[Patch v2] align crash_notes allocation to make it be inside one physical page

[Andrew Morton]

On Mon,  3 Aug 2015 20:50:43 +0800 Baoquan He <bhe at redhat.com> wrote:

> People reported that crash_notes in /proc/vmcore were corrupted and
> this cause crash kdump failure. With code debugging and log we got
> the root cause. This is because percpu variable crash_notes are
> allocated in 2 vmalloc pages. Currently percpu is based on vmalloc
> by default. Vmalloc can't guarantee 2 continuous vmalloc pages
> are also on 2 continuous physical pages. So when 1st kernel exports
> the starting address and size of crash_notes through sysfs like below:
> 
> /sys/devices/system/cpu/cpux/crash_notes
> /sys/devices/system/cpu/cpux/crash_notes_size
> 
> kdump kernel use them to get the content of crash_notes. However the
> 2nd part may not be in the next neighbouring physical page as we
> expected if crash_notes are allocated accross 2 vmalloc pages. That's
> why nhdr_ptr->n_namesz or nhdr_ptr->n_descsz could be very huge in
> update_note_header_size_elf64() and cause note header merging failure
> or some warnings.
> 
> In this patch change to call __alloc_percpu() to passed in the align
> value by rounding crash_notes_size up to the nearest power of two.
> This make sure the crash_notes is allocated inside one physical page
> since sizeof(note_buf_t) in all ARCHS is smaller than PAGE_SIZE.
> Meanwhile add a WARN_ON in case it grows to be bigger than PAGE_SIZE
> in the future.
> 
> ...
>
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -1620,7 +1620,16 @@ 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);
> +	size_t size, align;
> +	int order;
> +
> +	size = sizeof(note_buf_t);
> +	order = get_count_order(size);
> +	align = min_t(size_t, (1<<order), PAGE_SIZE);
> +
> +	WARN_ON(size > PAGE_SIZE);
> +
> +	crash_notes = __alloc_percpu(size, align);

A code comment would be helpful - the reason for this code's existence
is otherwise utterly unobvious.

I think it can be done this way:

	align = min(roundup_pow_of_two(sizeof(note_buf_t)), PAGE_SIZE);


I never noticed get_count_order() before.  afaict it does the same as
order_base_2(), except get_count_order() generates better code and has
a ridiculous name.

And I think the WARN_ON can be replaced with a
BUILD_BUG_ON(sizeof>PAGE_SIZE)?  That would avoid adding runtime
overhead.