From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 83F2FC169C4 for ; Mon, 11 Feb 2019 19:54:43 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 35B17218AD for ; Mon, 11 Feb 2019 19:54:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 35B17218AD Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id C90B68E014A; Mon, 11 Feb 2019 14:54:42 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id C401B8E0125; Mon, 11 Feb 2019 14:54:42 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B55FF8E014A; Mon, 11 Feb 2019 14:54:42 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from mail-qt1-f197.google.com (mail-qt1-f197.google.com [209.85.160.197]) by kanga.kvack.org (Postfix) with ESMTP id 8C9A78E0125 for ; Mon, 11 Feb 2019 14:54:42 -0500 (EST) Received: by mail-qt1-f197.google.com with SMTP id k5so159465qte.0 for ; Mon, 11 Feb 2019 11:54:42 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-original-authentication-results:x-gm-message-state:date:from:to :cc:subject:message-id:references:mime-version:content-disposition :in-reply-to; bh=3OctDhvHgItcpZp3HBjN3tE8ByKKVzxq76m9SutyXmc=; b=VgJII3xt9PWbaqgyKfK/IY3TCurHPZ30DvHpX3nZaOLzSXxY6FjjvVv1KhCiUR+tHy Tek8Xq6QKEBN+g/pua267SPEKQ37RPIsRU79qjBDD5jXYey2WUojJDaGEsskvRtlvbE4 my1ymCHOgX1sTZJ1LJb0xyKmViN9clLTUxEn6riilZXay/NLixG9r//grP0fdTBeRb9o 5PW0LBZC3s44vnzzaNnaQE4JSlb6yHY8bAdHdtdPJJqYkqTsLyqAW14ebCdBuE07QmEX uS/haZIUwpIAXBjKifh0U5cowCp54YFnZ3ATQeGusIkfAK7dXeVEQKWOcqaPLnMGgWW7 ce+w== X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of mst@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=mst@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com X-Gm-Message-State: AHQUAuaWLbsaDIKAfljmBdM5ImEQ8o7VXxt1QFpNj3FIBLG1If+EA9gX L5FkHPntrStkz03ne90KsYDGoL6U9NWg9bIorXF66/MM1xvsKhtABcEBM2qsYywJuGNT9+jxL3w 7qI96RhhiMp2XChMhuS4mwC9qcyfmXyNdFhna2+gJkRKf2CQlLJGRhSeZhLSf5xUGNg== X-Received: by 2002:ac8:962:: with SMTP id z31mr28003716qth.305.1549914882287; Mon, 11 Feb 2019 11:54:42 -0800 (PST) X-Google-Smtp-Source: AHgI3IYV1170AYMnl71l8pcV8lrM/OQd9fYXJwh961HPsQph31oIsxuO6NidMi1Uk7d5KpFyf0yu X-Received: by 2002:ac8:962:: with SMTP id z31mr28003683qth.305.1549914881586; Mon, 11 Feb 2019 11:54:41 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1549914881; cv=none; d=google.com; s=arc-20160816; b=wU2O6eCFMMrC/WNPQWw1WO4zQWeyJAmNbaOCAufOM1d7v9V2VTYG3QhyGKJR44UFv3 KlGS46ZNad1M7E0HvgmDjL2jZLnRePBeth7peuOUbf0cVG3hbpAh8s1+FD0Tq2hwnueS sAyUIaickmeL3CkBu9VEuh/K4MrSZTV1OTEKmBH2NiYrQoGwZ7Nc2ZAfT7dP6prlcvKQ bhwPyWSleampe4/SUzuc8zP0I/sV1mrH7Z8y4e8BPHBei9/xtbPmqC808uBznqbOOhqv oXwkJI3d05K7e8/jxqXdE9K9hgSs7L91O59e2ZyN/1na0GGuA9UzLTOg3rFQppCFJkLh SMCw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date; bh=3OctDhvHgItcpZp3HBjN3tE8ByKKVzxq76m9SutyXmc=; b=ZZX9z38oDODH8bFmelBlMiJFol3nCUfvOval06w9KX19duu+hyq+gdYP7AOCO7TmJM lvLCve6E9xyGcnMbs0h6XMHw22zVEdixwHFz7a4/ZGP4wppMNVszhb89Z6UGYNG5/Bry F9sEhNACJDQSmCA2J4gmESvmr/rfg3PvTjfqyQVEXyZCbavoWiakBYn6yAqcrr6yy5Lx 9g0kc5fGyIbVx0VCafvmitlWlF6hDaOionM2vbWJbyFy+xtQbE/FIPE/Y3cFyP2JtAi9 rGT1/OOlA9funD8yO7NGui+t+4Mmr3/Bmx2Kfh532FwUvZfnv+cFwthLNnI62vGggBdn b57w== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of mst@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=mst@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id l66si1748703qkc.26.2019.02.11.11.54.41 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Feb 2019 11:54:41 -0800 (PST) Received-SPF: pass (google.com: domain of mst@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of mst@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=mst@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 8F301432A2; Mon, 11 Feb 2019 19:54:40 +0000 (UTC) Received: from redhat.com (ovpn-120-40.rdu2.redhat.com [10.10.120.40]) by smtp.corp.redhat.com (Postfix) with SMTP id 592EB10013BB; Mon, 11 Feb 2019 19:54:31 +0000 (UTC) Date: Mon, 11 Feb 2019 14:54:30 -0500 From: "Michael S. Tsirkin" To: Alexander Duyck Cc: Alexander Duyck , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, rkrcmar@redhat.com, x86@kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, pbonzini@redhat.com, tglx@linutronix.de, akpm@linux-foundation.org Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints Message-ID: <20190211142902-mutt-send-email-mst@kernel.org> References: <20190204181118.12095.38300.stgit@localhost.localdomain> <20190204181552.12095.46287.stgit@localhost.localdomain> <20190209194437-mutt-send-email-mst@kernel.org> <869a170e9232ffbc8ddbcf3d15535e8c6daedbde.camel@linux.intel.com> <20190211122321-mutt-send-email-mst@kernel.org> <44d0848e62f6d5237b60d209265dbcdf58ade1b9.camel@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <44d0848e62f6d5237b60d209265dbcdf58ade1b9.camel@linux.intel.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Mon, 11 Feb 2019 19:54:40 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Feb 11, 2019 at 10:10:06AM -0800, Alexander Duyck wrote: > On Mon, 2019-02-11 at 12:36 -0500, Michael S. Tsirkin wrote: > > On Mon, Feb 11, 2019 at 08:31:34AM -0800, Alexander Duyck wrote: > > > On Sat, 2019-02-09 at 19:49 -0500, Michael S. Tsirkin wrote: > > > > On Mon, Feb 04, 2019 at 10:15:52AM -0800, Alexander Duyck wrote: > > > > > From: Alexander Duyck > > > > > > > > > > Add guest support for providing free memory hints to the KVM hypervisor for > > > > > freed pages huge TLB size or larger. I am restricting the size to > > > > > huge TLB order and larger because the hypercalls are too expensive to be > > > > > performing one per 4K page. > > > > > > > > Even 2M pages start to get expensive with a TB guest. > > > > > > Agreed. > > > > > > > Really it seems we want a virtio ring so we can pass a batch of these. > > > > E.g. 256 entries, 2M each - that's more like it. > > > > > > The only issue I see with doing that is that we then have to defer the > > > freeing. Doing that is going to introduce issues in the guest as we are > > > going to have pages going unused for some period of time while we wait > > > for the hint to complete, and we cannot just pull said pages back. I'm > > > not really a fan of the asynchronous nature of Nitesh's patches for > > > this reason. > > > > Well nothing prevents us from doing an extra exit to the hypervisor if > > we want. The asynchronous nature is there as an optimization > > to allow hypervisor to do its thing on a separate CPU. > > Why not proceed doing other things meanwhile? > > And if the reason is that we are short on memory, then > > maybe we should be less aggressive in hinting? > > > > E.g. if we just have 2 pages: > > > > hint page 1 > > page 1 hint processed? > > yes - proceed to page 2 > > no - wait for interrupt > > > > get interrupt that page 1 hint is processed > > hint page 2 > > > > > > If hypervisor happens to be running on same CPU it > > can process things synchronously and we never enter > > the no branch. > > > > Another concern I would have about processing this asynchronously is > that we have the potential for multiple guest CPUs to become > bottlenecked by a single host CPU. I am not sure if that is something > that would be desirable. Well with a hypercall per page the fix is to block VCPU completely which is also not for everyone. If you can't push a free page hint to host, then ideally you just won't. That's a nice property of hinting we have upstream right now. Host too busy - hinting is just skipped. > > > > > Using the huge TLB order became the obvious > > > > > choice for the order to use as it allows us to avoid fragmentation of higher > > > > > order memory on the host. > > > > > > > > > > I have limited the functionality so that it doesn't work when page > > > > > poisoning is enabled. I did this because a write to the page after doing an > > > > > MADV_DONTNEED would effectively negate the