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

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Message-ID: <694bb1ac-4f9c-a6a5-7c87-1fc0cdd948a6@oracle.com>
Date:   Tue, 14 Jul 2020 14:12:03 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc:     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 7/14/20 3:02 AM, Vlastimil Babka wrote:
> On 7/14/20 11:57 AM, Wei Yang wrote:
>> 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
> 
> Yes, by "we should switch to C99" I meant that the kernel kbuild system would
> need to switch. Not a trivial change...
> Without that, I don't see how your patch is possible to do safely.

Vlastimil, thanks for pointing out future potential issues with this patch.
I likely would have missed that.

Wei, thanks for taking the time to put together the patch.  However, I tend
to agree with Vlastimil's assesment.  The cleanup is not worth the risk of
running into issues if someone uses multiple instances of the macro.
-- 
Mike Kravetz