mmu_map_page writes page tables through their physical address, but only the first 4 MB are identity-mapped

[kudasaixc]

mmu_init() identity-maps only 0x0-0x400000. But mmu_init() and mmu_map_page() access the page directory / page tables via @ptrFromInt(<physical address>). This only works while pmm_alloc_frame() happens to return frames below 4 MB.

Once low memory fills up (heap growth) and a new page table gets a frame >= 0x400000, the @memset on the new table touches unmapped virtual memory and page-faults inside the mapping code itself -- with paging enabled this ends in a double fault and a dead machine.

Possible directions:

• identity-map all detected RAM at init,
• keep a dedicated low-memory pool for paging structures, or
• a recursive page directory mapping.

Can send a PR once a direction is picked.