[PATCH v8 1/4] printk: make dynamic kernel ring buffer alignment explicit

["Luis R. Rodriguez" <mcgrof@do-not-panic.com>]

From: "Luis R. Rodriguez" <mcgrof@suse.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).

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. This does that to be safe and make dynamic allocation
alignment explicit.

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Petr Mladek <pmladek@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Joe Perches <joe@perches.com>
Cc: Arun KS <arunks.linux@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: linux-kernel@vger.kernel.org
Acked-by: Petr Mladek <pmladek@suse.cz>
Tested-by: Petr Mladek <pmladek@suse.cz>
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
---
 kernel/printk/printk.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ea2d5f6..78ceccf 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -853,9 +853,10 @@ 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