Re: [PATCH] virtio_balloon: fix shrinker pages_to_free calculation

[Wei Wang <wei.w.wang@intel.com>]

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