Re: [PATCH] virtio_balloon: fix shrinker pages_to_free calculation

[Wei Wang]

On 11/18/2019 01:30 PM, Michael S. Tsirkin wrote:
> On Mon, Nov 18, 2019 at 12:01:08PM +0800, Wei Wang wrote:
> > 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;
> No let's do accounting in pages please. virtio page is a legacy
> thing we just did not fix it in time to get rid of it by now.
>
> > 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.
> OMG. should be a separate patch.
> But really this just shows why shifts are such a bad idea.
>
> Let's define
> VIRTIO_BALLOON_PAGES_PER_FREE_PAGE
>
> and use it with * and / consistently instead of shifts.

OK, will do (maybe call it VIRTIO_BALLOON_FREE_PAGES_PER_BLOCK).

Best,
Wei