Re: [RFC PATCH] mm: align anon mmap for THP

[Kirill A. Shutemov]

On Mon, Jan 14, 2019 at 10:54:45AM -0800, Mike Kravetz wrote:
> On 1/14/19 7:35 AM, Steven Sistare wrote:
> > On 1/11/2019 6:28 PM, Mike Kravetz wrote:
> >> On 1/11/19 1:55 PM, Kirill A. Shutemov wrote:
> >>> On Fri, Jan 11, 2019 at 08:10:03PM +0000, Mike Kravetz wrote:
> >>>> At LPC last year, Boaz Harrosh asked why he had to 'jump through hoops'
> >>>> to get an address returned by mmap() suitably aligned for THP.  It seems
> >>>> that if mmap is asking for a mapping length greater than huge page
> >>>> size, it should align the returned address to huge page size.
> > 
> > A better heuristic would be to return an aligned address if the length
> > is a multiple of the huge page size.  The gap (if any) between the end of
> > the previous VMA and the start of this VMA would be filled by subsequent
> > smaller mmap requests.  The new behavior would need to become part of the
> > mmap interface definition so apps can rely on it and omit their hoop-jumping
> > code.
> 
> Yes, the heuristic really should be 'length is a multiple of the huge page
> size'.  As you mention, this would still leave gaps.  I need to look closer
> but this may not be any worse than the trick of mapping an area with rounded
> up length and then unmapping pages at the beginning.

The question why is it any better. Virtual address space is generally
cheap, additional VMA maybe more signficiant due to find_vma() overhead.

And you don't *need* to unmap anything. Just use alinged pointer.

> 
> When I sent this out, the thought in the back of my mind was that this doesn't
> really matter unless there is some type of alignment guarantee.  Otherwise,
> user space code needs continue employing their code to check/force alignment.
> Making matters somewhat worse is that I do not believe there is C interface to
> query huge page size.  I thought there was discussion about adding one, but I
> can not find it.

We have posix_memalign(3).


-- 
 Kirill A. Shutemov