Re: [PATCH v2 5/5] printk/nmi: Increase the size of the temporary buffer

[yalin wang <yalin.wang2010@gmail.com>]

> On Nov 27, 2015, at 19:09, Petr Mladek <pmladek@suse.com> wrote:
> 
> Testing has shown that the backtrace sometimes does not fit
> into the 4kB temporary buffer that is used in NMI context.
> 
> The warnings are gone when I double the temporary buffer size.
> 
> Note that this problem existed even in the x86-specific
> implementation that was added by the commit a9edc8809328
> ("x86/nmi: Perform a safe NMI stack trace on all CPUs").
> Nobody noticed it because it did not print any warnings.
> 
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
> kernel/printk/nmi.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
> index 8af1e4016719..6111644d5f01 100644
> --- a/kernel/printk/nmi.c
> +++ b/kernel/printk/nmi.c
> @@ -42,7 +42,7 @@ atomic_t nmi_message_lost;
> struct nmi_seq_buf {
> 	atomic_t		len;	/* length of written data */
> 	struct irq_work		work;	/* IRQ work that flushes the buffer */
> -	unsigned char		buffer[PAGE_SIZE - sizeof(atomic_t) -
> +	unsigned char		buffer[2 * PAGE_SIZE - sizeof(atomic_t) -
> 				       sizeof(struct irq_work)];
> };
> 

why not define like this:

union {
struct {atomic_t		len;	
	struct irq_work		work;
}
unsigned char		buffer[PAGE_SIZE * 2] ;
}

we can make sure the union is 2 PAGE_SIZE .

Thanks