highmemory

Highmemory, or high memory, refers to the portion of a computer's physical RAM that is not permanently mapped into the kernel's address space. It is most commonly discussed in the context of 32-bit Linux kernels, where the virtual address space is limited and not all RAM can be directly addressed by the kernel.

In Linux, memory is divided into zones. The Normal (or LowMem) zone is permanently mapped into the kernel, enabling fast access to those pages. The HighMem zone comprises memory above the LowMem boundary and cannot be permanently mapped; pages in HighMem must be temporarily mapped using mechanisms such as kmap, kmap_atomic, or similar. Accessing HighMem thus incurs extra overhead compared to LowMem.

Purpose and trade-offs: HighMem allows systems to support larger amounts of RAM on 32-bit architectures despite limited virtual address space. However, it adds complexity, potential performance penalties, and limited direct access for kernel code and drivers. The kernel can allocate HighMem pages as needed and map them when necessary.

Historically, on i386 with 4 GB RAM, a significant portion of memory resided in HighMem, with only a portion allocated as LowMem. Modern 32-bit systems may use highmem if more than a few gigabytes of RAM are present. On 64-bit systems, the entire RAM is within a directly mapped zone, and highmem generally does not apply.

Configuration and visibility: HighMem status is exposed in kernel config symbols and in /proc and /sys zones (for example, /proc/zoneinfo). Administrators may encounter terms like HighMem and LowMem in system logs and diagnostic tools.