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Understanding the Linux® Virtual Memory Manager by Mel Gorman

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Chapter 9. High Memory Management

The kernel may only directly address memory for which it has set up a page table entry. In the most common case, the user/kernel address space split of 3GiB/1GiB implies that, at best, only 896MiB of memory may be directly accessed at any given time on a 32-bit machine as explained in Section 4.1. On 64-bit hardware, this is not really an issue because there is more than enough virtual address space. It is highly unlikely there will be machines running 2.4 kernels with more than terabytes of RAM.

Many high end 32-bit machines have more than 1GiB of memory, and the inconveniently located memory cannot be simply ignored. The solution Linux uses is to temporarily map pages from high memory into the lower page tables. ...

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