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

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

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@oracle.com> 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@oracle.com>
> >> ---
> >>  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?
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