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[79.181.91.42]) by smtp.gmail.com with ESMTPSA id t190sm24001915vkt.31.2019.07.29.13.49.34 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Mon, 29 Jul 2019 13:49:38 -0700 (PDT) Date: Mon, 29 Jul 2019 16:49:31 -0400 From: "Michael S. Tsirkin" To: Alexander Duyck Cc: wei.w.wang@intel.com, Nitesh Narayan Lal , Alexander Duyck , kvm list , David Hildenbrand , Dave Hansen , LKML , linux-mm , Andrew Morton , Yang Zhang , pagupta@redhat.com, Rik van Riel , Konrad Rzeszutek Wilk , lcapitulino@redhat.com, Andrea Arcangeli , Paolo Bonzini , dan.j.williams@intel.com Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting" Message-ID: <20190729163053-mutt-send-email-mst@kernel.org> References: <20190724165158.6685.87228.stgit@localhost.localdomain> <20190724171050.7888.62199.stgit@localhost.localdomain> <20190724150224-mutt-send-email-mst@kernel.org> <6218af96d7d55935f2cf607d47680edc9b90816e.camel@linux.intel.com> <20190724164023-mutt-send-email-mst@kernel.org> <20190729151805-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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, Jul 29, 2019 at 01:21:32PM -0700, Alexander Duyck wrote: > On Mon, Jul 29, 2019 at 12:25 PM Michael S. Tsirkin wrote: > > > > On Mon, Jul 29, 2019 at 09:58:04AM -0700, Alexander Duyck wrote: > > > On Wed, Jul 24, 2019 at 1:42 PM Michael S. Tsirkin wrote: > > > > > > > > On Wed, Jul 24, 2019 at 04:29:27PM -0400, Nitesh Narayan Lal wrote: > > > > > > > > > > On 7/24/19 4:18 PM, Alexander Duyck wrote: > > > > > > On Wed, 2019-07-24 at 15:02 -0400, Michael S. Tsirkin wrote: > > > > > >> On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote: > > > > > >>> From: Alexander Duyck > > > > > >>> > > > > > >>> Add support for what I am referring to as "bubble hinting". Basically the > > > > > >>> idea is to function very similar to how the balloon works in that we > > > > > >>> basically end up madvising the page as not being used. However we don't > > > > > >>> really need to bother with any deflate type logic since the page will be > > > > > >>> faulted back into the guest when it is read or written to. > > > > > >>> > > > > > >>> This is meant to be a simplification of the existing balloon interface > > > > > >>> to use for providing hints to what memory needs to be freed. I am assuming > > > > > >>> this is safe to do as the deflate logic does not actually appear to do very > > > > > >>> much other than tracking what subpages have been released and which ones > > > > > >>> haven't. > > > > > >>> > > > > > >>> Signed-off-by: Alexander Duyck > > > > > >>> --- > > > > > >>> hw/virtio/virtio-balloon.c | 40 +++++++++++++++++++++++ > > > > > >>> include/hw/virtio/virtio-balloon.h | 2 + > > > > > >>> include/standard-headers/linux/virtio_balloon.h | 1 + > > > > > >>> 3 files changed, 42 insertions(+), 1 deletion(-) > > > > > >>> > > > > > >>> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c > > > > > >>> index 2112874055fb..70c0004c0f88 100644 > > > > > >>> --- a/hw/virtio/virtio-balloon.c > > > > > >>> +++ b/hw/virtio/virtio-balloon.c > > > > > >>> @@ -328,6 +328,39 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v, > > > > > >>> balloon_stats_change_timer(s, 0); > > > > > >>> } > > > > > >>> > > > > > >>> +static void virtio_bubble_handle_output(VirtIODevice *vdev, VirtQueue *vq) > > > > > >>> +{ > > > > > >>> + VirtQueueElement *elem; > > > > > >>> + > > > > > >>> + while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) { > > > > > >>> + unsigned int i; > > > > > >>> + > > > > > >>> + for (i = 0; i < elem->in_num; i++) { > > > > > >>> + void *addr = elem->in_sg[i].iov_base; > > > > > >>> + size_t size = elem->in_sg[i].iov_len; > > > > > >>> + ram_addr_t ram_offset; > > > > > >>> + size_t rb_page_size; > > > > > >>> + RAMBlock *rb; > > > > > >>> + > > > > > >>> + if (qemu_balloon_is_inhibited()) > > > > > >>> + continue; > > > > > >>> + > > > > > >>> + rb = qemu_ram_block_from_host(addr, false, &ram_offset); > > > > > >>> + rb_page_size = qemu_ram_pagesize(rb); > > > > > >>> + > > > > > >>> + /* For now we will simply ignore unaligned memory regions */ > > > > > >>> + if ((ram_offset | size) & (rb_page_size - 1)) > > > > > >>> + continue; > > > > > >>> + > > > > > >>> + ram_block_discard_range(rb, ram_offset, size); > > > > > >> I suspect this needs to do like the migration type of > > > > > >> hinting and get disabled if page poisoning is in effect. > > > > > >> Right? > > > > > > Shouldn't something like that end up getting handled via > > > > > > qemu_balloon_is_inhibited, or did I miss something there? I assumed cases > > > > > > like that would end up setting qemu_balloon_is_inhibited to true, if that > > > > > > isn't the case then I could add some additional conditions. I would do it > > > > > > in about the same spot as the qemu_balloon_is_inhibited check. > > > > > I don't think qemu_balloon_is_inhibited() will take care of the page poisoning > > > > > situations. > > > > > If I am not wrong we may have to look to extend VIRTIO_BALLOON_F_PAGE_POISON > > > > > support as per Michael's suggestion. > > > > > > > > > > > > BTW upstream qemu seems to ignore VIRTIO_BALLOON_F_PAGE_POISON ATM. > > > > Which is probably a bug. > > > > Wei, could you take a look pls? > > > > > > So I was looking at sorting out this for the unused page reporting > > > that I am working on and it occurred to me that I don't think we can > > > do the free page hinting if any sort of poison validation is present. > > > The problem is that free page hinting simply stops the page from being > > > migrated. As a result if there was stale data present it will just > > > leave it there instead of zeroing it or writing it to alternating 1s > > > and 0s. > > > > stale data where? on source or on destination? > > do you mean the case where memory was corrupted? > > > > Actually I am getting my implementation and this one partially mixed > up again. I was thinking that the page just gets put back. However it > doesn't. Instead free_pages is called. As such it is going to dirty > the page by poisoning it as soon as the hinting is complete. > > In some ways it is worse because I think page poisoning combined with > free page hinting will make the VM nearly unusable because it will be > burning cycles allocating all memory, and then poisoning all those > pages on free. So it will be populating the dirty bitmap with all free > memory each time it goes through and attempts to determine what memory > is free. Right, it does make it useless. I really consider this a bug: page should be given to hypervisor after it's poisoned. Then at least with 0 poison we could just mark it clean. For non-zero poison, we could think about adding kvm APIs for aggressively mapping all freed pages to a single non-zero one with COW. I guess it's prudent to sacrifice another feature bit if/when we fix it properly, saving a feature bit isn't worth the risk that someone shipped a guest like this. But it does show why just using alloc/free for hinting isn't as great an idea as it seems on the surface. > > > > > > Also it looks like the VIRTIO_BALLOON_F_PAGE_POISON feature is > > > assuming that 0 means that page poisoning is disabled, > > > when in reality > > > it might just mean we are using the value zero to poison pages instead > > > of the 0xaa pattern. As such I think there are several cases where we > > > could incorrectly flag the pages with the hint and result in the > > > migrated guest reporting pages that contain non-poison values. > > > > > > > > > Well guest has this code: > > static int virtballoon_validate(struct virtio_device *vdev) > > { > > if (!page_poisoning_enabled()) > > __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON); > > > > __virtio_clear_bit(vdev, VIRTIO_F_IOMMU_PLATFORM); > > return 0; > > } > > > > So it seems that host can figure out what is going on easily enough. > > What did I miss? > > Okay. So it is clearing that feature bit. I didn't see that part. > However that leads to the question of where we should be setting that > feature bit in the QEMU side of things. I was looking at setting that > bit in virtio_balloon_get_features(). Would that be the appropriate > place to set that so that the feature flag is reset when we are > changing OSes or rebooting the guest? We have DEFINE_PROP_BIT("free-page-hint", VirtIOBalloon, host_features, VIRTIO_BALLOON_F_FREE_PAGE_HINT, false), and poison should be there, too. > > > The zero assumption works for unused page reporting since we will be > > > zeroing out the page when it is faulted back into the guest, however > > > the same doesn't work for the free page hint since it is simply > > > skipping the migration of the recently dirtied page. > > > > Right but the dirtied page is normally full of 0 since that is the > > poison value, if we just leave it there we still get 0s, right? > > So for the unused page reporting which I am working on we can still > hint the page away since it will be 0s, and us returning the pages > doesn't alter the page data. However for the free page hinting I don't > think we can. > > With page poisoning and free page hinting I am thinking we should just > disable the free page hinting as I don't see how there can be any > advantage to it if page poisoning is enbled. Basically the thing that > makes the hinting "safe" will sabotage it since for every page we > don't migrate we will also be marking as dirty for the next iteration > through migration. As such we are just pushing the migration of any > free pages until the VM has stopped since the VM will just keep > dirtying the free pages until it stops hinting. Right this was discussed at the time for non-zero poison. For hinting, my argument was simple: the reason it's useless is contained within the hypervisor. So don't put the logic in the guest: hypervisor can see poison is set and not request hints from guest. For reporting, as you say it works with 0s and we can thinkably make it work with non-0s down the road. Until we do we can always have the hypervisor do something like if (poisoning && poison != 0) return; madvise(MADV_FREE); -- MST