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

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Date:   Thu, 21 Jan 2021 21:02:09 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlbfs: make hugepage size conversion more readable

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@...wei.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.

-- 
Mike Kravetz