On Thu, Oct 11, 2012 at 01:22:55PM +0100, Mel Gorman wrote: > On Thu, Oct 04, 2012 at 01:50:48AM +0200, Andrea Arcangeli wrote: > > In the special "pmd" mode of knuma_scand > > (/sys/kernel/mm/autonuma/knuma_scand/pmd == 1), the pmd may be of numa > > type (_PAGE_PRESENT not set), however the pte might be > > present. Therefore, gup_pmd_range() must return 0 in this case to > > avoid losing a NUMA hinting page fault during gup_fast. > > > > So if gup_fast fails, presumably we fall back to taking the mmap_sem and > calling get_user_pages(). This is a heavier operation and I wonder if the > cost is justified. i.e. Is the performance loss from using get_user_pages() > offset by improved NUMA placement? I ask because we always incur the cost of > taking mmap_sem but only sometimes get it back from improved NUMA placement. > How bad would it be if gup_fast lost some of the NUMA hinting information? Good question indeed. Now, I agree it wouldn't be bad to skip NUMA hinting page faults in gup_fast for no-virt usage like O_DIRECT/ptrace, but the only problem is that we'd lose AutoNUMA on the memory touched by the KVM vcpus. I've been also asked if the vhost-net kernel thread (KVM in kernel virtio backend) will be controlled by autonuma in between use_mm/unuse_mm and answer is yes, but to do that, it also needs this. (see also the flush to task_autonuma_nid and mm/task statistics in unuse_mm to reset it back to regular kernel thread status, uncontrolled by autonuma) $ git grep get_user_pages tcm_vhost.c: ret = get_user_pages_fast((unsigned long)ptr, 1, write, &page); vhost.c: r = get_user_pages_fast(log, 1, 1, &page); -- 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>