There was a time when the Linux kernel was 32bit but hardware systems had much
more than 1GB of memory. A solution was developed to allow the use of high
memory (HIGHMEM). High memory was excluded from the kernel direct map and was
temporarily mapped into and out of the kernel as needed. These mappings were
made via kmap_*() calls.
With the prevalence of 64bit kernels the usefulness of this interface is
waning. But the idea of memory not being in the direct map (or having
permissions beyond the direct map mapping) has brought about the need to
rethink the HIGHMEM interface.
This talk will discuss the changes to the kmap_*() API and the motivations
driving them. This includes the status of a project to rework the HIGHMEM
interface as of the LPC conference.
Finally how does HIGHMEM affect the modern architectures in use? Is it finally
time to remove CONFIG_HIGHMEM? Or is there still a need for 32 bit systems to
support large amounts of memory in production environments?
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