't access it as
   kernel uses "void *" and/or  "unsigned long" to hold the address. But
it is not clear for me
   why kernel's can't directly access memory below PAGE_OFFSET ( 0-3GB)
directly.

Kernel can access 0-3GB memory, take an example of application sending
data to kernel via system call e.g. write.
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 ..

quoted

Regards

Kshemendra

_______________________________________________
Kernelnewbies mailing list
Kernelnewbies at kernelnewbies.org
http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

Chetan Nanda

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20121017/2bcebe46/attachment-0001.html