Re: [PATCH] virtio_balloon: fix shrinker pages_to_free calculation

[Wei Wang]

On 11/16/2019 06:55 AM, Khazhismel Kumykov wrote:
> To my reading, we're accumulating total freed pages in pages_freed, but
> subtracting it every iteration from pages_to_free, meaning we'll count
> earlier iterations multiple times, freeing fewer pages than expected.
> Just accumulate in pages_freed, and compare to pages_to_free.

Not sure about the above. But the following unit mismatch is a good 
capture, thanks!

> There's also a unit mismatch, where pages_to_free seems to be virtio
> balloon pages, and pages_freed is system pages (We divide by
> VIRTIO_BALLOON_PAGES_PER_PAGE), so sutracting pages_freed from
> pages_to_free may result in freeing too much.
>
> There also seems to be a mismatch between shrink_free_pages() and
> shrink_balloon_pages(), where in both pages_to_free is given as # of
> virtio pages to free, but free_pages() returns virtio pages, and
> balloon_pages returns system pages.
>
> (For 4K PAGE_SIZE, this mismatch wouldn't be noticed since
> VIRTIO_BALLOON_PAGES_PER_PAGE would be 1)
>
> Have both return virtio pages, and divide into system pages when
> returning from shrinker_scan()

Sounds good.

> Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> Cc: Wei Wang <wei.w.wang@xxxxxxxxx>
> Signed-off-by: Khazhismel Kumykov <khazhy@xxxxxxxxxx>
> ---
>
> Tested this under memory pressure conditions and the shrinker seemed to
> shrink.
>
>   drivers/virtio/virtio_balloon.c | 11 ++++-------
>   1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 226fbb995fb0..7951ece3fe24 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -782,11 +782,8 @@ 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_to_free > pages_freed)
> +		pages_freed += leak_balloon(vb, pages_to_free - pages_freed);
>   	update_balloon_size(vb);
>   
>   	return pages_freed;
> @@ -805,11 +802,11 @@ static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker,
>   		pages_freed = shrink_free_pages(vb, pages_to_free);

We also need a fix here then:

pages_freed = shrink_free_pages(vb, sc->nr_to_scan) * 
VIRTIO_BALLOON_PAGES_PER_PAGE;


Btw, there is another mistake, in virtio_balloon_shrinker_count:

-       count += vb->num_free_page_blocks >> VIRTIO_BALLOON_FREE_PAGE_ORDER;
+      count += vb->num_free_page_blocks << VIRTIO_BALLOON_FREE_PAGE_ORDER;

You may want to include it in this fix patch as well.

Best,
Wei