linux-kernel - Re: [PATCH]: add dmesg log symbols to /proc/vmcoreinfo lists

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Message-ID: <498A317A.8030707@mxs.nes.nec.co.jp>
Date:	Thu, 05 Feb 2009 09:23:22 +0900
From:	"Ken'ichi Ohmichi" <oomichi@....nes.nec.co.jp>
To:	Neil Horman <nhorman@...driver.com>
CC:	Eric Biederman <ebiederm@...sion.com>, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, hbabu@...ibm.com,
	Simon Horman <horms@...ge.net.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH]: add dmesg log symbols to /proc/vmcoreinfo lists


Hi Neil,

Thank you for a kernel patch and a makedumpfile patch.

Neil Horman wrote:
>> That aside we aren't currently exporting log_buf_len, so I don't
>> think this code works actually works.
>>
>> Neil can you add a comment in kernel/printk.c of the algorithm
>> necessary for external tools to decode the ring buffer?
>>
>> We need the comment because people working on kernel/printk.c
>> need to know what is happening and without having to review lots
>> of user space code, and we need the comment to verify that we
>> are exporting the right things.
>>
> 
> Ok, as per Erics comment, I've written this.  It applies on top of whats already
> in your tree Andrew.  It adds some comments on the function in question so that
> anyone working on printk.c will know why we're exporting their symbols.  It also
> modifies slightly the symbols we are exporting so that we can handle dmesg
> buffers that are longer than the standard PAGE_SIZE configuration, and lets us
> detect and handle buffer wraps.
> 
> Thanks!
> 
> printk.c |   12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
> 
> Signed-off-by: Neil Horman <nhorman@...driver.com>
> 
> diff --git a/kernel/printk.c b/kernel/printk.c
> index dd5cc7b..6a1163b 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -138,10 +138,20 @@ static int log_buf_len = __LOG_BUF_LEN;
>  static unsigned logged_chars; /* Number of chars produced since last read+clear operation */
>  
>  #ifdef CONFIG_KEXEC
> +/*
> + * This appends the listed symbols to /proc/vmcoreinfo
> + * /proc/vmcoreinfo is used by various utiilties, like
> + * crash and makedumpfile to obtain access to symbols that
> + * are otherwise very difficult to locate.  These symbols 
> + * are specifically used so that utilities 
> + * can access and extract the dmesg log from a vmcore file
> + * after a crash
> + */
>  void log_buf_kexec_setup(void)
>  {
>  	VMCOREINFO_SYMBOL(log_buf);
> -	VMCOREINFO_SYMBOL(log_end);
> +	VMCOREINFO_SYMBOL(log_buf_len);
> +	VMCOREINFO_SYMBOL(logged_chars)
>  }
>  #endif

Please do not remove VMCOREINFO_SYMBOL(log_end), because makedumpfile
needs it for knowing the latest log offset in ring buffer of dmesg.
I updated a makedumpfile patch for handling ring buffer, and it is
possible to download the latest patch from the following site:

https://sourceforge.net/tracker2/download.php?group_id=178938&atid=887142&file_id=312113&aid=2521075


Thanks
Ken'ichi Ohmichi
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