Commit-ID: 6979287a7df66a92d6f308338e972a406f9ef842 Gitweb: http://git.kernel.org/tip/6979287a7df66a92d6f308338e972a406f9ef842 Author: Yinghai Lu <ying...@kernel.org> AuthorDate: Fri, 6 Sep 2013 19:07:09 -0700 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Tue, 10 Sep 2013 09:51:34 +0200
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 <ying...@kernel.org> Cc: Pekka Enberg <penb...@kernel.org> Cc: Jacob Shin <jacob.s...@amd.com> Link: http://lkml.kernel.org/r/1378519629-10433-1-git-send-email-ying...@kernel.org Signed-off-by: Ingo Molnar <mi...@kernel.org> --- arch/x86/mm/init.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c index 04664cd..ce32017 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; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
