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

[Ingo Molnar]

* Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> 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?

Yes, please use IS_ERR_VALUE(), using PAGE_MASK is way too obfuscated.

Thanks,

	Ingo