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I’m developing a custom OS and facing a chicken-and-egg problem with my page frame allocator. I need to map a specific page, but if the corresponding PML4 entry is NULL, I must allocate a PDPT. Allocating a PDPT, however, requires allocating another page—which itself requires mapping. This creates a recursive dependency.

Has anyone encountered this issue? I’ve heard that using a self-referential entry in the PML4 might be a solution, but I’m not sure how it resolves the problem.

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    I recommend you read this article: os.phil-opp.com/paging-implementation . Although the author uses Rust, the ways to achieve what you are doing are well laid out and can be applied to any language: Commented Mar 2 at 4:25
  • @MichaelPetch I read the article but they don’t seem to cover this particular problem. It seems that my approach is completely different, but I don’t know why. Commented Mar 2 at 11:28
  • Every method mentioned there is a solution to this chicken and egg problem that every OS designer comes up against when dealing with paging. Since you are on x86_64 the easiest thing for you to do is map the entire 39 bits of physical address space into the higher half of the 48-bit virtual memory space (I recommend using 2MiB pages for that). A convenient place is to map it all starting at address 0xffff800000000000. The advantage is you can gain access to every physical address through a corresponding virtual memory offset by just adding 0xffff800000000000 to the physical address. Commented Mar 2 at 13:28
  • @MichaelPetch so if im understanding this correctly I need to allocate and fill (whichever are needed to cover the physical address range I want to access) every table (PML4, PDPT, PD, PT). Am I understanding it correctly? Commented Mar 2 at 13:56
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