* Yinghai Lu <[email protected]> wrote:
> As request by hpa, add comments for why we choose 5 for
> step size shift.
>
> Signed-off-by: Yinghai Lu <[email protected]>
> Reviewed-by: Tang Chen <[email protected]>
> Tested-by: Tang Chen <[email protected]>
>
> ---
> arch/x86/mm/init.c | 21 ++++++++++++++++++---
> 1 file changed, 18 insertions(+), 3 deletions(-)
>
> Index: linux-2.6/arch/x86/mm/init.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/mm/init.c
> +++ linux-2.6/arch/x86/mm/init.c
> @@ -395,8 +395,23 @@ static unsigned long __init init_range_m
> 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)
> +{
> + /*
> + * initial mapped size is PMD_SIZE, aka 2M.
> + * We can not set step_size to be PUD_SIZE aka 1G yet.
> + * In worse case, when 1G is cross the 1G boundary, and
> + * PG_LEVEL_2M is not set, we will need 1+1+512 pages (aka 2M + 8k)
> + * to map 1G range with PTE. Use 5 as shift for now.
> + */
This is much more readable:
+ /*
+ * initial mapped size is PMD_SIZE (2M).
+ * We can not set step_size to be PUD_SIZE (1G) yet.
+ * In the worst 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.
+ */
> + unsigned long new_step_size = step_size << 5;
> +
> + if (new_step_size > step_size)
> + step_size = new_step_size;
> +
> + return step_size;
> +}
> +
> void __init init_mem_mapping(void)
> {
> unsigned long end, real_end, start, last_start;
> @@ -445,7 +460,7 @@ 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;
> }
As-is the changelog claims it only adds comments - but it
obviously does more than that ...
Yinghai, for the 1001st time, please use the customary
changelog style we use in the kernel:
" Current code does (A), this has a problem when (B).
We can improve this doing (C), because (D)."
I'm also going to suggest something radical: how about you
keep this sugestion of mine in mind for _all_ future
patches so I don't have to repeat it for every 3rd patch
like I had to for the past 4 years, non-stop? Okay?
Thanks,
Ingo
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