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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: Zhang 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: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Zhang Yanfei 2013-05-09 23:57:42 +08:00 committed by Ingo Molnar
parent 074d72ff57
commit cf8b166d5c
1 changed files with 18 additions and 1 deletions
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19
arch/x86/mm/init.c
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@ -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);