on MAXMEM_PFN and VMALLOC_RESERVE

on MAXMEM_PFN and VMALLOC_RESERVE

[Kapish K]

Hello,
 What does this code ( in arch/i386/kernel/setup.c ), actually 
imply?
/* 
 *Determine low and high memory ranges: 
 */

max_low_pfn=max_pfn;
if ( max_low_pfn > MAXMEM_PFN ){
      max_low_pfn = MAXMEM_PFN;
#ifndef CONFIG_HIGHMEM
    /* Maximum memory usable is what is directlt addressable */
Now here, what does this imply, and the significance of 
VMALLOC_RESERVE in the MAXMEM_PFN calculations ( as in setup.c ) 
:MAXMEM_PFN PFN_DOWN(MAXMEM)
where MAXMEM = (unsigned long) ( -PAGE_OFFSET - VMALLOC_RESERVE 
)
Also, what is the significance of this in terms of physical RAM 
sizes of 128 mb or more ( even greater than 1 GB ). I assume 
that still will not be high mem.
Any hints or pointers would be welcome.
Thanks



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Re: on MAXMEM_PFN and VMALLOC_RESERVE

[Joseph A. Knapka]

Kapish K wrote:
> 
> Hello,
>  What does this code ( in arch/i386/kernel/setup.c ), actually
> imply?
> /*
>  *Determine low and high memory ranges:
>  */
> 
> max_low_pfn=max_pfn;
> if ( max_low_pfn > MAXMEM_PFN ){
>       max_low_pfn = MAXMEM_PFN;
> #ifndef CONFIG_HIGHMEM
>     /* Maximum memory usable is what is directlt addressable */
> Now here, what does this imply, and the significance of
> VMALLOC_RESERVE in the MAXMEM_PFN calculations ( as in setup.c )
> :MAXMEM_PFN PFN_DOWN(MAXMEM)
> where MAXMEM = (unsigned long) ( -PAGE_OFFSET - VMALLOC_RESERVE
> )
> Also, what is the significance of this in terms of physical RAM
> sizes of 128 mb or more ( even greater than 1 GB ). I assume
> that still will not be high mem.
> Any hints or pointers would be welcome.

Have a look at http://home.earthlink.net/~jknapka/linux-mm/kmap.html

Basically, MAX_MEM is the amount of address space available between
PAGE_OFFSET and the beginning of the VMALLOC_RESERVE area just
below 4GB. Thus, it's the maximum amount of physical RAM that
can be permanently mapped into kernel VM. max_low_pfn is the
highest page frame number of permanently-mapped RAM.

HTH,

-- Joe


-- Joe Knapka
"You know how many remote castles there are along the gorges? You
 can't MOVE for remote castles!" -- Lu Tze re. Uberwald
// Linux MM Documentation in progress:
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