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

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Date:   Fri, 22 Jan 2021 09:42:08 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.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

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 compiler will actually reduce the size of the routine by one instruction.
> 
Many thanks.