[PATCH 10/10] Print warning about cyclic buffer overrun if it can happen

[HATAYAMA Daisuke]

Clearling bits on cyclic buffer can overrun the cyclic buffer
according to some combination of MAX_ORDER and cyclic buffer size.

This patch warns this possibility but continues processing.

Signed-off-by: HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com>
---

 makedumpfile.c |   48 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 47 insertions(+), 1 deletions(-)

diff --git a/makedumpfile.c b/makedumpfile.c
index d1efc09..98eb640 100644
--- a/makedumpfile.c
+++ b/makedumpfile.c
@@ -58,6 +58,7 @@ do { \
 		*ptr_long_table = value; \
 } while (0)
 
+static void check_cyclic_buffer_overrun(void);
 static void setup_page_is_buddy(void);
 
 void
@@ -2806,6 +2807,9 @@ out:
 		    !sadump_generate_elf_note_from_dumpfile())
 			return FALSE;
 
+		if (info->flag_cyclic && info->dump_level & DL_EXCLUDE_FREE)
+			check_cyclic_buffer_overrun();
+
 	} else {
 		if (!get_mem_map_without_mm())
 			return FALSE;
@@ -3642,6 +3646,38 @@ exclude_free_page(void)
 	return TRUE;
 }
 
+static void
+check_cyclic_buffer_overrun(void)
+{
+	int max_order = ARRAY_LENGTH(zone.free_area);
+	int max_order_nr_pages = 1 << (max_order - 1);
+
+	/*
+	 * Let C be a cyclic buffer size and B a bitmap size used for
+	 * representing maximum block size managed by buddy
+	 * allocator. Then, if C mod B != 0, i.e., if there exists n >
+	 * 0, B > b > 0 such that C = n x B + b, then clearing
+	 * operation overruns cyclic buffer (B - b)-bytes.
+	 *
+	 * The bitmap size used for maximum block size B is calculated
+	 * from MAX_ORDER as:
+	 *
+	 *   B := ROUND_UP((1 << (MAX_ORDER - 1)), / BITS_PER_BYTE)
+	 *
+	 * Normally, MAX_ORDER is 11 at default. This is configurable
+	 * through CONFIG_FORCE_MAX_ZONEORDER.
+	 *
+	 * Note that If MAX_ORDER is default 11, this overrun issue
+	 * never happens. First, if MAX_ORDER is 11, B is 128
+	 * bytes. Cyclic buffer size is always specified in kilobytes,
+	 * i.e. C = 1024 x c for some c > 0. Hence, C mod B = 0
+	 * because 1024 mod 128 = 0.
+	 */
+	if (info->bufsize_cyclic %
+	    roundup(max_order_nr_pages, BITPERBYTE))
+		MSG("Some free pages are not filtered.\n");
+}
+
 /*
  * For the kernel versions from v2.6.15 to v2.6.17.
  */
@@ -3978,8 +4014,18 @@ __exclude_unnecessary_pages(unsigned long mem_map,
 		    && info->page_is_buddy(flags, _mapcount, private, _count)) {
 			int i;
 
-			for (i = 0; i < (1 << private); ++i)
+			for (i = 0; i < (1 << private); ++i) {
+				/*
+				 * According to combination of
+				 * MAX_ORDER and size of cyclic
+				 * buffer, this clearing bit operation
+				 * can overrun the cyclic buffer.
+				 *
+				 * See check_cyclic_buffer_overrun()
+				 * for the detail.
+				 */
 				clear_bit_on_2nd_bitmap_for_kernel(pfn + i);
+			}
 			pfn_free += i;
 		}
 		/*