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Commit cf8b166d authored by Zhang Yanfei's avatar Zhang Yanfei Committed by Ingo Molnar
Browse files

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 <yinghai@kernel.org>
Signed-off-by: default avatarZhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/518BC776.7010506@gmail.com


[ Tidied it all up a bit. ]
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 074d72ff

arch/x86/mm/init.c

+18 −1
Original line number Diff line number Diff line
@@ -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);