Re: [PATCH v2 0/5] Add movablecore_map boot option

[Tang Chen <tangchen@cn.fujitsu.com>]

Hi Bob, Liu Jiang,

About CMA, could you give me more info ?
Thanks for your patent and nice advice. :)


1) I saw the following on http://lwn.net/Articles/447405/:

The "CMA" type is sticky; pages which are marked as being for CMA
should never have their migration type changed by the kernel.

As Wen said, we now support a user interface to change movable memory
into kernel memory. But seeing from above, the memory specified as
CMA will not be able to be changed, right ?  If so, I don't think
using CMA is a good idea.


2) Is CMA just implemented on ARM platform ?  I found the following in
kernel-parameters.txt.

cma=nn[MG]      [ARM,KNL]
         Sets the size of kernel global memory area for contiguous
         memory allocations. For more information, see
         include/linux/dma-contiguous.h

We are developing on x86. Could we use it ?


3) Is CMA just used for DMA ? I am a little confused here. :)
I found the main code of CMA is implemented in dma-contiguous.c.


4) The boot options cma=xxx and movablecore_map=xxx have different
meanings for user. Reusing CMA could make user confused, I'm afraid.

And, even if we reuse "cma=" option, we still need to do the work
in patch 3~5, right ?


Thanks. :)



On 11/28/2012 12:08 PM, Jiang Liu wrote:
> On 2012-11-28 11:24, Bob Liu wrote:
>> On Tue, Nov 27, 2012 at 8:49 PM, Tang Chen<tangchen@cn.fujitsu.com>  wrote:
>>> On 11/27/2012 08:09 PM, Bob Liu wrote:
>>>>
>>>> On Tue, Nov 27, 2012 at 4:29 PM, Tang Chen<tangchen@cn.fujitsu.com>
>>>> wrote:
>>>>>
>>>>> Hi Liu,
>>>>>
>>>>>
>>>>> This feature is used in memory hotplug.
>>>>>
>>>>> In order to implement a whole node hotplug, we need to make sure the
>>>>> node contains no kernel memory, because memory used by kernel could
>>>>> not be migrated. (Since the kernel memory is directly mapped,
>>>>> VA = PA + __PAGE_OFFSET. So the physical address could not be changed.)
>>>>>
>>>>> User could specify all the memory on a node to be movable, so that the
>>>>> node could be hot-removed.
>>>>>
>>>>
>>>> Thank you for your explanation. It's reasonable.
>>>>
>>>> But i think it's a bit duplicated with CMA, i'm not sure but maybe we
>>>> can combine it with CMA which already in mainline?
>>>>
>>> Hi Liu,
>>>
>>> Thanks for your advice. :)
>>>
>>> CMA is Contiguous Memory Allocator, right?  What I'm trying to do is
>>> controlling where is the start of ZONE_MOVABLE of each node. Could
>>> CMA do this job ?
>>
>> cma will not control the start of ZONE_MOVABLE of each node, but it
>> can declare a memory that always movable
>> and all non movable allocate request will not happen on that area.
>>
>> Currently cma use a boot parameter "cma=" to declare a memory size
>> that always movable.
>> I think it might fulfill your requirement if extending the boot
>> parameter with a start address.
>>
>> more info at http://lwn.net/Articles/468044/
>>>
>>> And also, after a short investigation, CMA seems need to base on
>>> memblock. But we need to limit memblock not to allocate memory on
>>> ZONE_MOVABLE. As a result, we need to know the ranges before memblock
>>> could be used. I'm afraid we still need an approach to get the ranges,
>>> such as a boot option, or from static ACPI tables such as SRAT/MPST.
>>>
>>
>> Yes, it's based on memblock and with boot option.
>> In setup_arch32()
>>      dma_contiguous_reserve(0);   =>  will declare a cma area using
>> memblock_reserve()
>>
>>> I'm don't know much about CMA for now. So if you have any better idea,
>>> please share with us, thanks. :)
>>
>> My idea is reuse cma like below patch(even not compiled) and boot with
>> "cma=size@start_address".
>> I don't know whether it can work and whether suitable for your
>> requirement, if not forgive me for this noises.
>>
>> diff --git a/drivers/base/dma-contiguous.c b/drivers/base/dma-contiguous.c
>> index 612afcc..564962a 100644
>> --- a/drivers/base/dma-contiguous.c
>> +++ b/drivers/base/dma-contiguous.c
>> @@ -59,11 +59,18 @@ struct cma *dma_contiguous_default_area;
>>    */
>>   static const unsigned long size_bytes = CMA_SIZE_MBYTES * SZ_1M;
>>   static long size_cmdline = -1;
>> +static long cma_start_cmdline = -1;
>>
>>   static int __init early_cma(char *p)
>>   {
>> +       char *oldp;
>>          pr_debug("%s(%s)\n", __func__, p);
>> +       oldp = p;
>>          size_cmdline = memparse(p,&p);
>> +
>> +       if (*p == '@')
>> +               cma_start_cmdline = memparse(p+1,&p);
>> +       printk("cma start:0x%x, size: 0x%x\n", size_cmdline, cma_start_cmdline);
>>          return 0;
>>   }
>>   early_param("cma", early_cma);
>> @@ -127,8 +134,10 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>>          if (selected_size) {
>>                  pr_debug("%s: reserving %ld MiB for global area\n", __func__,
>>                           selected_size / SZ_1M);
>> -
>> -               dma_declare_contiguous(NULL, selected_size, 0, limit);
>> +               if (cma_size_cmdline != -1)
>> +                       dma_declare_contiguous(NULL, selected_size,
>> cma_start_cmdline, limit);
>> +               else
>> +                       dma_declare_contiguous(NULL, selected_size, 0, limit);
>>          }
>>   };
> Seems a good idea to reserve memory by reusing CMA logic, though need more
> investigation here. One of CMA goal is to ensure pages in CMA are really
> movable, and this patchset tries to achieve the same goal at a first glance.
>
>
>
>
>

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