linux-kernel - Re: [PATCH] mm/zswap.c: add BUG() for default case in zswap_writeback

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Date:	Mon, 09 Dec 2013 18:11:06 +0800
From:	Chen Gang <gang.chen.5i5j@...il.com>
To:	James Hogan <james.hogan@...tec.com>
CC:	Seth Jennings <sjenning@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/zswap.c: add BUG() for default case in zswap_writeback_entry()

On 12/09/2013 05:54 PM, James Hogan wrote:
> On 09/12/13 02:51, Chen Gang wrote:
>> Recommend to add default case to avoid compiler's warning, although at
>> present, the original implementation is still correct.
>>
>> The related warning (with allmodconfig for metag):
>>
>>     CC      mm/zswap.o
>>   mm/zswap.c: In function 'zswap_writeback_entry':
>>   mm/zswap.c:537: warning: 'ret' may be used uninitialized in this function
>>
>>
>> Signed-off-by: Chen Gang <gang.chen.5i5j@...il.com>
>> ---
>>  mm/zswap.c |    2 ++
>>  1 files changed, 2 insertions(+), 0 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 5a63f78..bfd1807 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -585,6 +585,8 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
>>  
>>  		/* page is up to date */
>>  		SetPageUptodate(page);
>> +	default:
>> +		BUG();
> 
> This doesn't hide the warning when CONFIG_BUG=n since BUG() optimises
> out completely.
> 

When "CONFIG_BUG=n", it will report many related warnings (for me,
CONFIG_BUG need be as architecture specific config feature, not a
generic config feature -- most architectures always enable it).

So for common generic users, we can assume it will always have effect.


> Since the metag compiler is stuck on an old version (gcc 4.2.4), which
> is wrong to warn in this case, and newer versions of gcc don't appear to
> warn about it anyway (I just checked with gcc 4.7.2 x86_64), I have no
> objection to this warning remaining in the metag build.
> 

Do you try "EXTRA_CFLAGS=-W" with gcc 4.7.2? I guess it will report the
warning too, I don't feel the compiler is smart enough (except it lets
the long function zswap_get_swap_cache_page really inline)  :-)


Thanks.
-- 
Chen Gang

Open, share, and attitude like air, water and life which God blessed
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