From: Zhang Yanfei Date: Thu, 9 May 2013 15:57:42 +0000 (+0800) Subject: x86/mm: Add missing comments for initial kernel direct mapping X-Git-Tag: v3.10-rc2~23^2 X-Git-Url: http://pileus.org/git/?a=commitdiff_plain;h=cf8b166d5c1c89aad6c436a954fa40fd18a75bfb;p=~andy%2Flinux x86/mm: Add missing comments for initial kernel direct mapping Two sets of comments were lost during patch-series shuffling: - comments for init_range_memory_mapping() - comments in init_mem_mapping that is helpful for reminding people that the pagetable is setup top-down The comments were written by Yinghai in his patch in: https://lkml.org/lkml/2012/11/28/620 This patch reintroduces them. Originally-From: Yinghai Lu Signed-off-by: Zhang Yanfei Cc: Yasuaki Ishimatsu Cc: Konrad Rzeszutek Wilk Cc: Andrew Morton Link: http://lkml.kernel.org/r/518BC776.7010506@gmail.com [ Tidied it all up a bit. ] Signed-off-by: Ingo Molnar --- diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c index fdc5dca14fb..eaac1743def 100644 --- a/arch/x86/mm/init.c +++ b/arch/x86/mm/init.c @@ -359,7 +359,17 @@ unsigned long __init_refok init_memory_mapping(unsigned long start, } /* - * would have hole in the middle or ends, and only ram parts will be mapped. + * We need to iterate through the E820 memory map and create direct mappings + * for only E820_RAM and E820_KERN_RESERVED regions. We cannot simply + * create direct mappings for all pfns from [0 to max_low_pfn) and + * [4GB to max_pfn) because of possible memory holes in high addresses + * that cannot be marked as UC by fixed/variable range MTRRs. + * Depending on the alignment of E820 ranges, this may possibly result + * in using smaller size (i.e. 4K instead of 2M or 1G) page tables. + * + * init_mem_mapping() calls init_range_memory_mapping() with big range. + * That range would have hole in the middle or ends, and only ram parts + * will be mapped in init_range_memory_mapping(). */ static unsigned long __init init_range_memory_mapping( unsigned long r_start, @@ -419,6 +429,13 @@ void __init init_mem_mapping(void) max_pfn_mapped = 0; /* will get exact value next */ min_pfn_mapped = real_end >> PAGE_SHIFT; last_start = start = real_end; + + /* + * We start from the top (end of memory) and go to the bottom. + * The memblock_find_in_range() gets us a block of RAM from the + * end of RAM in [min_pfn_mapped, max_pfn_mapped) used as new pages + * for page table. + */ while (last_start > ISA_END_ADDRESS) { if (last_start > step_size) { start = round_down(last_start - 1, step_size);