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

[Harrosh, Boaz]

 Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> On Fri, Jan 11, 2019 at 03:28:37PM -0800, Mike Kravetz wrote:
>> Ok, I just wanted to ask the question.  I've seen application code doing
>> the 'mmap sufficiently large area' then unmap to get desired alignment
>> trick.  Was wondering if there was something we could do to help.
>
> Application may want to get aligned allocation for different reasons.
> It should be okay for userspace to ask for size + (alignment - PAGE_SIZE)
> and then round up the address to get the alignment. We basically do the
> same on kernel side.
>

This is what we do and will need to keep doing for old Kernels.
But it is a pity that those holes can not be reused for small maps, and most important
that we cannot have "mapping holes" around the mapping that catch memory
overruns

> For THP, I believe, kernel already does The Right Thing™ for most users.
> User still may want to get speific range as THP (to avoid false sharing or
> something).

I'm an OK Kernel programmer.  But I was not able to create a HugePage mapping
against /dev/shm/ in a reliable way. I think it only worked on Fedora 28/29
but not on any other distro/version. (MMAP_HUGE)

We run with our own compiled Kernel on various distros, THP is configured
in but mmap against /dev/shm/ never gives me Huge pages. Does it only
work with unanimous mmap ? (I think it is mount dependent which is not
in the application control)

Just a rant. One day I will figure this out. Meanwhile I do this ugly
user mode aligns the pointers, and try to sleep at night ...

> But still I believe userspace has all required tools to get it
> right.
>

I still wish that if I ask for an mmap size aligned on 2M that I would automatically
get a 2M pointer. I don't see how the system can benefit from having both ends
of the VMA cross Huge page boundary.

> --
> Kirill A. Shutemov

Thanks
Boaz