Subject: Re: [RFC 2/4] powerpc/mm: Add comments to the vmemmap layout

From: Anshuman Khandual <[email-masked]>
Date: Fri, 19 Feb 2016 10:45:48 +0530

On 02/18/2016 07:52 PM, Michael Ellerman wrote:
> On Wed, 2016-02-17 at 17:42 +0530, Anshuman Khandual wrote:
> 
>> Add some explaination to the layout of vmemmap virtual address
>> space and how physical page mapping is only used for valid PFNs
>> present at any point on the system.
>>
>> Signed-off-by: Anshuman Khandual <[email-masked]>
>> ---
>>  arch/powerpc/include/asm/book3s/64/pgtable.h | 41 ++++++++++++++++++++++++++++
>>  1 file changed, 41 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index 8d1c41d..9db4a86 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -26,6 +26,47 @@
>>  #define IOREMAP_BASE	(PHB_IO_END)
>>  #define IOREMAP_END	(KERN_VIRT_START + KERN_VIRT_SIZE)
>>  
>> +/*
>> + * Starting address of the virtual address space where all page structs
> 
> This is so far from the variable it's referring to that it's not clear what it
> refers to. So you should say "vmemmap is the starting ..."
> 
>> + * for the system physical memory are stored under the vmemmap sparse
>                                               ^
> 					      , when using the SPARSEMEM_VMEMMAP
>> + * memory model. All possible struct pages are logically stored in a
>> + * sequence in this virtual address space irrespective of the fact
>> + * whether any given PFN is valid or even the memory section is valid
>> + * or not.
> 
> I know what you mean but I think that could be worded better. But it's too late
> for me to reword it :)
> 
> The key point is that we allocate space for a page struct for each PFN that
> could be present in the system, including holes in the address space (hence
> sparse). That has the nice property of meaning there is a constant relationship
> between the address of a struct page and it's PFN.
> 
>> + * During boot and memory hotplug add operation when new memory
>                   ^                               ^
> 		  or				  ,
>> + * sections are added, real physical allocation and hash table bolting
>                                                   ^
> 						  of struct pages
> 
>> + * will be performed. This saves precious physical memory when the system
>> + * really does not have valid PFNs in some address ranges.
> 
> 
>> + *
>> + *  vmemmap +--------------+
>> + *     +    |  page struct +----------+  PFN is valid
>> + *     |    +--------------+          |
>> + *     |    |  page struct |          |  PFN is invalid
>> + *     |    +--------------+          |
>> + *     |    |  page struct +------+   |
>> + *     |    +--------------+      |   |
>> + *     |    |  page struct |      |   |
>> + *     |    +--------------+      |   |
>> + *     |    |  page struct |      |   |
>> + *     |    +--------------+      |   |
>> + *     |    |  page struct +--+   |   |
>> + *     |    +--------------+  |   |   |
>> + *     |    |  page struct |  |   |   |       +-------------+
>> + *     |    +--------------+  |   |   +-----> |     PFN     |
>> + *     |    |  page struct |  |   |           +-------------+
>> + *     |    +--------------+  |   +---------> |     PFN     |
>> + *     |    |  page struct |  |               +-------------+
>> + *     |    +--------------+  +-------------> |     PFN     |
>> + *     |    |  page struct |                  +-------------+
>> + *     |    +--------------+           +----> |     PFN     |
>> + *     |    |  page struct |           |      +-------------+
>> + *     |    +--------------+           |    Bolted in hash table
>> + *     |    |  page struct +-----------+
>> + *     v    +--------------+
> 
> 
> The things on the right are not PFNs, they're struct pages. Each one
> corresponds to a PFN, but that relationship is derived from the vmemap layout,
> not the physical layout.
> 
> I think it's more like:
> 
> 	f000000000000000		  c000000000000000 (and also 0x0)
> vmemmap +--------------+                  +--------------+
>    +    |  page struct | +--------------> |  page struct |
>    |    +--------------+                  +--------------+
>    |    |  page struct | +--------------> |  page struct |
>    |    +--------------+ |                +--------------+
>    |    |  page struct | +       +------> |  page struct |
>    |    +--------------+         |        +--------------+
>    |    |  page struct |         |   +--> |  page struct |
>    |    +--------------+         |   |    +--------------+
>    |    |  page struct |         |   |
>    |    +--------------+         |   |
>    |    |  page struct |         |   |
>    |    +--------------+         |   |
>    |    |  page struct |         |   |
>    |    +--------------+         |   |
>    |    |  page struct |         |   |
>    |    +--------------+         |   |
>    |    |  page struct | +-------+   |
>    |    +--------------+             |
>    |    |  page struct | +-----------+
>    |    +--------------+
>    |    |  page struct | No mapping
>    |    +--------------+
>    |    |  page struct | No mapping
>    v    +--------------+
> 
> 
> 
> Then there's the relationship between struct pages and PFNs:
> 
> 
>                            page_to_pfn
>                            +--------->
>   vmemmap +--------------+             +-------------+
>      +    |  page struct | +---------> |     PFN     |
>      |    +--------------+             +-------------+
>      |    |  page struct | +---------> |     PFN     |
>      |    +--------------+             +-------------+
>      |    |  page struct | +---------> |     PFN     |
>      |    +--------------+             +-------------+
>      |    |  page struct | +---------> |     PFN     |
>      |    +--------------+             +-------------+
>      |    |              |
>      |    +--------------+
>      |    |              |
>      |    +--------------+
>      |    |              |
>      |    +--------------+             +-------------+
>      |    |  page struct | +---------> |     PFN     |
>      |    +--------------+             +-------------+
>      |    |              |
>      |    +--------------+
>      |    |              |
>      |    +--------------+             +-------------+
>      |    |  page struct | +---------> |     PFN     |
>      |    +--------------+             +-------------+
>      |    |  page struct | +---------> |     PFN     |
>      v    +--------------+             +-------------+

Awesome, this conveys the message better. Will change it next time
around. Thanks !