What is meant by holes in the memory Linux?

Question

I have come across a term - holes in the memory in Linux. I believe this is the memory that is I/O remapped. Is my understanding correct?

Answer 1

Holes in memory can mean different things:

1. It can refer to physical memory addressing: For historical and boot-strapping reasons, in the "standard PC" (x86) architecture, all of system RAM is not contiguous. There are "holes" in the address space where memory-mapped I/O resides. For example, from the earliest days, there has been an area reserved for boot ROM (BIOS) and video memory. Also, there is a large area of the address space which is reserved for dynamic assignment to PCI (and PCI-X or PCI-Express) peripherals. These areas are often mapped as needed into kernel virtual address space by device drivers (which may be referred to as "I/O remapping").

   Memory controllers built-in to the motherboard allow the physical address of the RAM to be configured (this is typically handled by the BIOS in the standard PC architecture). Other [non-x86] architectures often have similar holes in the physical address space.

2. The term can also refer to unassigned regions in the virtual address space. Both kernel virtual address space and user processes' virtual address space typically have "holes" in them. For example, Linux doesn't map any physical memory corresponding to virtual address 0 (i.e. the first page of the address space never has valid memory) -- this allows null pointer references to be trapped.

   In some kinds of memory allocations, the Linux kernel maintains unmapped areas between properly allocated virtual memory regions in order to trap faulty memory references (i.e. that stray beyond the end of the allocated space).