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

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Date:	Mon, 30 Nov 2015 08:42:04 -0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Daniel Thompson <daniel.thompson@...aro.org>,
	Jiri Kosina <jkosina@...e.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-cris-kernel@...s.com, linux-mips@...ux-mips.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	linux-sh@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v2 5/5] printk/nmi: Increase the size of the temporary buffer


> On Nov 27, 2015, at 19:09, Petr Mladek <pmladek@...e.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@...e.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

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