Re: [RFC PATCH 2/2] x86/hugetlb: Attempt PUD_SIZE mapping alignment if PMD sharing enabled

[Mike Kravetz]

On 03/30/2016 07:26 PM, Naoya Horiguchi wrote:
> On Tue, Mar 29, 2016 at 10:05:31AM -0700, Mike Kravetz wrote:
>> On 03/29/2016 01:35 AM, Ingo Molnar wrote:
>>>
>>> * Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>>
>>>> When creating a hugetlb mapping, attempt PUD_SIZE alignment if the
>>>> following conditions are met:
>>>> - Address passed to mmap or shmat is NULL
>>>> - The mapping is flaged as shared
>>>> - The mapping is at least PUD_SIZE in length
>>>> If a PUD_SIZE aligned mapping can not be created, then fall back to a
>>>> huge page size mapping.
>>>>
>>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>>> ---
>>>>  arch/x86/mm/hugetlbpage.c | 64 ++++++++++++++++++++++++++++++++++++++++++++---
>>>>  1 file changed, 61 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
>>>> index 42982b2..4f53af5 100644
>>>> --- a/arch/x86/mm/hugetlbpage.c
>>>> +++ b/arch/x86/mm/hugetlbpage.c
>>>> @@ -78,14 +78,39 @@ static unsigned long hugetlb_get_unmapped_area_bottomup(struct file *file,
>>>>  {
>>>>  	struct hstate *h = hstate_file(file);
>>>>  	struct vm_unmapped_area_info info;
>>>> +	bool pud_size_align = false;
>>>> +	unsigned long ret_addr;
>>>> +
>>>> +	/*
>>>> +	 * If PMD sharing is enabled, align to PUD_SIZE to facilitate
>>>> +	 * sharing.  Only attempt alignment if no address was passed in,
>>>> +	 * flags indicate sharing and size is big enough.
>>>> +	 */
>>>> +	if (IS_ENABLED(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) &&
>>>> +	    !addr && flags & MAP_SHARED && len >= PUD_SIZE)
>>>> +		pud_size_align = true;
>>>>  
>>>>  	info.flags = 0;
>>>>  	info.length = len;
>>>>  	info.low_limit = current->mm->mmap_legacy_base;
>>>>  	info.high_limit = TASK_SIZE;
>>>> -	info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>>>> +	if (pud_size_align)
>>>> +		info.align_mask = PAGE_MASK & (PUD_SIZE - 1);
>>>> +	else
>>>> +		info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>>>>  	info.align_offset = 0;
>>>> -	return vm_unmapped_area(&info);
>>>> +	ret_addr = vm_unmapped_area(&info);
>>>> +
>>>> +	/*
>>>> +	 * If failed with PUD_SIZE alignment, try again with huge page
>>>> +	 * size alignment.
>>>> +	 */
>>>> +	if ((ret_addr & ~PAGE_MASK) && pud_size_align) {
>>>> +		info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>>>> +		ret_addr = vm_unmapped_area(&info);
>>>> +	}
>>>
>>> So AFAICS 'ret_addr' is either page aligned, or is an error code. Wouldn't it be a 
>>> lot easier to read to say:
>>>
>>> 	if ((long)ret_addr > 0 && pud_size_align) {
>>> 		info.align_mask = PAGE_MASK & ~huge_page_mask(h);
>>> 		ret_addr = vm_unmapped_area(&info);
>>> 	}
>>>
>>> 	return ret_addr;
>>>
>>> to make it clear that it's about error handling, not some alignment 
>>> requirement/restriction?
>>
>> Yes, I agree that is easier to read.  However, it assumes that process
>> virtual addresses can never evaluate to a negative long value.  This may
>> be the case for x86_64 today.  But, there are other architectures where
>> this is not the case.  I know this is x86 specific code, but might it be
>> possible that x86 virtual addresses could be negative longs in the future?
>>
>> It appears that all callers of vm_unmapped_area() are using the page aligned
>> check to determine error.   I would prefer to do the same, and can add
>> comments to make that more clear.
> 
> IS_ERR_VALUE() might be helpful?
> 

Thanks Naoya,  I'll change all this to use IS_ERR_VALUE().

-- 
Mike Kravetz