hint, so it would be wasting > > > > > cycles to do so. > > > > > > > > Again that's leaking host implementation detail into guest interface. > > > > > > > > We are giving guest page hints to host that makes sense, > > > > weird interactions with other features due to host > > > > implementation details should be handled by host. > > > > > > I don't view this as a host implementation detail, this is guest > > > feature making use of all pages for debugging. If we are placing poison > > > values in the page then I wouldn't consider them an unused page, it is > > > being actively used to store the poison value. > > > > Well I guess it's a valid point of view for a kernel hacker, but they are > > unused from application's point of view. > > However poisoning is transparent to users and most distro users > > are not aware of it going on. They just know that debug kernels > > are slower. > > User loading a debug kernel and immediately breaking overcommit > > is an unpleasant experience. > > How would that be any different then a user loading an older kernel > that doesn't have this feature and breaking overcommit as a result? Well old kernel does not have the feature so nothing to debug. When we have a new feature that goes away in the debug kernel, that's a big support problem since this leads to heisenbugs. > I still think it would be better if we left the poisoning enabled in > such a case and just displayed a warning message if nothing else that > hinting is disabled because of page poisoning. > > One other thought I had on this is that one side effect of page > poisoning is probably that KSM would be able to merge all of the poison > pages together into a single page since they are all set to the same > values. So even with the poisoned pages it would be possible to reduce > total memory overhead. Right. And BTW one thing that host can do is pass the hinted area to KSM for merging. That requires an alloc hook to free it though. Or we could add a per-VMA byte with the poison value and use that on host to populate pages on fault. > > > If we can achieve this > > > and free the page back to the host then even better, but until the > > > features can coexist we should not use the page hinting while page > > > poisoning is enabled. > > > > Existing hinting in balloon allows them to coexist so I think we > > need to set the bar just as high for any new variant. > > That is what I heard. I will have to look into this. It's not doing anything smart right now, just checks that poison == 0 and skips freeing if not. But it can be enhanced transparently to guests. > > > This is one of the reasons why I was opposed to just disabling page > > > poisoning when this feature was enabled in Nitesh's patches. If the > > > guest has page poisoning enabled it is doing something with the page. > > > It shouldn't be prevented from doing that because the host wants to > > > have the option to free the pages. > > > > I agree but I think the decision belongs on the host. I.e. > > hint the page but tell the host it needs to be careful > > about the poison value. It might also mean we > > need to make sure poisoning happens after the hinting, not before. > > The only issue with poisoning after instead of before is that the hint > is ignored and we end up triggering a page fault and zero as a result. > It might make more sense to have an architecture specific call that can > be paravirtualized to handle the case of poisoning the page for us if > we have the unused page hint enabled. Otherwise the write to the page > is a given to invalidate the hint. Sounds interesting. So the arch hook will first poison and then pass the page to the host? Or we can also ask the host to poison for us, problem is this forces host to either always write into page, or call MADV_DONTNEED, without it could do MADV_FREE. Maybe that is not a big issue. -- MST