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x86/mm: Add 'step_size' comments to init_mem_mapping()

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Current code uses macro to shift by 5, but there is no explanation
why there's no worry about an overflow there.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Jacob Shin <jacob.shin@amd.com>
Link: http://lkml.kernel.org/r/1378519629-10433-1-git-send-email-yinghai@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Yinghai Lu 2013-09-06 19:07:09 -07:00 committed by Ingo Molnar
parent 816434ec4a
commit 6979287a7d
1 changed files with 20 additions and 3 deletions
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23
arch/x86/mm/init.c
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@ -399,8 +399,25 @@ static unsigned long __init init_range_memory_mapping(
return mapped_ram_size;
}
/* (PUD_SHIFT-PMD_SHIFT)/2 */
#define STEP_SIZE_SHIFT 5
static unsigned long __init get_new_step_size(unsigned long step_size)
{
/*
* Explain why we shift by 5 and why we don't have to worry about
* 'step_size << 5' overflowing:
*
* initial mapped size is PMD_SIZE (2M).
* We can not set step_size to be PUD_SIZE (1G) yet.
* In worse case, when we cross the 1G boundary, and
* PG_LEVEL_2M is not set, we will need 1+1+512 pages (2M + 8k)
* to map 1G range with PTE. Use 5 as shift for now.
*
* Don't need to worry about overflow, on 32bit, when step_size
* is 0, round_down() returns 0 for start, and that turns it
* into 0x100000000ULL.
*/
return step_size << 5;
}
void __init init_mem_mapping(void)
{
unsigned long end, real_end, start, last_start;
@ -449,7 +466,7 @@ void __init init_mem_mapping(void)
min_pfn_mapped = last_start >> PAGE_SHIFT;
/* only increase step_size after big range get mapped */
if (new_mapped_ram_size > mapped_ram_size)
step_size <<= STEP_SIZE_SHIFT;
step_size = get_new_step_size(step_size);
mapped_ram_size += new_mapped_ram_size;
}