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

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Date:   Tue, 14 Jul 2020 12:02:38 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc:     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 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.