ARM: 6890/1: memmap: only free allocated memmap entries when using SPARSEMEM

The SPARSEMEM code allocates memmap entries only for sections which are
present (i.e. those which contain some valid memory). The membank checks
in free_unused_memmap do not take this into account and can incorrectly
attempt to free memory which is not allocated, resulting in a BUG() in
the bootmem code.

However, if memory is configured as follows:

|<----section---->|<----hole---->|<----section---->|
+--------+--------+--------------+--------+--------+
| bank 0 | unused | | bank 1 | unused |
+--------+--------+--------------+--------+--------+

where a bank only occupies part of a section, the memmap allocated for
the remainder of the section *can* be freed.

This patch modifies the checks in free_unused_memmap so that only valid
memmap entries are considered for removal.

Acked-by: Catalin Marinas <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

arch/arm/mm/init.c

···
 	 * Convert start_pfn/end_pfn to a struct page pointer.
 	 */
 	start_pg = pfn_to_page(start_pfn - 1) + 1;
-	end_pg = pfn_to_page(end_pfn);
+	end_pg = pfn_to_page(end_pfn - 1) + 1;
 
 	/*
 	 * Convert to physical addresses, and
···
 
 		bank_start = bank_pfn_start(bank);
 
+#ifdef CONFIG_SPARSEMEM
+		/*
+		 * Take care not to free memmap entries that don't exist
+		 * due to SPARSEMEM sections which aren't present.
+		 */
+		bank_start = min(bank_start,
+				 ALIGN(prev_bank_end, PAGES_PER_SECTION));
+#endif
 		/*
 		 * If we had a previous bank, and there is a space
 		 * between the current bank and the previous, free it.
···
 		 */
 		prev_bank_end = ALIGN(bank_pfn_end(bank), MAX_ORDER_NR_PAGES);
 	}
+
+#ifdef CONFIG_SPARSEMEM
+	if (!IS_ALIGNED(prev_bank_end, PAGES_PER_SECTION))
+		free_memmap(prev_bank_end,
+			    ALIGN(prev_bank_end, PAGES_PER_SECTION));
+#endif
 }
 
 static void __init free_highpages(void)