[PATCH] Tight check of pfn_valid on sparsemem - v2

[Minchan Kim]

Kukjin reported oops happen while he change min_free_kbytes
http://www.spinics.net/lists/arm-kernel/msg92894.html
It happen by memory map on sparsemem.

The system has a memory map following as. 
     section 0             section 1              section 2
0x20000000-0x25000000, 0x40000000-0x50000000, 0x50000000-0x58000000
SECTION_SIZE_BITS 28(256M)

It means section 0 is an incompletely filled section.
Nontheless, current pfn_valid of sparsemem checks pfn loosely. 
It checks only mem_section's validation but ARM can free mem_map on hole 
to save memory space. So in above case, pfn on 0x25000000 can pass pfn_valid's 
validation check. It's not what we want. 

We can match section size to smallest valid size.(ex, above case, 16M)
But Russell doesn't like it due to mem_section's memory overhead with different
configuration(ex, 512K section).

I tried to add valid pfn range in mem_section but everyone doesn't like it 
due to size overhead. This patch is suggested by KAMEZAWA-san. 
I just fixed compile error and change some naming. 

This patch registers address of mem_section to memmap itself's page struct's
pg->private field. This means the page is used for memmap of the section.
Otherwise, the page is used for other purpose and memmap has a hole.

This patch is based on mmotm-2010-07-01-12-19.

Signed-off-by: Minchan Kim <minchan.kim at gmail.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
Reported-by: Kukjin Kim <kgene.kim at samsung.com>
---
 arch/arm/mm/init.c     |    9 ++++++++-
 include/linux/mmzone.h |   21 ++++++++++++++++++++-
 mm/Kconfig             |    5 +++++
 mm/sparse.c            |   41 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 74 insertions(+), 2 deletions(-)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index cfe4c5e..4586f40 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -234,6 +234,11 @@ static void __init arm_bootmem_free(struct meminfo *mi)
 	arch_adjust_zones(zone_size, zhole_size);
 
 	free_area_init_node(0, zone_size, min, zhole_size);
+
+	for_each_bank(i, mi) {
+		mark_memmap_hole(bank_pfn_start(&mi->bank[i]),
+				bank_pfn_end(&mi->bank[i]), true);
+	}
 }
 
 #ifndef CONFIG_SPARSEMEM
@@ -386,8 +391,10 @@ free_memmap(unsigned long start_pfn, unsigned long end_pfn)
 	 * If there are free pages between these,
 	 * free the section of the memmap array.
 	 */
-	if (pg < pgend)
+	if (pg < pgend) {
+		mark_memmap_hole(pg >> PAGE_SHIFT, pgend >> PAGE_SHIFT, false);
 		free_bootmem(pg, pgend - pg);
+	}
 }
 
 /*
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 9ed9c45..2ed9728 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -15,6 +15,7 @@
 #include <linux/seqlock.h>
 #include <linux/nodemask.h>
 #include <linux/pageblock-flags.h>
+#include <linux/mm_types.h>
 #include <generated/bounds.h>
 #include <asm/atomic.h>
 #include <asm/page.h>
@@ -1047,11 +1048,29 @@ static inline struct mem_section *__pfn_to_section(unsigned long pfn)
 	return __nr_to_section(pfn_to_section_nr(pfn));
 }
 
+void mark_memmap_hole(unsigned long start, unsigned long end, bool valid);
+
+#ifdef CONFIG_SPARSEMEM_HAS_HOLE
+static inline int page_valid(struct mem_section *ms, unsigned long pfn)
+{
+	struct page *page = pfn_to_page(pfn);
+	struct page *__pg = virt_to_page(page);
+	return __pg->private == (unsigned long)ms;
+}
+#else
+static inline int page_valid(struct mem_section *ms, unsigned long pfn)
+{
+	return 1;
+}
+#endif
+
 static inline int pfn_valid(unsigned long pfn)
 {
+	struct mem_section *ms;
 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
 		return 0;
-	return valid_section(__nr_to_section(pfn_to_section_nr(pfn)));
+	ms = __nr_to_section(pfn_to_section_nr(pfn));
+	return valid_section(ms) && page_valid(ms, pfn);
 }
 
 static inline int pfn_present(unsigned long pfn)
diff --git a/mm/Kconfig b/mm/Kconfig
index 527136b..959ac1d 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -128,6 +128,11 @@ config SPARSEMEM_VMEMMAP
 	 pfn_to_page and page_to_pfn operations.  This is the most
 	 efficient option when sufficient kernel resources are available.
 
+config SPARSEMEM_HAS_HOLE
+	bool "allow holes in sparsemem's memmap"
+	depends on ARM && SPARSEMEM && !SPARSEMEM_VMEMMAP
+	default n
+
 # eventually, we can have this option just 'select SPARSEMEM'
 config MEMORY_HOTPLUG
 	bool "Allow for memory hot-add"
diff --git a/mm/sparse.c b/mm/sparse.c
index 95ac219..76d5012 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -615,6 +615,47 @@ void __init sparse_init(void)
 	free_bootmem(__pa(usemap_map), size);
 }
 
+#ifdef CONFIG_SPARSEMEM_HAS_HOLE
+/*
+ * Fill memmap's pg->private with a pointer to mem_section.
+ * pfn_valid() will check this later. (see include/linux/mmzone.h)
+ * Evenry arch should call
+ * 	mark_memmap_hole(start, end, true) # for all allocated mem_map
+ * 	and, after that,
+ * 	mark_memmap_hole(start, end, false) # for all holes in mem_map.
+ * please see usage in ARM.
+ */
+void mark_memmap_hole(unsigned long start, unsigned long end, bool valid)
+{
+	struct mem_section *ms;
+	unsigned long pos, next;
+	struct page *pg;
+	void *memmap, *mapend;
+
+	for (pos = start; pos < end; pos = next) {
+		next = (pos + PAGES_PER_SECTION) & PAGE_SECTION_MASK;
+		ms = __pfn_to_section(pos);
+		if (!valid_section(ms))
+			continue;
+
+		for (memmap = (void*)pfn_to_page(pos),
+			/* The last page in section */
+			mapend = pfn_to_page(next-1);
+			memmap < mapend; memmap += PAGE_SIZE) {
+			pg = virt_to_page(memmap);
+			if (valid)
+				pg->private = (unsigned long)ms;
+			else
+				pg->private = 0;
+		}
+	}
+}
+#else
+void mark_memmap_hole(unsigned long start, unsigned long end, bool valid)
+{
+}
+#endif
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 static inline struct page *kmalloc_section_memmap(unsigned long pnum, int nid,
-- 
1.7.0.5