linux-kernel - Re: [PATCH] x86: remove wrong -1 in calling init_memory

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Date:	Tue, 28 Oct 2008 09:42:55 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shaohua Li <shaohua.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Yasunori Goto <y-goto@...fujitsu.com>
Subject: Re: [PATCH] x86: remove wrong -1 in calling init_memory_mapping


* Yinghai Lu <yinghai@...nel.org> wrote:

> From: Shaohua Li <shaohua.li@...el.com>
> 
> impact: make memory hot plug got last page mapped.
> 
> Shuahua Li found:
> Round up address to a page, otherwise the last page isn't mapped.
> 
> No, I just did some experiments on a desktop for memory hotplug and this bug
> triggered a crash in my test.
> Yinghai's suggestion also fixed the bug. I just want to have safer method. Anyway, either approach is ok to me.
> 
> So acctually we don't need to round it.
> just remove that extra -1
> 
> Signed-off-by: Yinghai <yinghai@...nel.org>

applied to tip/x86/urgent, thanks guys!

Note, i changed the "Impact:" line. We try to use it to provide a 
single-line description of what practical impact a patch has. For 
example if it's a bugfix, we try to describe what type of bug effect 
it fixes. If it's a cleanup, we mark it as a cleanup. If it introduces 
a new feature or changes existing behavior to improve the code, we 
describe that.

There's no hard rules, so here are a few common examples:

 Impact: fix crash with memory hotplug
 Impact: cleanup
 Impact: fix bootup crash on certain types of hardware
 Impact: improve printk output
 Impact: add (default-off) debug feature
 Impact: make the implementation faster
 Impact: cleanup (no object code changed)
 Impact: change the implementation to be more robust
 Impact: fix bootup hang on certain systems
 Impact: add new system call
 Impact: enable hardware feature and make use of it
 Impact: fix rare crash under high DB load

etc. etc.

"Wrong" impact lines are the ones which just describe the change 
itself, not the general impact:

 Impact: add +1 to condition
 Impact: change foo() to call bar() as well
 Impact: flush the TLB when tearing down a mapping
 Impact: check for NULL

these are less useful because they just duplicate the information 
already available in the patch itself.

	Ingo

----------------->
>From 60817c9b31ef7897d60bca2f384cbc316a3fdd8b Mon Sep 17 00:00:00 2001
From: Shaohua Li <shaohua.li@...el.com>
Date: Mon, 27 Oct 2008 13:03:18 -0700
Subject: [PATCH] x86, memory hotplug: remove wrong -1 in calling init_memory_mapping()

Impact: fix crash with memory hotplug

Shuahua Li found:

| I just did some experiments on a desktop for memory hotplug and this bug
| triggered a crash in my test.
|
| Yinghai's suggestion also fixed the bug.

We don't need to round it, just remove that extra -1

Signed-off-by: Yinghai <yinghai@...nel.org>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 arch/x86/mm/init_64.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index c7a4c5a..f79a02f 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -837,7 +837,7 @@ int arch_add_memory(int nid, u64 start, u64 size)
 	unsigned long nr_pages = size >> PAGE_SHIFT;
 	int ret;
 
-	last_mapped_pfn = init_memory_mapping(start, start + size-1);
+	last_mapped_pfn = init_memory_mapping(start, start + size);
 	if (last_mapped_pfn > max_pfn_mapped)
 		max_pfn_mapped = last_mapped_pfn;
 
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