[PATCH] mm/hotplug: remove unnecessary BUG_ON in __offline_pages()

[PATCH] mm/hotplug: remove unnecessary BUG_ON in __offline_pages()

[Xishi Qiu]

I think we can remove "BUG_ON(start_pfn >= end_pfn)" in __offline_pages(),
because in memory_block_action() "nr_pages = PAGES_PER_SECTION * sections_per_block" 
is always greater than 0.

memory_block_action()
	offline_pages()
		__offline_pages()
			BUG_ON(start_pfn >= end_pfn)

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
---
 mm/memory_hotplug.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index ca1dd3a..8e333f9 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1472,7 +1472,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
 	struct zone *zone;
 	struct memory_notify arg;
 
-	BUG_ON(start_pfn >= end_pfn);
 	/* at least, alignment against pageblock is necessary */
 	if (!IS_ALIGNED(start_pfn, pageblock_nr_pages))
 		return -EINVAL;
-- 
1.8.2.2

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Re: [PATCH] mm/hotplug: remove unnecessary BUG_ON in __offline_pages()

[Toshi Kani]

On Tue, 2013-07-30 at 14:49 +0800, Xishi Qiu wrote:
> I think we can remove "BUG_ON(start_pfn >= end_pfn)" in __offline_pages(),
> because in memory_block_action() "nr_pages = PAGES_PER_SECTION * sections_per_block" 
> is always greater than 0.

BUG_ON() is used for checking a condition that should never happen,
unless there is a bug.  So, to me, what you described seems to match
with the use of BUG_ON() to prevent a potential bug in the caller.

Thanks,
-Toshi


> memory_block_action()
> 	offline_pages()
> 		__offline_pages()
> 			BUG_ON(start_pfn >= end_pfn)
> 
> Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
> ---
>  mm/memory_hotplug.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index ca1dd3a..8e333f9 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1472,7 +1472,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  	struct zone *zone;
>  	struct memory_notify arg;
>  
> -	BUG_ON(start_pfn >= end_pfn);
>  	/* at least, alignment against pageblock is necessary */
>  	if (!IS_ALIGNED(start_pfn, pageblock_nr_pages))
>  		return -EINVAL;


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Re: [PATCH] mm/hotplug: remove unnecessary BUG_ON in __offline_pages()

[Dave Hansen]

On 07/29/2013 11:49 PM, Xishi Qiu wrote:
> I think we can remove "BUG_ON(start_pfn >= end_pfn)" in __offline_pages(),
> because in memory_block_action() "nr_pages = PAGES_PER_SECTION * sections_per_block" 
> is always greater than 0.
...
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1472,7 +1472,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  	struct zone *zone;
>  	struct memory_notify arg;
>  
> -	BUG_ON(start_pfn >= end_pfn);
>  	/* at least, alignment against pageblock is necessary */
>  	if (!IS_ALIGNED(start_pfn, pageblock_nr_pages))
>  		return -EINVAL;

I think you're saying that you don't see a way to hit this BUG_ON() in
practice.  That does appear to be true, unless sections_per_block ended
up 0 or negative.  The odds of getting in to this code if
'sections_per_block' was bogus are pretty small.

Or, is this a theoretical thing that folks might run in to when adding
new features or developing?  It's in a cold path and the cost of the
check is miniscule.  The original author (cc'd) also saw a need to put
this in probably because he actually ran in to this.

In any case, it looks fairly safe to me:

Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

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Re: [PATCH] mm/hotplug: remove unnecessary BUG_ON in __offline_pages()

[Xishi Qiu]

On 2013/8/1 0:55, Dave Hansen wrote:

> On 07/29/2013 11:49 PM, Xishi Qiu wrote:
>> I think we can remove "BUG_ON(start_pfn >= end_pfn)" in __offline_pages(),
>> because in memory_block_action() "nr_pages = PAGES_PER_SECTION * sections_per_block" 
>> is always greater than 0.
> ...
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1472,7 +1472,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  	struct zone *zone;
>>  	struct memory_notify arg;
>>  
>> -	BUG_ON(start_pfn >= end_pfn);
>>  	/* at least, alignment against pageblock is necessary */
>>  	if (!IS_ALIGNED(start_pfn, pageblock_nr_pages))
>>  		return -EINVAL;
> 
> I think you're saying that you don't see a way to hit this BUG_ON() in
> practice.  That does appear to be true, unless sections_per_block ended
> up 0 or negative.  The odds of getting in to this code if
> 'sections_per_block' was bogus are pretty small.
> 

Yes, I find there is an only to hit this BUG_ON() in v3.11, and "sections_per_block"
seems to be always greater than 0.

> Or, is this a theoretical thing that folks might run in to when adding
> new features or developing?  It's in a cold path and the cost of the
> check is miniscule.  The original author (cc'd) also saw a need to put
> this in probably because he actually ran in to this.
> 

In v2.6.32, If info->length==0, this way may hit this BUG_ON().
acpi_memory_disable_device()
	remove_memory(info->start_addr, info->length)
		offline_pages()

Later Fujitsu's patch rename this function and the BUG_ON() is unnecessary.

Thanks,
Xishi Qiu

> In any case, it looks fairly safe to me:
> 
> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
> .
> 


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