On 17.07.19 16:34, Michael S. Tsirkin wrote: > On Wed, Jul 17, 2019 at 04:10:47PM +0200, David Hildenbrand wrote: >> On 17.07.19 13:20, Michael S. Tsirkin wrote: >>> Wei, others, >>> >>> ATM virtio_balloon_shrinker_scan will only get registered >>> when deflate on oom feature bit is set. >>> >>> Not sure whether that's intentional. Assuming it is: >>> >>> virtio_balloon_shrinker_scan will try to locate and free >>> pages that are processed by host. >>> The above seems broken in several ways: >>> - count ignores the free page list completely >>> - if free pages are being reported, pages freed >>> by shrinker will just get re-allocated again >> >> Trying to answer your questions (not sure if I fully understood what you >> mean) >> >> virtio_balloon_shrinker_scan() will not be called due to inflation >> requests (balloon_page_alloc()). It will be called whenever the system >> is OOM, e.g., when starting a new application. >> >> I assume you were expecting the shrinker getting called due to >> balloon_page_alloc(). however, that is not the case as we pass >> "__GFP_NORETRY". > > Right but it's possible we exhaust all memory, then > someone else asks for a single page and that invokes > the shrinker. Yes, I think that can happen. > >> >> To test, something like: >> >> 1. Start a VM with >> >> -device virtio-balloon-pci,deflate-on-oom=true >> >> 2. Inflate the balloon, e.g., >> >> QMP: balloon 1024 >> QMP: info balloon >> -> 1024 >> >> See how "MemTotal" in /proc/meminfo in the guest won't change >> >> 3. Run a workload that exhausts memory in the guest (OOM). >> >> See how the balloon was automatically deflated >> >> QMP: info balloon >> -> Something bigger than 1024 >> >> >> Not sure if it is broken, last time I played with it, it worked, but >> that was ~1-2 years ago. >> >> -- >> >> Thanks, >> >> David / dhildenb > > Sorry I was unclear. The question was about > VIRTIO_BALLOON_F_FREE_PAGE_HINT specifically. Ah, I see. Never used both things together. -- Thanks, David / dhildenb
