On Mon 14-01-19 16:29:29, Harrosh, Boaz wrote: > Kirill A. Shutemov <kir...@shutemov.name> 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 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) 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
