How Linux kernel decide to which memory zone to use?

Question

When I check pagetypeinfo

cat /proc/pagetypeinfo

I see three types of memory zones;

• DMA
• DMA32
• Normal

How Linux choose a memory zone to allocate a new page?

Answer 1

These memory zones are defined only for the 32 bit systems and not in the 64 bit.

Rememember these are the kernel accessible main memory we are talking about. In a 32 bit (4GB) system, the split between the kernel and the user space is 1:3 . Meaning kernel can access 1GB and the user space 3GB. The kernel's 1GB is split as follows:

Zone_DMA (0-16MB): Permanently mapped into the kernel address space.
For compatibility reasons for older ISA devices that can address only the lower 16MB of main memory.

Zone_Normal (16MB-896MB): Permanently mapped into the kernel address space.
Many kernel operations can only take place using ZONE_NORMAL so it is the most performance critical zone and is the memory mostly allocated by the kernel.

ZONE_HIGH_MEM (896MB-above): not permanently mapped into the kernel's address space.
Kernel can access entire 4GB main memory. kernel's 1GB through Zone_DMA & Zone_Normal and user's 3GB through ZONE_HIGH_MEM . With Intel's Physical Address Extension (PAE) , one gets 4 extra bits to address the main memory resulting in 36 bits, a total of 64GB of memory that can be accessed. The delta address space (36 bit address - 32 bit address) is where ZONE_HIGH_MEM is used to map to the user accessed main memory (ie between 2GB - 4GB).

Read more:

http://www.quora.com/Linux-Kernel/Why-is-there-ZONE_HIGHMEM-in-the-x86-32-Linux-kernel-but-not-in-the-x86-64-kernel
http://www.quora.com/Linux-Kernel/What-is-the-difference-between-high-memory-and-normal-memory
Linux 3/1 virtual address split

Answer 2

For every memory allocation request (for eg via kmalloc), based on the flags passed to the function,kernel selects the memory zone. these requests internally triggers the kernel function alloc_pages().

zonelist is an argument that gets passed to alloc_pages(), that Points to a zonelist data structure describing, in order of preference, the mem- ory zones suitable for the memory allocation.

refer the memory management chapter in book Understanding the Linux kernel