Re: [RFT 1/2] printk: make dynamic kernel ring buffer alignemnt explicit

[Petr Mládek]

On Wed 2014-06-18 04:14:24, Luis R. Rodriguez wrote:
> From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> 
> We have to consider alignment for the ring buffer both for the
> default static size, and then also for when an dynamic allocation
> is made when the log_buf_len=n kernel parameter is passed to set
> the size specifically to a size larger than the default size set
> by the architecture through CONFIG_LOG_BUF_SHIFT.
> 
> The default static kernel ring buffer can be aligned properly if
> architectures set CONFIG_LOG_BUF_SHIFT properly, we provide ranges
> for the size though so even if CONFIG_LOG_BUF_SHIFT has a sensible
> aligned value it can be reduced to a non aligned value. Commit
> 6ebb017de9 by Andrew ensures the static buffer is always aligned
> and the decision of alignment is done by the compiler by using
> __alignof__(struct log) (curious what value caused the crash?).
> 
> When log_buf_len=n is used we allocate the ring buffer dynamically.
> Dynamic allocation varies, for the early allocation called
> before setup_arch() memblock_virt_alloc() requests a page aligment
> and for the default kernel allocation memblock_virt_alloc_nopanic()
> requests no special alignment, which in turn ends up aligning the
> allocation to SMP_CACHE_BYTES, which is L1 cache aligned.
> 
> Since we already have the required alignment for the kernel ring
> buffer though we can do better and request explicit alignment for
> LOG_ALIGN. Do that and also put the power of 2 practice of
> the ring buffer size into a helper which we'll use later.
> 
> Cc: Andrew Lunn <andrew@xxxxxxx>
> Cc: Stephen Warren <swarren@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: Petr Mladek <pmladek@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Joe Perches <joe@xxxxxxxxxxx>
> Cc: Arun KS <arunks.linux@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Davidlohr Bueso <davidlohr@xxxxxx>
> Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>

The change makes sense to me.

Acked-by: Petr Mladek <pmladek@xxxxxxx>
Tested-by: Petr Mladek <pmladek@xxxxxxx>

> This is perhaps not required given that we stick to powers of 2
> and the min LOG_BUF_SHIFT is 12, if the min LOG_BUF_SHIFT is
> aligned then I think any passed log_buf_len=n would be aligned
> as well as we don't make log_buf_len=n take effect unless
> its > than the default size, and we round to the produced size
> to the next power of 2. If the min length produced by
> LOG_BUF_SHIFT is aligned, multiples of 2 of this should be as
> well I think.
> 
> This might be perhaps safest thing to do though given we'll
> add other alloc entries next.
> 
>  kernel/printk/printk.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index ea2d5f6..af164a7 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -828,15 +828,21 @@ void log_buf_kexec_setup(void)
>  /* requested log_buf_len from kernel cmdline */
>  static unsigned long __initdata new_log_buf_len;
>  
> -/* save requested log_buf_len since it's too early to process it */
> -static int __init log_buf_len_setup(char *str)
> +/* we practice scaling the ring buffer by powers of 2 */
> +static void __init log_buf_len_update(unsigned size)
>  {
> -	unsigned size = memparse(str, &str);
> -
>  	if (size)
>  		size = roundup_pow_of_two(size);
>  	if (size > log_buf_len)
>  		new_log_buf_len = size;
> +}
> +
> +/* save requested log_buf_len since it's too early to process it */
> +static int __init log_buf_len_setup(char *str)
> +{
> +	unsigned size = memparse(str, &str);
> +
> +	log_buf_len_update(size);
>  
>  	return 0;
>  }
> @@ -853,9 +859,10 @@ void __init setup_log_buf(int early)
>  
>  	if (early) {
>  		new_log_buf =
> -			memblock_virt_alloc(new_log_buf_len, PAGE_SIZE);
> +			memblock_virt_alloc(new_log_buf_len, LOG_ALIGN);
>  	} else {
> -		new_log_buf = memblock_virt_alloc_nopanic(new_log_buf_len, 0);
> +		new_log_buf = memblock_virt_alloc_nopanic(new_log_buf_len,
> +							  LOG_ALIGN);
>  	}
>  
>  	if (unlikely(!new_log_buf)) {
> -- 
> 1.9.3
> 
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