MIPS: bootmem: Don't use memory holes for page bitmap

Commit f9a7febd leads to a fact that mapstart and therefore a page bitmap for
bootmem allocator immediately follows initrd_end. This doesn't always work
well on Octeon, where there are holes in PFN ranges (refer to 5b3b1688 and
4MB-aligned PFN allocation). Depending on the inird location it could happen,
that mapstart would be in an area not allocated by plat_mem_setup() in
arch/mips/cavium-octeon/setup.c, but in the alignment hole between initrd and
the next PFN area. Later on this memory will be unconditionally made available
to buddy allocator at the end of free_all_bootmem_core() (mm/bootmem.c).
All of this results in Linux using the memory not designated for Linux in
Octeon's plat_mem_setup(), which in turn means corruption of the memory used
by another OS/baremetal code on the same SoC.

It doesn't look to me as a problem of Octeon platform code, but rather as an
inability of f9a7febd to deal correctly with the fragmented memory-mappings.
Proposed fix moves the check for initrd address to the same calculation-loop
in bootmem_init() (arch/mips/kernel/setup.c), which also accounts for kernel
code location. This should result in mapstart located starting from the first
PFN area after kernel code AND initrd.
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Cc: linux-mips@linux-mips.org
Cc: David Daney <david.daney@cavium.com>
Cc: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Cc: Joe Perches <joe@perches.com>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Yusuf Khan <yusuf.khan@nokia.com>
Cc: Michael Kreuzer <michael.kreuzer@nokia.com>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Patchwork: https://patchwork.linux-mips.org/patch/10594/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/setup.c

@@ -337,6 +337,11 @@ static void __init bootmem_init(void)
 			min_low_pfn = start;
 		if (end <= reserved_end)
 			continue;
+#ifdef CONFIG_BLK_DEV_INITRD
+		/* mapstart should be after initrd_end */
+		if (initrd_end && end <= (unsigned long)PFN_UP(__pa(initrd_end)))
+			continue;
+#endif
 		if (start >= mapstart)
 			continue;
 		mapstart = max(reserved_end, start);
@@ -366,14 +371,6 @@ static void __init bootmem_init(void)
 		max_low_pfn = PFN_DOWN(HIGHMEM_START);
 	}
 
-#ifdef CONFIG_BLK_DEV_INITRD
-	/*
-	 * mapstart should be after initrd_end
-	 */
-	if (initrd_end)
-		mapstart = max(mapstart, (unsigned long)PFN_UP(__pa(initrd_end)));
-#endif
-
 	/*
 	 * Initialize the boot-time allocator with low memory only.
 	 */