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@xxxxxxxxxx>
>> ---
>> 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.
