Re: [patch 2/3] kdump: add is_vmcore_usable() and vmcore_unusable()

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Jul 29, 2008 at 06:12:37PM +1000, Simon Horman wrote:
> 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@verge.net.au>
> 
> 
> Index: linux-2.6/include/linux/crash_dump.h
> ===================================================================
> --- linux-2.6.orig/include/linux/crash_dump.h	2008-07-29 17:27:43.000000000 +1000
> +++ linux-2.6/include/linux/crash_dump.h	2008-07-29 17:53:13.000000000 +1000
> @@ -8,6 +8,7 @@
>  #include <linux/proc_fs.h>
>  
>  #define ELFCORE_ADDR_MAX	(-1ULL)
> +#define ELFCORE_ADDR_ERR	(-2ULL)
>  
>  #ifdef CONFIG_PROC_VMCORE
>  extern unsigned long long elfcorehdr_addr;
> @@ -36,5 +37,30 @@ 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;
> +}

Hi Simon,

Should above condition be "if(is_kdump_kernel())" instead of
"if(!is_kdump_kernel())?

I would think that you would like to mark a vmcore unusable only if, to 
begin with you were booting after a panic.

If we being marking vmcore_unusable in case of normal kernel
(!is_kdump_kernel()), then is_kdump_kernel() will start reporting
normal kernel as kdump kernel?

Thanks
Vivek