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References: <20191119101718.38976-1-mst@redhat.com>
In-Reply-To: <20191119101718.38976-1-mst@redhat.com>
From:   Khazhismel Kumykov <khazhy@google.com>
Date:   Tue, 19 Nov 2019 16:48:41 -0800
Message-ID: <CACGdZYLzo1LUjJuKgZrqaq8FJGVp+QUPs2pjCEnAHyMeTfuKfQ@mail.gmail.com>
Subject: Re: [PATCH] virtio_balloon: fix shrinker scan number of pages
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        stable@vger.kernel.org, Wei Wang <wei.w.wang@intel.com>,
        Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org
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On Tue, Nov 19, 2019 at 2:17 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> virtio_balloon_shrinker_scan should return number of system pages freed,
> but because it's calling functions that deal with balloon pages, it gets
> confused and sometimes returns the number of balloon pages.
>
> It does not matter practically as the exact number isn't
> used, but it seems better to be consistent in case someone
> starts using this API.
>
> Further, if we ever tried to iteratively leak pages as
> virtio_balloon_shrinker_scan tries to do, we'd run into issues - this is
> because freed_pages was accumulating total freed pages, but was also
> subtracted on each iteration from pages_to_free, which can result in
> either leaking less memory than we were supposed to free, or or more if
> pages_to_free underruns.
>
> On a system with 4K pages we are lucky that we are never asked to leak
> more than 128 pages while we can leak up to 256 at a time,
> but it looks like a real issue for systems with page size != 4K.
>
> Cc: stable@vger.kernel.org
> Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> Reported-by: Khazhismel Kumykov <khazhy@google.com>
> Reviewed-by: Wei Wang <wei.w.wang@intel.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  drivers/virtio/virtio_balloon.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 226fbb995fb0..7cee05cdf3fb 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -772,6 +772,13 @@ static unsigned long shrink_free_pages(struct virtio_balloon *vb,
>         return blocks_freed << VIRTIO_BALLOON_FREE_PAGE_ORDER;
>  }
>
> +static unsigned long leak_balloon_pages(struct virtio_balloon *vb,
> +                                          unsigned long pages_to_free)
> +{
> +       return leak_balloon(vb, pages_to_free * VIRTIO_BALLOON_PAGES_PER_PAGE) /
> +               VIRTIO_BALLOON_PAGES_PER_PAGE;
> +}
> +
>  static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
>                                           unsigned long pages_to_free)
>  {
> @@ -782,11 +789,9 @@ static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
>          * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
>          * multiple times to deflate pages till reaching pages_to_free.
>          */
> -       while (vb->num_pages && pages_to_free) {
> -               pages_freed += leak_balloon(vb, pages_to_free) /
> -                                       VIRTIO_BALLOON_PAGES_PER_PAGE;
> -               pages_to_free -= pages_freed;
> -       }
> +       while (vb->num_pages && pages_freed < pages_to_free)
> +               pages_freed += leak_balloon_pages(vb, pages_to_free);

pages_to_free - pages_freed ?

> +
>         update_balloon_size(vb);
>
>         return pages_freed;
> @@ -799,7 +804,7 @@ static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker,
>         struct virtio_balloon *vb = container_of(shrinker,
>                                         struct virtio_balloon, shrinker);
>
> -       pages_to_free = sc->nr_to_scan * VIRTIO_BALLOON_PAGES_PER_PAGE;
> +       pages_to_free = sc->nr_to_scan;
>
>         if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
>                 pages_freed = shrink_free_pages(vb, pages_to_free);
> --
> MST
>

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