Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro
On 03/18/2013 02:19 PM, Yinghai Lu wrote: > On Mon, Mar 18, 2013 at 12:14 PM, H. Peter Anvin wrote: > >> Instead, try to explain why 5 is the correct value in the current code >> and how it is (or should be!) derived. > > initial mapped size is PMD_SIZE, aka 2M. > if we use step_size to be PUD_SIZE aka 1G, as most worse case > that 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. > So i picked (30-21)/2 to get 5. > > Please check attached patch. > > Thanks > > Yinghai > This still seems very opaque. I need to look at it and see if it makes more sense in context. -hpa -- 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/
Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro
On Mon, Mar 18, 2013 at 12:14 PM, H. Peter Anvin wrote: > Instead, try to explain why 5 is the correct value in the current code > and how it is (or should be!) derived. initial mapped size is PMD_SIZE, aka 2M. if we use step_size to be PUD_SIZE aka 1G, as most worse case that 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. So i picked (30-21)/2 to get 5. Please check attached patch. Thanks Yinghai add_comment_for_step_size.patch Description: Binary data
Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro
On 03/18/2013 12:13 PM, Yinghai Lu wrote: >> >> No, it doesn't. This is C, not elementary school Now I'm really bothered. >> >> The comment doesn't say *why* (PUD_SHIFT-PMD_SHIFT)/2 or any other >> variant is correct, furthermore I suspect that the +1 is misplaced. >> However, what is really needed is: >> >> 1. Someone needs to explain what the logic should be and why, and >> 2. replace the macro with a symbolic macro, not with a constant and a >>comment explaining, incorrectly, how that value was derived. > > yes, we should find out free_mem_size instead to decide next step size. > > But that will come out page table size estimation problem again. > Sorry, that comment is double nonsense for someone who isn't intimately familiar with the code, and it sounds like it is just plain wrong. Instead, try to explain why 5 is the correct value in the current code and how it is (or should be!) derived. -hpa -- 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/
Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro
On Mon, Mar 18, 2013 at 11:59 AM, H. Peter Anvin wrote:
> On 03/18/2013 11:53 AM, Yinghai Lu wrote:
>> On Mon, Mar 18, 2013 at 3:21 AM, Lin Feng wrote:
>>> For x86 PUD_SHIFT is 30 and PMD_SHIFT is 21, so the consequence of
>>> (PUD_SHIFT-PMD_SHIFT)/2 is 4. Update the comments to the code.
>>>
>>> Cc: Yinghai Lu
>>> Signed-off-by: Lin Feng
>>> ---
>>> arch/x86/mm/init.c | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>>> index 59b7fc4..637a95b 100644
>>> --- a/arch/x86/mm/init.c
>>> +++ b/arch/x86/mm/init.c
>>> @@ -389,7 +389,7 @@ static unsigned long __init init_range_memory_mapping(
>>> return mapped_ram_size;
>>> }
>>>
>>> -/* (PUD_SHIFT-PMD_SHIFT)/2 */
>>> +/* (PUD_SHIFT-PMD_SHIFT)/2+1 */
>>> #define STEP_SIZE_SHIFT 5
>>> void __init init_mem_mapping(void)
>>> {
>>
>> 9/2=4.5, so it becomes 5.
>>
>
> No, it doesn't. This is C, not elementary school Now I'm really bothered.
>
> The comment doesn't say *why* (PUD_SHIFT-PMD_SHIFT)/2 or any other
> variant is correct, furthermore I suspect that the +1 is misplaced.
> However, what is really needed is:
>
> 1. Someone needs to explain what the logic should be and why, and
> 2. replace the macro with a symbolic macro, not with a constant and a
>comment explaining, incorrectly, how that value was derived.
yes, we should find out free_mem_size instead to decide next step size.
But that will come out page table size estimation problem again.
Yinghai
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Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro
On 03/18/2013 11:53 AM, Yinghai Lu wrote:
> On Mon, Mar 18, 2013 at 3:21 AM, Lin Feng wrote:
>> For x86 PUD_SHIFT is 30 and PMD_SHIFT is 21, so the consequence of
>> (PUD_SHIFT-PMD_SHIFT)/2 is 4. Update the comments to the code.
>>
>> Cc: Yinghai Lu
>> Signed-off-by: Lin Feng
>> ---
>> arch/x86/mm/init.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>> index 59b7fc4..637a95b 100644
>> --- a/arch/x86/mm/init.c
>> +++ b/arch/x86/mm/init.c
>> @@ -389,7 +389,7 @@ static unsigned long __init init_range_memory_mapping(
>> return mapped_ram_size;
>> }
>>
>> -/* (PUD_SHIFT-PMD_SHIFT)/2 */
>> +/* (PUD_SHIFT-PMD_SHIFT)/2+1 */
>> #define STEP_SIZE_SHIFT 5
>> void __init init_mem_mapping(void)
>> {
>
> 9/2=4.5, so it becomes 5.
>
No, it doesn't. This is C, not elementary school Now I'm really bothered.
The comment doesn't say *why* (PUD_SHIFT-PMD_SHIFT)/2 or any other
variant is correct, furthermore I suspect that the +1 is misplaced.
However, what is really needed is:
1. Someone needs to explain what the logic should be and why, and
2. replace the macro with a symbolic macro, not with a constant and a
comment explaining, incorrectly, how that value was derived.
-hpa
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Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro
On Mon, Mar 18, 2013 at 3:21 AM, Lin Feng wrote:
> For x86 PUD_SHIFT is 30 and PMD_SHIFT is 21, so the consequence of
> (PUD_SHIFT-PMD_SHIFT)/2 is 4. Update the comments to the code.
>
> Cc: Yinghai Lu
> Signed-off-by: Lin Feng
> ---
> arch/x86/mm/init.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index 59b7fc4..637a95b 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -389,7 +389,7 @@ static unsigned long __init init_range_memory_mapping(
> return mapped_ram_size;
> }
>
> -/* (PUD_SHIFT-PMD_SHIFT)/2 */
> +/* (PUD_SHIFT-PMD_SHIFT)/2+1 */
> #define STEP_SIZE_SHIFT 5
> void __init init_mem_mapping(void)
> {
9/2=4.5, so it becomes 5.
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[PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro
For x86 PUD_SHIFT is 30 and PMD_SHIFT is 21, so the consequence of
(PUD_SHIFT-PMD_SHIFT)/2 is 4. Update the comments to the code.
Cc: Yinghai Lu
Signed-off-by: Lin Feng
---
arch/x86/mm/init.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 59b7fc4..637a95b 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -389,7 +389,7 @@ static unsigned long __init init_range_memory_mapping(
return mapped_ram_size;
}
-/* (PUD_SHIFT-PMD_SHIFT)/2 */
+/* (PUD_SHIFT-PMD_SHIFT)/2+1 */
#define STEP_SIZE_SHIFT 5
void __init init_mem_mapping(void)
{
--
1.8.0.1
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