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Date: Wed, 18 Jun 2014 17:40:23 +0200
From: Petr Mládek <pmladek@...e.cz>
To: "Luis R. Rodriguez" <mcgrof@...not-panic.com>
Cc: hpa@...ux.intel.com, akpm@...ux-foundation.org,
linux-kernel@...r.kernel.org,
"Luis R. Rodriguez" <mcgrof@...e.com>,
Andrew Lunn <andrew@...n.ch>,
Stephen Warren <swarren@...dotorg.org>,
Michal Hocko <mhocko@...e.cz>, Joe Perches <joe@...ches.com>,
Arun KS <arunks.linux@...il.com>,
Kees Cook <keescook@...omium.org>,
Davidlohr Bueso <davidlohr@...com>,
Chris Metcalf <cmetcalf@...era.com>
Subject: Re: [RFT 1/2] printk: make dynamic kernel ring buffer alignemnt
explicit
On Wed 2014-06-18 04:14:24, Luis R. Rodriguez wrote:
> From: "Luis R. Rodriguez" <mcgrof@...e.com>
>
> 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@...n.ch>
> Cc: Stephen Warren <swarren@...dotorg.org>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Petr Mladek <pmladek@...e.cz>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Joe Perches <joe@...ches.com>
> Cc: Arun KS <arunks.linux@...il.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Davidlohr Bueso <davidlohr@...com>
> Cc: Chris Metcalf <cmetcalf@...era.com>
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Luis R. Rodriguez <mcgrof@...e.com>
The change makes sense to me.
Acked-by: Petr Mladek <pmladek@...e.cz>
Tested-by: Petr Mladek <pmladek@...e.cz>
> 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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