Messages in this thread | | | Date | Mon, 18 Nov 2019 15:42:10 +0800 | From | Wei Wang <> | Subject | Re: [PATCH] virtio_balloon: fix shrinker pages_to_free calculation |
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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@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. >
OK, will do (maybe call it VIRTIO_BALLOON_FREE_PAGES_PER_BLOCK).
Best, Wei
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