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

[Mike Kravetz]

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.

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.

> Personally I would like to see a new MAP_ALIGN flag and treat the addr
> argument as the alignment (like Solaris), but I am told that adding flags
> is problematic because old kernels accept undefined flag bits from userland
> without complaint, so their behavior would change.

Well, a flag would clearly define desired behavior.

As others have been mentioned, there are mechanisms in place that allow user
space code to get the alignment it wants.  However, it is at the expense of
an additional system call or two.  Perhaps the question is, "Is it worth
defining new behavior to eliminate this overhead?".

One other thing to consider is that at mmap time, we likely do not know if
the vma will/can use THP.  We would know if system wide THP configuration
is set to never or always.  However, I 'think' the default for most distros
is madvize.  Therefore, it is not until a subsequent madvise call that we
know THP will be employed.  If the application code will need to make this
separate madvise call, then perhaps it is not too much to expect that it
take explicit action to optimally align the mapping.

-- 
Mike Kravetz