linux-kernel - Re: [PATCH] tty: hvc: don't allocate a buffer for console print on stack

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Message-ID: <18f113d4-9c37-0114-d637-3720ad093722@gmail.com>
Date:   Mon, 20 Mar 2017 18:55:15 +0300
From:   Jan Dakinevich <jan.dakinevich@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.com>,
        Peter Hurley <peter@...leysoftware.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Arnd Bergmann <arnd@...db.de>, Amit Shah <amit@...nel.org>,
        virtualization@...ts.linux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: hvc: don't allocate a buffer for console print on
 stack

> You shouldn't use 'static' data either, that's not always guaranteed to
> be DMA-able, right?

I naively thought all 'static' data was linked into .data and the
section was always kmapped.

> What about allocating it dynamically?  That's the correct thing to do.

Yep, the solution was more welcomed and happened earlier:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c4baad50297d84bde1a7ad45e50c73adae4a2192

On 03/17/2017 08:09 AM, Greg Kroah-Hartman wrote:
> On Fri, Feb 17, 2017 at 11:42:45PM +0300, Jan Dakinevich wrote:
>> The buffer is used by virtio console driver as DMA buffer. Since v4.9
>> (if VMAP_STACK is enabled) we shouldn't use the stack for DMA.
> 
> You shouldn't use 'static' data either, that's not always guaranteed to
> be DMA-able, right?
> 
>>
>> Signed-off-by: Jan Dakinevich <jan.dakinevich@...il.com>
>> ---
>>  drivers/tty/hvc/hvc_console.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
>> index 9b5c0fb..1ce6aaf 100644
>> --- a/drivers/tty/hvc/hvc_console.c
>> +++ b/drivers/tty/hvc/hvc_console.c
>> @@ -143,10 +143,15 @@ static struct hvc_struct *hvc_get_by_index(int index)
>>  static void hvc_console_print(struct console *co, const char *b,
>>  			      unsigned count)
>>  {
>> -	char c[N_OUTBUF] __ALIGNED__;
>>  	unsigned i = 0, n = 0;
>>  	int r, donecr = 0, index = co->index;
>>  
>> +	/*
>> +	 * Access to the buffer is serialized by console_sem in caller code from
>> +	 * kernel/printk/printk.c
>> +	 */
>> +	static char c[N_OUTBUF] __ALIGNED__;
> 
> What about allocating it dynamically?  That's the correct thing to do.
> 
> thanks,
> 
> greg k-h
> 

-- 
Best regards
Jan Dakinevich