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

[Michal Hocko]

On Mon 14-01-19 16:29:29, Harrosh, Boaz wrote:
>  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

What does prevent you from mapping a larger area and MAP_FIXED,
PROT_NONE over it to get the protection?
 
> > For THP, I believe, kernel already does The Right Thingâ 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)

Are you mixing hugetlb rather than THP?

> 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)

If you are talking about THP then you have to enable huge pages for the
mapping AFAIR.
-- 
Michal Hocko
SUSE Labs