Re: [PATCH] hugetlbfs: make hugepage size conversion more readable

[Miaohe Lin <linmiaohe@huawei.com>]

Hi:
On 2021/1/22 13:02, Mike Kravetz wrote:
> On 1/21/21 5:42 PM, Miaohe Lin wrote:
>> Hi:
>> On 2021/1/22 3:00, Mike Kravetz wrote:
>>> On 1/20/21 1:23 AM, Miaohe Lin wrote:
>>>> The calculation 1U << (h->order + PAGE_SHIFT - 10) is actually equal to
>>>> (PAGE_SHIFT << (h->order)) >> 10. So we can make it more readable by
>>>> replace it with huge_page_size(h) / SZ_1K.
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>> ---
>>>>  fs/hugetlbfs/inode.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>>>> index 25c1857ff45d..f94b8f6553fa 100644
>>>> --- a/fs/hugetlbfs/inode.c
>>>> +++ b/fs/hugetlbfs/inode.c
>>>> @@ -1519,8 +1519,8 @@ static struct vfsmount *__init mount_one_hugetlbfs(struct hstate *h)
>>>>  		put_fs_context(fc);
>>>>  	}
>>>>  	if (IS_ERR(mnt))
>>>> -		pr_err("Cannot mount internal hugetlbfs for page size %uK",
>>>> -		       1U << (h->order + PAGE_SHIFT - 10));
>>>> +		pr_err("Cannot mount internal hugetlbfs for page size %luK",
>>>> +		       huge_page_size(h) / SZ_1K);
>>>
>>> I appreciate the effort to make the code more readable.  The existing
>>> calculation does take a minute to understand.  However, it is correct and
>>> anyone modifying the code should be able to understand.
>>>
>>> With my compiler, your proposed change adds an additional instruction to
>>> the routine mount_one_hugetlbfs.  I know this is not significant, but still
>>
>> I thought compiler would generate the same code...
>>
>>> it does increase the kernel size for a change that is of questionable value.
>>>
>>> In the kernel, size in KB is often calculated as (size << (PAGE_SHIFT - 10)).
>>> If you change the calculation in the hugetlb code to be:
>>>> 			huge_page_size(h) << (PAGE_SHIFT - 10)
>>
>> I'am sorry but this looks not really correct. I think the calculation shoud be
>> huge_page_size(h) >> 10. What do you think?
> 
> My bad!  I was looking at code that converts page counts to KB.  Sorry.
> 
> Yes, huge_page_size(h) >> 10 is correct.
> 

So I will send v2 with this change. Many thanks.