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
