Messages in this thread | | | Date | Mon, 18 Nov 2019 00:30:21 -0500 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH] virtio_balloon: fix shrinker pages_to_free calculation |
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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@intel.com> > > Signed-off-by: Khazhismel Kumykov <khazhy@google.com> > > --- > > > > 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.
> Best, > Wei >
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