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[209.132.183.28]) by mx.google.com with ESMTPS id g124si5368706qkc.76.2019.02.11.14.52.34 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Feb 2019 14:52:34 -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-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 20ADA80F83; Mon, 11 Feb 2019 22:52:34 +0000 (UTC) Received: from redhat.com (ovpn-121-111.rdu2.redhat.com [10.10.121.111]) by smtp.corp.redhat.com (Postfix) with SMTP id D898D17CDD; Mon, 11 Feb 2019 22:52:30 +0000 (UTC) Date: Mon, 11 Feb 2019 17:52:30 -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: <20190211174256-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> <20190211142902-mutt-send-email-mst@kernel.org> <770615ef2db838775fb68130ca60711c6e593f3d.camel@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <770615ef2db838775fb68130ca60711c6e593f3d.camel@linux.intel.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Mon, 11 Feb 2019 22:52:34 +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 01:00:53PM -0800, Alexander Duyck wrote: > On Mon, 2019-02-11 at 14:54 -0500, Michael S. Tsirkin wrote: > > 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. > > Right, but if you do that then there is a potential to end up missing > hints for a large portion of memory. It seems like you would end up > with even bigger issues since then at that point you have essentially > leaked memory. > I would think you would need a way to resync the host and the guest > after something like that. Otherwise you can have memory that will just > go unused for an extended period if a guest just goes idle. Yes and that is my point. Existing hints code will just take a page off the free list in that case so it resyncs using the free list. Something like this could work then: mark up hinted pages with a flag (its easy to find unused flags for free pages) then when you get an interrupt because outstanding hints have been consumed, get unflagged/unhinted pages from buddy and pass them to host. > > > > > > > > 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. > > Trying to debug host features from the guest would be a pain anyway as > a guest shouldn't even really know what the underlying setup of the > guest is supposed to be. I'm talking about debugging the guest though. > > > 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. > > Okay, so it probably should be extended to add something like poison > page that could replace the zero page for reads to a page that has been > unmapped. > > > > > > 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. > > I would think we would ask the host to poison for us. If I am not > mistaken both solutions right now are using MADV_DONTNEED. I would tend > to lean that way if we are doing page poisoning since the cost for > zeroing/poisoning the page on the host could be canceled out by > dropping the page poisoning on the guest. > > Then again since we are doing higher order pages only, and the > poisoning is supposed to happen before we get into __free_one_page we > would probably have to do both the poisoning, and the poison on fault. Oh that's a nice trick. So in fact if we just make sure we never report PAGE_SIZE pages then poisoning will automatically happen before reporting? So we just need to teach host to poison on fault. Sounds cool and we can always optimize further later. -- MST