Re: [RFC PATCH] virtio_balloon: add param to skip adjusting pages

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 18, 2021 at 10:25:45AM +0900, David Stevens wrote:
> On Wed, Nov 17, 2021 at 10:32 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Wed, Nov 17, 2021 at 07:06:34PM +0900, David Stevens wrote:
> > > From: David Stevens <stevensd@chromium.org>
> > >
> > > Add a module parameters to virtio_balloon to allow specifying whether or
> > > not the driver should call adjust_managed_page_count. If the parameter
> > > is set, it overrides the default behavior inferred from the deflate on
> > > OOM flag. This allows the balloon to operate without changing the amount
> > > of memory visible to userspace via /proc/meminfo or sysinfo, even on a
> > > system that cannot set the default on OOM flag.
> > >
> > > The motivation for this patch is to allow userspace to more accurately
> > > take advantage of virtio_balloon's cooperative memory control on a
> > > system without the ability to use deflate on OOM. As it stands,
> > > userspace has no way to know how much memory may be available on such a
> > > system, which makes tasks such as sizing caches impossible.
> > >
> > > When deflate on OOM is not enabled, the current behavior of the
> > > virtio_balloon more or less resembles hotplugging individual pages, at
> > > least from an accounting perspective. This is basically hardcoding the
> > > requirement that totalram_pages must be available to the guest
> > > immediately, regardless of what the host does. While that is a valid
> > > policy, on Linux (which supports memory overcommit) with virtio_balloon
> > > (which is designed to facilitate overcommit in the host), it is not the
> > > only possible policy.
> > >
> > > The param added by this patch allows the guest to operate under the
> > > assumption that pages in the virtio_balloon will generally be made
> > > available when needed. This assumption may not always hold, but when it
> > > is violated, the guest will just fall back to the normal mechanisms for
> > > dealing with overcommitted memory.
> > >
> > > Signed-off-by: David Stevens <stevensd@chromium.org>
> > > ---
> > >
> > > Another option to achieve similar behavior would be to add a new feature
> > > flag that would be used in conjunction with DEFLATE_ON_OOM to determine
> > > whether or not adjust_managed_page_count should be called. However, I
> > > feel that this sort of policy decision is a little outside the scope of
> > > what the virtio_balloon API deals with.
> > >
> > > ---
> > >  drivers/virtio/virtio_balloon.c | 23 ++++++++++++++++++-----
> > >  1 file changed, 18 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > > index c22ff0117b46..17dac286899c 100644
> > > --- a/drivers/virtio/virtio_balloon.c
> > > +++ b/drivers/virtio/virtio_balloon.c
> > > @@ -133,6 +133,21 @@ static const struct virtio_device_id id_table[] = {
> > >       { 0 },
> > >  };
> > >
> > > +static char *adjust_pages = "";
> > > +module_param(adjust_pages, charp, 0);
> > > +MODULE_PARM_DESC(adjust_pages, "Whether or not pages in the balloon should be removed from the managed page count");
> > > +
> > > +static bool should_adjust_pages(struct virtio_balloon *vb)
> > > +{
> > > +     if (!strcmp(adjust_pages, "always"))
> > > +             return true;
> > > +     else if (!strcmp(adjust_pages, "never"))
> > > +             return false;
> > > +
> > > +     return !virtio_has_feature(vb->vdev,
> > > +                                VIRTIO_BALLOON_F_DEFLATE_ON_OOM);
> > > +}
> > > +
> > >  static u32 page_to_balloon_pfn(struct page *page)
> > >  {
> > >       unsigned long pfn = page_to_pfn(page);
> > > @@ -243,8 +258,7 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
> > >
> > >               set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> > >               vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
> > > -             if (!virtio_has_feature(vb->vdev,
> > > -                                     VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> > > +             if (should_adjust_pages(vb))
> > >                       adjust_managed_page_count(page, -1);
> > >               vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
> > >       }
> > > @@ -264,8 +278,7 @@ static void release_pages_balloon(struct virtio_balloon *vb,
> > >       struct page *page, *next;
> > >
> > >       list_for_each_entry_safe(page, next, pages, lru) {
> > > -             if (!virtio_has_feature(vb->vdev,
> > > -                                     VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> > > +             if (should_adjust_pages(vb))
> > >                       adjust_managed_page_count(page, 1);
> > >               list_del(&page->lru);
> > >               put_page(page); /* balloon reference */
> > > @@ -775,7 +788,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
> > >         * managed page count when inflating, we have to fixup the count of
> > >         * both involved zones.
> > >         */
> > > -     if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM) &&
> > > +     if (should_adjust_pages(vb) &&
> > >           page_zone(page) != page_zone(newpage)) {
> > >               adjust_managed_page_count(page, 1);
> > >               adjust_managed_page_count(newpage, -1);
> >
> > A problem here is that the host also cares: IIUC
> > with VIRTIO_BALLOON_F_STATS_VQ we are sending the info
> > about page counts to host, changing what the stats
> > mean.
> >
> > So I suspect we need a device feature for this at least
> > to control this aspect.
> >
> 
> The only stat this would affect is VIRTIO_BALLOON_S_MEMTOT, I think.
> If that's not supposed to include memory held by the balloon, then we
> can just subtract the balloon's size from i.totalram in
> update_balloon_stats if should_adjust_pages() is true but
> VIRTIO_BALLOON_F_DEFLATE_ON_OOM is not set. That should preserve the
> current behavior.

OK, that sounds reasonable.

> >
> > > --
> > > 2.34.0.rc2.393.gf8c9666880-goog
> >

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