From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qg0-f70.google.com (mail-qg0-f70.google.com [209.85.192.70]) by kanga.kvack.org (Postfix) with ESMTP id DC9366B007E for ; Fri, 15 Apr 2016 09:42:39 -0400 (EDT) Received: by mail-qg0-f70.google.com with SMTP id b32so135484164qgf.0 for ; Fri, 15 Apr 2016 06:42:39 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id c76si12717128qkj.22.2016.04.15.06.42.38 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 15 Apr 2016 06:42:39 -0700 (PDT) Date: Fri, 15 Apr 2016 14:42:33 +0100 From: "Dr. David Alan Gilbert" Subject: Re: post-copy is broken? Message-ID: <20160415134233.GG2229@work-vm> References: <20160412175501.GB6415@work-vm> <20160413080545.GA2270@work-vm> <20160413114103.GB2270@work-vm> <20160413125053.GC2270@work-vm> <20160413205132.GG26364@redhat.com> <20160414123441.GF2252@work-vm> <20160414162230.GC9976@redhat.com> <20160415125236.GA3376@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160415125236.GA3376@node.shutemov.name> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Andrea Arcangeli , kirill.shutemov@linux.intel.com, "Li, Liang Z" , Amit Shah , "qemu-devel@nongnu.org" , "quintela@redhat.com" , linux-mm@kvack.org * Kirill A. Shutemov (kirill@shutemov.name) wrote: > On Thu, Apr 14, 2016 at 12:22:30PM -0400, Andrea Arcangeli wrote: > > Adding linux-mm too, > > > > On Thu, Apr 14, 2016 at 01:34:41PM +0100, Dr. David Alan Gilbert wrote: > > > * Andrea Arcangeli (aarcange@redhat.com) wrote: > > > > > > > The next suspect is the massive THP refcounting change that went > > > > upstream recently: > > > > > > > As further debug hint, can you try to disable THP and see if that > > > > makes the problem go away? > > > > > > Yep, this seems to be the problem (cc'ing in Kirill). > > > > > > 122afea9626ab3f717b250a8dd3d5ebf57cdb56c - works (just before Kirill disables THP) > > > 61f5d698cc97600e813ca5cf8e449b1ea1c11492 - breaks (when THP is reenabled) > > > > > > It's pretty reliable; as you say disabling THP makes it work again > > > and putting it back to THP/madvise mode makes it break. And you need > > > to test on a machine with some free ram to make sure THP has a chance > > > to have happened. > > > > > > I'm not sure of all of the rework that happened in that series, > > > but my reading of it is that splitting of THP pages gets deferred; > > > so I wonder if when I do the madvise to turn THP off, if it's actually > > > still got THP pages and thus we end up with a whole THP mapped > > > when I'm expecting to be userfaulting those pages. > > > > Good thing at least I didn't make UFFDIO_COPY THP aware yet so there's > > less variables (as no user was interested to handle userfaults at THP > > granularity yet, and from userland such an improvement would be > > completely invisible in terms of API, so if an user starts doing that > > we can just optimize the kernel for it, criu restore could do that as > > the faults will come from disk-I/O, when network is involved THP > > userfaults wouldn't have a great tradeoff with regard to the increased > > fault latency). > > > > I suspect there is an handle_userfault missing somewhere in connection > > with trans_huge_pmd splits (not anymore THP splits) that you're doing > > with MADV_DONTNEED to zap those pages in the destination that got > > redirtied in source during the last precopy stage. Or more simply > > MADV_DONTNEED isn't zapping all the right ptes after the trans huge > > pmd got splitted. > > > > The fact the page isn't splitted shouldn't matter too much, all we care > > about is the pte triggers handle_userfault after MADV_DONTNEED. > > > > The userfaultfd testcase in the kernel isn't exercising this case > > unfortunately, that should probably be improved too, so there is a > > simpler way to reproduce than running precopy before postcopy in qemu. > > I've tested current Linus' tree and v4.5 using qemu postcopy test case for > both x86-64 and i386 and it never failed for me: > > /x86_64/postcopy: first_byte = 7e last_byte = 7d hit_edge = 1 OK > OK > /i386/postcopy: first_byte = f6 last_byte = f5 hit_edge = 1 OK > OK > > I've run it directly, setting relevant QTEST_QEMU_BINARY. Interesting; it's failing reliably for me - but only with a reasonably freshly booted machine (so that the pages get THPd). Dave > > -- > Kirill A. Shutemov -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org