linux-kernel - Re: [PATCH] mm/hugetlb: hide nr_nodes in the internal of for_each_node_mask_to

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Message-ID: <20200714095713.GA86690@L-31X9LVDL-1304.local>
Date:   Tue, 14 Jul 2020 17:57:13 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        mike.kravetz@...cle.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: hide nr_nodes in the internal of
 for_each_node_mask_to_[alloc|free]

On Tue, Jul 14, 2020 at 11:22:03AM +0200, Vlastimil Babka wrote:
>On 7/14/20 11:13 AM, Vlastimil Babka wrote:
>> On 7/14/20 9:34 AM, Wei Yang wrote:
>>> The second parameter of for_each_node_mask_to_[alloc|free] is a loop
>>> variant, which is not used outside of loop iteration.
>>> 
>>> Let's hide this.
>>> 
>>> Signed-off-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
>>> ---
>>>  mm/hugetlb.c | 38 ++++++++++++++++++++------------------
>>>  1 file changed, 20 insertions(+), 18 deletions(-)
>>> 
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 57ece74e3aae..9c3d15fb317e 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -1196,17 +1196,19 @@ static int hstate_next_node_to_free(struct hstate *h, nodemask_t *nodes_allowed)
>>>  	return nid;
>>>  }
>>>  
>>> -#define for_each_node_mask_to_alloc(hs, nr_nodes, node, mask)		\
>>> -	for (nr_nodes = nodes_weight(*mask);				\
>>> -		nr_nodes > 0 &&						\
>>> +#define for_each_node_mask_to_alloc(hs, node, mask)			\
>>> +	int __nr_nodes;							\
>>> +	for (__nr_nodes = nodes_weight(*mask);				\
>> 
>> The problem with this is that if I use the macro twice in the same block, this
>> will redefine __nr_nodes and fail to compile, no?
>> In that case it's better to avoid setting up this trap, IMHO.
>
>Ah, and it will also generate the following warning, if the use of for_each*
>macro is not the first thing after variable declarations, but there's another
>statement before:
>
>warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
>
>Instead we should switch to C99 and declare it as "for (int __nr_nodes" :P

Hmm... I tried what you suggested, but compiler complains.

'for' loop initial declarations are only allowed in C99 or C11 mode

-- 
Wei Yang
Help you, Help me