Question: Whether a bank must be fully contained by a section?

I have sent this question to linux-kernel at vger.kernel.org, but receive no answer. I think these should be a better place to discuss this question. Apologies if you got multiple copies of this email.

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By reading the code of show_mem(), I found that there is an assumption that the page structs must be continuous for each bank:

	for_each_bank (i, mi) {
		...
		page = pfn_to_page(pfn1);
		end  = pfn_to_page(pfn2 - 1) + 1;
	
		do {
			...
			page++;  <-- pageframe must be continuous
			...
		} while (page < end);
		...
	}

Therefore, a bank must be fully contained in a section in sparse memory mode, because page frames are allocated section by section (in sparse_init()).

However, I didn't find other code which enforces this assumption. Instead, in arm_memory_present (arch/arm/mm/init.c), it seems that a bank may contain more than one section:

arm_memory_present:

	...
	for_each_memblock(memory, reg)
		memory_present(0, memblock_region_memory_base_pfn(reg),
			       memblock_region_memory_end_pfn(reg));
	...

memory_present:

	...
	for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION) {
		...
	}
	...

Therefore, would you please consider the following patch, which removes the assumption that a bank must be fully contained in one section?

===========================

From: Wang Nan <[email protected]>
Date: Thu, 2 Jan 2014 13:20:02 +0800
Subject: [PATCH] use pfn_to_page in show_mem

If a bank spans into different sections, the page structures of the bank may not continous.

This patch uses pfn_to_page to recompute the address of struct page in show_mem from pfn, makes it to collect correct information even if a bank spans into different sections.

---
 arch/arm/mm/init.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

index 1f7b19a..3078e5a 100644
@@ -97,16 +97,14 @@ void show_mem(unsigned int filter)
 
 	for_each_bank (i, mi) {
 		struct membank *bank = &mi->bank[i];
-		unsigned int pfn1, pfn2;
-		struct page *page, *end;
+		unsigned int pfn, pfn_end;
+		struct page *page;
 
-		pfn1 = bank_pfn_start(bank);
-		pfn2 = bank_pfn_end(bank);
-
-		page = pfn_to_page(pfn1);
-		end  = pfn_to_page(pfn2 - 1) + 1;
+		pfn = bank_pfn_start(bank);
+		pfn_end = bank_pfn_end(bank);
 
 		do {
+			page = pfn_to_page(pfn);
 			total++;
 			if (PageReserved(page))
 				reserved++;

@@ -118,8 +116,8 @@ void show_mem(unsigned int filter)
 				free++;
 			else
 				shared += page_count(page) - 1;
-			page++;
-		} while (page < end);
+			pfn++;
+		} while (pfn < pfn_end);
 	}
 
 	printk("%d pages of RAM\n", total);

1.8.4