Re: [PATCH] Put each per-cpu kdump ELF notes into a single page

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Sep 05, 2014 at 06:33:14PM +0200, Petr Tesarik wrote:
> On architectures that use percpu-vm, the percpu region is not guaranteed
> to be contiguous in physical space.

Petr,

Which are those arches?

> However, fs/proc/vmcore.c expects
> all ELF notes to be contiguous. If the ELF note happens to occupy
> two non-adjacent physical pages, part of the note may be read from an
> incorrect memory location by the kdump kernel, resulting in failure to
> initialize /proc/vmcore (if the content of the following physical page,
> incorrectly interpreted as an ELF note specifies a large number), wrong
> register values or other apparent random memory corruption.
> 
> There is currently no mechanism to pass the virtual-to-physical mapping
> of the percpu allocation to the kdump kernel. So, instead, I'm changing
> the alignment of the ELF note buffer. Since sizeof(note_buf_t) is less
> than PAGE_SIZE, aligning the buffer to the nearest higher power of 2
> is enough to make sure that the buffer cannot cross a page boundary,
> effectively ensuring that the whole buffer is contiguous in physical
> space.
> 
> Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
> ---
>  kernel/kexec.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index 2bee072..cdab59d 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -1610,7 +1610,8 @@ 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);
> +	crash_notes = __alloc_percpu(sizeof(note_buf_t),
> +				     roundup_pow_of_two(sizeof(note_buf_t)));

I think some of the changelog should show up here as comment in short
form. I don't think it is obvious that why we are using __alloc_percpu()
and why aligning to nearst higher power of 2 is needed here. Please also
mention here which arches run into issues.

Thanks
Vivek

>  	if (!crash_notes) {
>  		pr_warn("Kexec: Memory allocation for saving cpu register states failed\n");
>  		return -ENOMEM;
> -- 
> 1.8.4.5
> 
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