Suppose your machine has 1 GB of RAM with a physical addresses of
0x40000000 thru 0x7FFFFFFF

Let's suppose that PHYS_OFFSET = 0xC0000000
Let's suppose that VMALLOC_START is set to 0xE0000000 and VMALLOC_END
is at 0xEFFFFFFF (256 Mb)

is this PAGE_OFFSET or PHYS_OFFSET?

So what we have is that memory from 0xC0000000 thru 0xDFFFFFFF (512
Mb) is kernel direct mapped memory and maps to physical addresses
0x40000000 thru 0x5FFFFFFF

Physical memory from 0x60000000 thru 0x7FFFFFFF (512Mb) is considered
high-mem. For the most part this is only usable by user-space. From
kernel space, you need to use the kmap/kunmap functions to even access
the memory.

i don't think high-memory is only used by user-space.
vmalloc() funciton will try to allocate physical memory from high_memory first, if there is no high_memory, then allocate from normal region.
 
 

However, if you call vmalloc and lets suppose that vmalloc just
happens to return 0xE0000000. The physical address of the first page
might be 0xD2345000.

suppose vmalloc() return 0xE0000000, which is the VMALLOC_START, then the physical memory address
should be 0x60000000 thru 0x7FFFFFFF in your example.

What's important is that the physical pages which back up the vmalloc
area all come from the kernel direct mapped area. They won't ever be
backed by pages from high-memory. So the physical addresses will all
be in the range 0x40000000 thru 0x5FFFFFFF.

what do you mean "back up", maybe i have some misunderstanding.
in my opnion, vmalloc area should map to physical address 0x60000000 thru 0x7FFFFFFF first, 
if this resion is exhausted, then normal area will be allocated.

  		 	   		  
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