[patch 5/6] kdump: add is_vmcore_usable() and vmcore_unusable()

[Simon Horman]

The usage of elfcorehdr_addr has changed recently
such that being set to ELFCORE_ADDR_MAX is used by
is_kdump_kernel() to indicate if the code is executing
in a kernel executed as a crash kernel.

However, arch/ia64/kernel/setup.c:reserve_elfcorehdr will
rest elfcorehdr_addr to ELFCORE_ADDR_MAX on error,
which means any subsequent calls to is_kdump_kernel()
will return 0, even though they should return 1.

Ok, at this point in time there are no subsequent calls,
but I think its fair to say that there is ample scope for error
or at the very least confusion.

This patch add an extra state, ELFCORE_ADDR_ERR, which
indicates that elfcorehdr_addr was passed on the command line,
and thus execution is taking place in a crashdump kernel,
but vmcore can't be used for some reason. This is tested
for using is_vmcore_usable() and set using vmcore_unusable().
A subsequent patch makes use of this new code.

To summarise, the states that elfcorehdr_addr can now be in are as follows:

ELFCORE_ADDR_MAX: not a crashdump kernel
ELFCORE_ADDR_ERR: crashdump kernel but vmcore is unusable
any other value:  crash dump kernel and vmcore is usable

Signed-off-by: Simon Horman <horms@xxxxxxxxxxxx>

--- 

 fs/proc/vmcore.c           |    2 +-
 include/linux/crash_dump.h |   26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

Wed, 30 Jul 2008 09:10:37 +1000
* Fixed logic bug in vmcore_unusable().
  if (!is_kdump_kernel()) is correct
  if (is_kdump_kernel()) is not

Andrew, this patch can wait until after 2.6.27 as it is (minor)
infrastructure that only has one usage case, and that usage case
is not buggy at this time.

is_vmcore_usable() and vmcore_unusable() are used in a subsequent patch
in this series

Index: linux-2.6/include/linux/crash_dump.h
===================================================================
--- linux-2.6.orig/include/linux/crash_dump.h	2008-07-30 09:05:50.000000000 +1000
+++ linux-2.6/include/linux/crash_dump.h	2008-07-30 09:08:58.000000000 +1000
@@ -8,6 +8,7 @@
 #include <linux/proc_fs.h>
 
 #define ELFCORE_ADDR_MAX	(-1ULL)
+#define ELFCORE_ADDR_ERR	(-2ULL)
 
 extern unsigned long long elfcorehdr_addr;
 
@@ -42,5 +43,30 @@ static inline int is_kdump_kernel(void)
 static inline int is_kdump_kernel(void) { return 0; }
 #endif /* CONFIG_CRASH_DUMP */
 
+#ifdef CONFIG_PROC_VMCORE
+/* is_vmcore_usable() checks if the kernel is booting after a panic and
+ * the vmcore region is usable.
+ *
+ * This makes use of the fact that due to alignment 1 is not
+ * a valid pointer, much in the vain of IS_ERR(), except
+ * dealing directly with an unsigned long long rather than a pointer.
+ */
+
+static inline int is_vmcore_usable(void)
+{
+	return is_kdump_kernel() && elfcorehdr_addr != ELFCORE_ADDR_ERR ? 1 : 0;
+}
+
+/* vmcore_unusable() marks the vmcore as unusable,
+ * without disturbing the logic of is_kdump_kernel()
+ */
+
+static inline void vmcore_unusable(void)
+{
+	if (is_kdump_kernel())
+		elfcorehdr_addr = ELFCORE_ADDR_ERR;
+}
+#endif /* CONFIG_PROC_VMCORE */
+
 extern unsigned long saved_max_pfn;
 #endif /* LINUX_CRASHDUMP_H */
Index: linux-2.6/fs/proc/vmcore.c
===================================================================
--- linux-2.6.orig/fs/proc/vmcore.c	2008-07-30 09:05:50.000000000 +1000
+++ linux-2.6/fs/proc/vmcore.c	2008-07-30 09:07:07.000000000 +1000
@@ -650,7 +650,7 @@ static int __init vmcore_init(void)
 	int rc = 0;
 
 	/* If elfcorehdr= has been passed in cmdline, then capture the dump.*/
-	if (!(elfcorehdr_addr < ELFCORE_ADDR_MAX))
+	if (!(is_vmcore_usable()))
 		return rc;
 	rc = parse_crash_elf_headers();
 	if (rc) {

-- 

-- 
Horms

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