Big memory, no struct page allocation

Big memory, no struct page allocation

[David Chow]

Dear all,

Hi, I've got a silly but serious question. I want to allocate a large 
buffer (>512MB) in kernel. Normally you use __get_free_page and handle 
it with page pointers. But when get to very large (say 1024MB), I will 
need to use 2 level of page pointer indirection to carry the page 
pointer array. I also find the total size of page struct is quite large 
when using lots of pages, what I want is to use memory pages without 
struct page, is this possible? By the way, can I use lots of memory in 
the kernel, something like 1GB of memory allocation when physically RAM 
available? Please give advise. Thanks.

regards,
David

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Re: Big memory, no struct page allocation

[William Lee Irwin III]

On Sun, Jun 23, 2002 at 04:55:58PM +0800, David Chow wrote:
> Hi, I've got a silly but serious question. I want to allocate a large 
> buffer (>512MB) in kernel. Normally you use __get_free_page and handle 
> it with page pointers. But when get to very large (say 1024MB), I will 
> need to use 2 level of page pointer indirection to carry the page 
> pointer array. I also find the total size of page struct is quite large 
> when using lots of pages, what I want is to use memory pages without 
> struct page, is this possible? By the way, can I use lots of memory in 
> the kernel, something like 1GB of memory allocation when physically RAM 
> available? Please give advise. Thanks.

Try allocating it at boot-time with the bootmem allocator.


Cheers,
Bill
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Re: Big memory, no struct page allocation

[David Chow]

William Lee Irwin III wrote:

>On Sun, Jun 23, 2002 at 04:55:58PM +0800, David Chow wrote:
>  
>
>>Hi, I've got a silly but serious question. I want to allocate a large 
>>buffer (>512MB) in kernel. Normally you use __get_free_page and handle 
>>it with page pointers. But when get to very large (say 1024MB), I will 
>>need to use 2 level of page pointer indirection to carry the page 
>>pointer array. I also find the total size of page struct is quite large 
>>when using lots of pages, what I want is to use memory pages without 
>>struct page, is this possible? By the way, can I use lots of memory in 
>>the kernel, something like 1GB of memory allocation when physically RAM 
>>available? Please give advise. Thanks.
>>    
>>
>
>Try allocating it at boot-time with the bootmem allocator.
>
>
>Cheers,
>Bill
>  
>
Thanks for suggestions, you mean this will allow no struct page or can 
use memory more than 1GB? Please make clear on direction, I would love 
to know. Thanks.

regards,
David


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Re: Big memory, no struct page allocation

[William Lee Irwin III]

William Lee Irwin III wrote:
>> Try allocating it at boot-time with the bootmem allocator.

On Sun, Jun 23, 2002 at 11:31:28PM +0800, David Chow wrote:
> Thanks for suggestions, you mean this will allow no struct page or can 
> use memory more than 1GB? Please make clear on direction, I would love 
> to know. Thanks.

On 32-bit machines with 3:1 process address space splits yes.

In this case you're far better off playing games with the highmem
initialization in order to slice the memory out of there and kmap it.


Cheers,
Bill
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Re: Big memory, no struct page allocation

[David Chow]

William Lee Irwin III wrote:

>William Lee Irwin III wrote:
>  
>
>>>Try allocating it at boot-time with the bootmem allocator.
>>>      
>>>
>
>On Sun, Jun 23, 2002 at 11:31:28PM +0800, David Chow wrote:
>  
>
>>Thanks for suggestions, you mean this will allow no struct page or can 
>>use memory more than 1GB? Please make clear on direction, I would love 
>>to know. Thanks.
>>    
>>
>
>On 32-bit machines with 3:1 process address space splits yes.
>
>In this case you're far better off playing games with the highmem
>initialization in order to slice the memory out of there and kmap it.
>
>
>Cheers,
>Bill
>  
>
In other words, even I have 2G physical memory, I cannot have benefits 
of using all memory for pagecache, this also means I cannot create any 
cache beyong a 1G size in kernel. That's a pitty for 32-bit systems, 
with himem, how does it work?

David


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Re: Big memory, no struct page allocation

[Rik van Riel]

On Mon, 1 Jul 2002, David Chow wrote:

> In other words, even I have 2G physical memory, I cannot have benefits
> of using all memory for pagecache, this also means I cannot create any
> cache beyong a 1G size in kernel. That's a pitty for 32-bit systems,
> with himem, how does it work?

Pagecache can use highmem just fine.

regards,

Rik
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Re: Big memory, no struct page allocation

[William Lee Irwin III]

On Mon, 1 Jul 2002, David Chow wrote:
>> In other words, even I have 2G physical memory, I cannot have benefits
>> of using all memory for pagecache, this also means I cannot create any
>> cache beyong a 1G size in kernel. That's a pitty for 32-bit systems,
>> with himem, how does it work?

On Mon, Jul 01, 2002 at 02:48:00PM -0300, Rik van Riel wrote:
> Pagecache can use highmem just fine.
> regards,
> Rik

Yes, pagecache doesn't care where it is, it just works with the
struct pages for the memory. Things that are more internal like
dcache and buffer cache need to be allocated from ZONE_NORMAL,
as the kernel actually touches that memory directly.


Cheers,
Bill
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Re: Big memory, no struct page allocation

[David Chow]

William Lee Irwin III wrote:

>On Mon, 1 Jul 2002, David Chow wrote:
>  
>
>>>In other words, even I have 2G physical memory, I cannot have benefits
>>>of using all memory for pagecache, this also means I cannot create any
>>>cache beyong a 1G size in kernel. That's a pitty for 32-bit systems,
>>>with himem, how does it work?
>>>      
>>>
>
>On Mon, Jul 01, 2002 at 02:48:00PM -0300, Rik van Riel wrote:
>  
>
>>Pagecache can use highmem just fine.
>>regards,
>>Rik
>>    
>>
>
>Yes, pagecache doesn't care where it is, it just works with the
>struct pages for the memory. Things that are more internal like
>dcache and buffer cache need to be allocated from ZONE_NORMAL,
>as the kernel actually touches that memory directly.
>
>
>Cheers,
>Bill
>  
>
Thanks for advice, that means allocation of slab cache never gets over 
1G? Since you mention dcache, where dcache uses kmem_cache_create() 
calls, or it depends on the flags pass to kmem_cache_create()? What 
about kmalloc()?

Since before access a page, we have to do kmap(page), how does this 
pointer address work? I found that if my machine have less than physical 
1G RAM (actually somewhere between 900-940M), I don't have to call 
kmap() before really accessing the page data, if more than this amount 
of memory, it will result an oops. It seems for system that has more 
than 900M memory, kernel handle page data differently (need to use kmap 
before accessing the page data). Is it true that kmap only translate the 
page into a virtual address if more than 1G RAM or leave it physical is 
less than 1G RAM? I am a bit confuse in this behaviour of kmap().

regards,
David


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