Re: Re : sparsemem usage

From: Andy Whitcroft
Date: Thu Aug 10 2006 - 11:21:37 EST

moreau francis wrote:
KAMEZAWA Hiroyuki wrote:
On Thu, 10 Aug 2006 14:40:52 +0200 (CEST)
moreau francis <francis_moreau2000@xxxxxxxx> wrote:
BTW, ioresouce information (see kernel/resouce.c)

[kamezawa@aworks Development]$ cat /proc/iomem | grep RAM
00000000-0009fbff : System RAM
000a0000-000bffff : Video RAM area
00100000-2dfeffff : System RAM

is not enough ?

well actually you show that to get a really simple information, ie does
a page exist ?, we need to parse some kernel data structures like ioresource (which is, IMHO, hackish) or duplicate in each architecture
some data to keep track of existing pages.

becasue memory map from e820(x86) or efi(ia64) are registered to iomem_resource,
we should avoid duplicates that information. kdump and memory hotplug uses
this information. (memory hotplug updates this iomem_resource.)

Implementing "page_is_exist" function based on ioresouce is one of generic
and rubust way to go, I think.
(if performance of list walking is problem, enhancing ioresouce code is

Why not implementing page_exist() by simply using mem_map[] ? When
allocating mem_map[], we can just fill it with a special value. And
then when registering memory area, we clear this special value with
the "reserved" value. Hence for flatmem model, we can have:

#define page_exist(pfn) (mem_map[pfn] != SPECIAL_VALUE)

and it should work for sparsemem too and other models that will use

The mem_map isn't a pointer, its a physical structure. We have a special value to tell you if the page is usable within that, thats called PG_reserved. If this page is reserved the kernel can't touch it, can't look at it.

Another point, is page_exist() going to replace page_valid() ?
I mean page_exist() is going to be something more accurate than
page_valid(). All tests on page_valid() _only_ will be fine to test
page_exist(). But all tests such:

if (page_valid(x) && page_is_ram(x))

can be replaced by

if (page_exist(x))

So, again, why not simply improving page_valid() definition rather
than introduce a new service ?

Whilst I can understand that not knowing if a page is real or not is perhaps unappealing, I've yet to see any case where we need or care. Changing things to make things 'nicer' interlectually is sometimes worthwhile. But what is the user here.

The only consumer you have shown is show_mem() which is a debug function, and that only dumps out the current memory counts. Its not clear it cares to really know if a page is real or not.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at