On Thu, 22 Jun 2006, Russell King wrote:
On Thu, Jun 22, 2006 at 04:54:06PM +0100, Mel Gorman wrote:
On Thu, 22 Jun 2006, Franck Bui-Huu wrote:
It's the default value (see memory_model.h). It means that pfn start
for node 0 is 0, therefore your physical memory address starts at 0.
I know, but what I'm getting at is that ARCH_PFN_OFFSET may be unnecessary
with flatmem-relax-requirement-for-memory-to-start-at-pfn-0.patch applied.
ARCH_PFN_OFFSET is used as
#define page_to_pfn(page) ((unsigned long)((page) - mem_map) + \
ARCH_PFN_OFFSET)
because it knew that the map may not start at PFN 0. With
flatmem-relax-requirement-for-memory-to-start-at-pfn-0.patch, the map will
start at PFN 0 even if physical memory does not start until later.
Doesn't that result in a massive array of struct pages if your memory
starts a 3GB physical and has 4K pages?
No, I should have been clear. The size of the map remains the same but
mem_map does not point directly to NODE_DATA(0)->node_mem_map when the PFN
of node 0 is not 0. Instead mem_map points to
NODE_DATA(0)->node_mem_map - NODE_DATA(0)->node_start_pfn
The relevant bit of code is
map = alloc_remap(pgdat->node_id, size);
if (!map)
map = alloc_bootmem_node(pgdat, size);
pgdat->node_mem_map = map + (pgdat->node_start_pfn - start);
/*
* With FLATMEM the global mem_map is used. This is assumed
* to be based at pfn 0 such that 'pfn = page* - mem_map'
* is true. Adjust map relative to node_mem_map to
* maintain this relationship.
*/
map -= pgdat->node_start_pfn;
and later
if (pgdat == NODE_DATA(0))
mem_map = map;
So memory is wasted and ARCH_PFN_OFFSET isn't needed in the case where it
is working around NODE_DATA(0)->node_start_pfn != 0
If you have only 32MB in that
scenario, and that was correct, you'd gobble 25MB of that just to
store that array. Ouch.
It would be a kick in the shins all right if that was the case.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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