mm: init hole_pfn from first memblock region instead of 0#641
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In memmap_init(), hole_pfn starts at 0 and is used to track the start of the next hole in init_unavailable_range(). When the first memblock memory region does not start at PFN 0 (e.g., LKL with MMU where memory starts at ARCH_PFN_OFFSET), hole_pfn = 0 causes init_unavailable_range(0, start_pfn, ...) to be called for PFNs that have no corresponding struct page allocated. Currently this is safe because pfn_valid() returns false for those PFNs and the pageblock-skip logic in init_unavailable_range() prevents pfn_to_page() from being called. However, it wastes boot time iterating through empty pageblocks. Fix by initializing hole_pfn to the start_pfn of the first memblock memory region. With this fix, unnecessary init_unavailable_range() call is skipped entirely. Signed-off-by: Cheng Lingfei <chenglingfei@foxmail.com>
tavip
approved these changes
Jul 8, 2026
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memmap_init() initializes hole_pfn to 0. On LKL with MMU, the first memblock memory range starts at ARCH_PFN_OFFSET, and PFNs below that offset have no corresponding struct pages in the FLATMEM memory map.
As a result, memmap_init_zone_range() calls init_unavailable_range() for [0, start_pfn). This is currently harmless because the pageblock validity check skips these PFNs before pfn_to_page() is called, but it unnecessarily scans all pageblocks below ARCH_PFN_OFFSET.
Initialize hole_pfn from the first memblock memory range so that the scan starts at the first PFN represented by the memory map.