linux-kernel - Re: [PATCH] virtio_balloon: fix shrinker scan number of pages

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Date:   Tue, 19 Nov 2019 12:39:51 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org, Khazhismel Kumykov <khazhy@...gle.com>,
        Wei Wang <wei.w.wang@...el.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio_balloon: fix shrinker scan number of pages

On 19.11.19 11:17, Michael S. Tsirkin 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.

If it doesn't matter, why cc: stable?

> 
> 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@...r.kernel.org
> Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> Reported-by: Khazhismel Kumykov <khazhy@...gle.com>
> Reviewed-by: Wei Wang <wei.w.wang@...el.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.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);
> +
>   	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);
> 


-- 

Thanks,

David / dhildenb