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

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Date:	Wed, 18 Jun 2014 13:45:34 -0700
From:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
To:	gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
	hpa@...ux.intel.com
Cc:	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>, Petr Mladek <pmladek@...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: [PATCH v8 1/4] printk: make dynamic kernel ring buffer alignment explicit

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).

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@...n.ch>
Cc: Stephen Warren <swarren@...dotorg.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.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
Acked-by: Petr Mladek <pmladek@...e.cz>
Tested-by: Petr Mladek <pmladek@...e.cz>
Signed-off-by: Luis R. Rodriguez <mcgrof@...e.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

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