Date:   Mon, 29 Jul 2019 13:21:32 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     wei.w.wang@...el.com, Nitesh Narayan Lal <nitesh@...hat.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        kvm list <kvm@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Zhang <yang.zhang.wz@...il.com>, pagupta@...hat.com,
        Rik van Riel <riel@...riel.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        lcapitulino@...hat.com, Andrea Arcangeli <aarcange@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, dan.j.williams@...el.com
Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting"

On Mon, Jul 29, 2019 at 12:25 PM Michael S. Tsirkin <mst@...hat.com> 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 <mst@...hat.com> 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 <alexander.h.duyck@...ux.intel.com>
> > > > >>>
> > > > >>> 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 <alexander.h.duyck@...ux.intel.com>
> > > > >>> ---
> > > > >>>  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.

> >
> > 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?

> > 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.

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