On Fri, 20 Dec 2013 10:58:10 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote: > On Thu, Dec 19, 2013 at 05:02:02PM -0800, Andrew Morton wrote: > > On Wed, 18 Dec 2013 15:53:59 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote: > > > > > If parallel fault occur, we can fail to allocate a hugepage, > > > because many threads dequeue a hugepage to handle a fault of same address. > > > This makes reserved pool shortage just for a little while and this cause > > > faulting thread who can get hugepages to get a SIGBUS signal. > > > > > > > So if I'm understanding this correctly... if N threads all generate a > > fault against the same address, they will all dive in and allocate a > > hugepage, will then do an enormous memcpy into that page and will then > > attempt to instantiate the page in pagetables. All threads except one > > will lose the race and will free the page again! This sounds terribly > > inefficient; it would be useful to write a microbenchmark which > > triggers this scenario so we can explore the impact. > > Yes, you understand correctly, I think. > > I have an idea to prevent this overhead. It is that marking page when it > is zeroed and unmarking when it is mapped to page table. If page mapping > is failed due to current thread, the zeroed page will keep the marker and > later we can determine if it is zeroed or not. Well OK, but the other threads will need to test that in-progress flag and then do <something>. Where <something> will involve some form of open-coded sleep/wakeup thing. To avoid all that wheel-reinventing we can avoid using an internal flag and use an external flag instead. There's one in struct mutex! I doubt if the additional complexity of the external flag is worth it, but convincing performance testing results would sway me ;) Please have a think about it all. > If you want to include this functionality in this series, I can do it ;) > Please let me know your decision. > > > I'm wondering if a better solution to all of this would be to make > > hugetlb_instantiation_mutex an array of, say, 1024 mutexes and index it > > with a hash of the faulting address. That will 99.9% solve the > > performance issue which you believe exists without introducing this new > > performance issue? > > Yes, that approach would solve the performance issue. > IIRC, you already suggested this idea roughly 6 months ago and it is > implemented by Davidlohr. I remembered that there is a race issue on > COW case with this approach. See following link for more information. > https://lkml.org/lkml/2013/8/7/142 That seems to be unrelated to hugetlb_instantiation_mutex? > And we need 1-3 patches to prevent other theorectical race issue > regardless any approaches. Yes, I'll be going through patches 1-12 very soon, thanks. And to reiterate: I'm very uncomfortable mucking around with performance patches when we have run no tests to measure their magnitude, or even whether they are beneficial at all! -- 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>