On Fri, Jan 21, 2011 at 2:20 AM, Dave Hansen [off-list ref] wrote:

On Thu, 2011-01-20 at 18:45 +0900, KyongHo Cho wrote:

quoted

Sparsemem allows that a bank of memory spans over several adjacent
sections if the start address and the end address of the bank
belong to different sections.
When gathering statictics of physical memory in mem_init() and
show_mem(), this possiblity was not considered.

This patch guarantees that simple increasing the pointer to page
descriptors does not exceed the boundary of a section

...

quoted

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 57c4c5c..6ccecbe 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c

@@ -93,24 +93,38 @@ void show_mem(void)

? ? ? ? ? ? ? pfn1 = bank_pfn_start(bank);
? ? ? ? ? ? ? pfn2 = bank_pfn_end(bank);
-
+#ifndef CONFIG_SPARSEMEM
? ? ? ? ? ? ? page = pfn_to_page(pfn1);
? ? ? ? ? ? ? end ?= pfn_to_page(pfn2 - 1) + 1;
-
+#else
+ ? ? ? ? ? ? pfn2--;
? ? ? ? ? ? ? do {
- ? ? ? ? ? ? ? ? ? ? total++;
- ? ? ? ? ? ? ? ? ? ? if (PageReserved(page))
- ? ? ? ? ? ? ? ? ? ? ? ? ? ? reserved++;
- ? ? ? ? ? ? ? ? ? ? else if (PageSwapCache(page))
- ? ? ? ? ? ? ? ? ? ? ? ? ? ? cached++;
- ? ? ? ? ? ? ? ? ? ? else if (PageSlab(page))
- ? ? ? ? ? ? ? ? ? ? ? ? ? ? slab++;
- ? ? ? ? ? ? ? ? ? ? else if (!page_count(page))
- ? ? ? ? ? ? ? ? ? ? ? ? ? ? free++;
- ? ? ? ? ? ? ? ? ? ? else
- ? ? ? ? ? ? ? ? ? ? ? ? ? ? shared += page_count(page) - 1;
- ? ? ? ? ? ? ? ? ? ? page++;
- ? ? ? ? ? ? } while (page < end);
+ ? ? ? ? ? ? ? ? ? ? page = pfn_to_page(pfn1);
+ ? ? ? ? ? ? ? ? ? ? if (pfn_to_section_nr(pfn1) < pfn_to_section_nr(pfn2)) {
+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? pfn1 += PAGES_PER_SECTION;
+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? pfn1 &= PAGE_SECTION_MASK;
+ ? ? ? ? ? ? ? ? ? ? } else {
+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? pfn1 = pfn2;
+ ? ? ? ? ? ? ? ? ? ? }
+ ? ? ? ? ? ? ? ? ? ? end = pfn_to_page(pfn1) + 1;
+#endif

This problem actually exists without sparsemem, too. ?Discontigmem (at
least) does it as well.

Actually, as long as a bank in meminfo only resides in a pgdat, no
problem happens
because there is no restriction of size of area in a pgdat.
That's why I just considered about sparsemem.

The x86 version of show_mem() actually manages to do this without any
#ifdefs, and works for a ton of configuration options. ?It uses
pfn_valid() to tell whether it can touch a given pfn.

Long-term, it might be a good idea to convert arm's show_mem() over to
use pgdat's like everything else. ?But, for now, you should just be able
to do something roughly like this:

- ? ? ? ? ? ? ? page = pfn_to_page(pfn1);
- ? ? ? ? ? ? ? end ?= pfn_to_page(pfn2 - 1) + 1;
-
- ? ? ? ? ? ? ? do {
+ ? ? ? ? ? ? ? for (pfn = pfn1; pfn < pfn2; pfn++) {
+ ? ? ? ? ? ? ? ? ? ? ? if (!pfn_valid(pfn))
+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? continue;
+ ? ? ? ? ? ? ? ? ? ? ? page = pfn_to_page(pfn);
+
? ? ? ? ? ? ? ? ? ? ? ?total++;
? ? ? ? ? ? ? ? ? ? ? ?if (PageReserved(page))
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?reserved++;
? ? ? ? ? ? ? ? ? ? ? ?else if (PageSwapCache(page))
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?cached++;
? ? ? ? ? ? ? ? ? ? ? ?else if (PageSlab(page))
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?slab++;
? ? ? ? ? ? ? ? ? ? ? ?else if (!page_count(page))
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?free++;
? ? ? ? ? ? ? ? ? ? ? ?else
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?shared += page_count(page) - 1;
? ? ? ? ? ? ? ? ? ? ? ?page++;
- ? ? ? ? ? ? ? } while (page < end);
+ ? ? ? ? ? ? ? }

That should work for sparsemem, or any other crazy memory models that we
come up with. ?pfn_to_page() is pretty quick, especially when doing it
in a tight loop like that.

That's true.
I worried that pfn_to_page() in sparsemem is a bit slower than that in flatmem.
Moreover, the previous one didn't use pfn_to_page() but page++ for the
performance.
Nevertheless, I also think that pfn_to_page() make the code neat.