On Thu, 2016-04-07 at 13:41 -0400, Matthew Wilcox wrote: > On Wed, Apr 06, 2016 at 11:44:32AM -0600, Toshi Kani wrote: > > > > > > The NVML chooses appropriate addresses and gets a properly aligned > > > address without any kernel code. > > > > An application like NVML can continue to specify a specific address to > > mmap(). Most existing applications, however, do not specify an address > > to mmap(). With this patch, specifying an address will remain > > optional. > > The point is that this *can* be done in userspace. You need to sell us > on the advantages of doing it in the kernel. Sure. As I said, the point is that we do not need to modify existing applications for using DAX PMD mappings. For instance, fio with "ioengine=mmap" performs I/Os with mmap(). https://github.com/caius/fio/blob/master/engines/mmap.c With this change, unmodified fio can be used for testing with DAX PMD mappings. There are many examples like this, and I do not think we want to modify all applications that we want to evaluate/test with. > > > I think this is the wrong place for it, if we decide that this is the > > > right thing to do. The filesystem has a get_unmapped_area() which > > > should be used instead. > > > > Yes, I considered adding a filesystem entry point, but decided going > > this way because: > > - arch_get_unmapped_area() and arch_get_unmapped_area_topdown() are > > arch-specific code. Therefore, this filesystem entry point will need > > arch-specific implementation. > > - There is nothing filesystem specific about requesting PMD alignment. > > See http://article.gmane.org/gmane.linux.kernel.mm/149227 for Hugh's > approach for shmem. I strongly believe that if we're going to do this > i the kernel, we should build on this approach, and not hack something > into each architecture's generic get_unmapped_area. Thanks for the pointer. Yes, we can call current->mm->get_unmapped_area() with size + PMD_SIZE, and adjust with the alignment in a filesystem entry point. I will update the patch with this approach. -Toshi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>