diff options
| author | Yinghai Lu <yinghai@kernel.org> | 2013-09-06 19:07:09 -0700 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2013-09-10 09:51:34 +0200 |
| commit | 6979287a7df66a92d6f308338e972a406f9ef842 (patch) | |
| tree | c04cb205eb63143b9d41894e190008fe575ac66d /arch/x86/mm/init.c | |
| parent | 816434ec4a674fcdb3c2221a6dffdc8f34020550 (diff) | |
x86/mm: Add 'step_size' comments to init_mem_mapping()
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>
Diffstat (limited to 'arch/x86/mm/init.c')
| -rw-r--r-- | arch/x86/mm/init.c | 23 |
1 files changed, 20 insertions, 3 deletions
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c index 04664cdb7fda..ce32017c5e38 100644 --- a/arch/x86/mm/init.c +++ b/arch/x86/mm/init.c @@ -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; } |