Re: [PATCH] kdump: force page alignment for per-CPU crash notes.

[Simon Horman <horms@verge.net.au>]

On Wed, Feb 29, 2012 at 09:21:23AM -0800, Eugene Surovegin wrote:
> Per-CPU allocations are not guaranteed to be physically contiguous.
> However, kdump kernel and user-space code assumes that per-CPU
> memory, used for saving CPU registers on crash, is.
> This can cause corrupted /proc/vmcore in some cases - the main
> symptom being huge ELF note section.
> 
> Force page alignment for note_buf_t to ensure that this assumption holds.

Ouch. I'm surprised there is an allocation on crash, perhaps
it could at least be done earlier? And am I right in thinking
that this change increases the likely hood that the allocation
could fail?

> 
> Signed-off-by: Eugene Surovegin <surovegin@google.com>
> CC: Eric Biederman <ebiederm@xmission.com>
> CC: Vivek Goyal <vgoyal@redhat.com>
> CC: kexec-list <kexec@lists.infradead.org>
> ---
>  kernel/kexec.c |    9 +++++++--
>  1 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index 7b08867..e641b5c 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -1232,8 +1232,13 @@ void crash_save_cpu(struct pt_regs *regs, int cpu)
>  
>  static int __init crash_notes_memory_init(void)
>  {
> -	/* Allocate memory for saving cpu registers. */
> -	crash_notes = alloc_percpu(note_buf_t);
> +	/* Allocate memory for saving cpu registers.
> +	 * Force page alignment to avoid crossing physical page boundary -
> +	 * kexec-tools and kernel /proc/vmcore handler assume these per-CPU
> +	 * chunks are physically contiguous.
> +	 */
> +	crash_notes = (note_buf_t __percpu *)__alloc_percpu(sizeof(note_buf_t),
> +							    PAGE_SIZE);
>  	if (!crash_notes) {
>  		printk("Kexec: Memory allocation for saving cpu register"
>  		" states failed\n");
> -- 
> 1.7.9.1
> 
> 
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