On 21.06.2017 14:56, Christian Borntraeger wrote: > On 06/20/2017 06:49 PM, David Hildenbrand wrote: >> On 20.06.2017 18:44, Rik van Riel wrote: >>> On Mon, 2017-06-12 at 07:10 -0700, Dave Hansen wrote: >>> >>>> The hypervisor is going to throw away the contents of these pages, >>>> right? As soon as the spinlock is released, someone can allocate a >>>> page, and put good data in it. What keeps the hypervisor from >>>> throwing >>>> away good data? >>> >>> That looks like it may be the wrong API, then? >>> >>> We already have hooks called arch_free_page and >>> arch_alloc_page in the VM, which are called when >>> pages are freed, and allocated, respectively. >>> >>> Nitesh Lal (on the CC list) is working on a way >>> to efficiently batch recently freed pages for >>> free page hinting to the hypervisor. >>> >>> If that is done efficiently enough (eg. with >>> MADV_FREE on the hypervisor side for lazy freeing, >>> and lazy later re-use of the pages), do we still >>> need the harder to use batch interface from this >>> patch? >>> >> David's opinion incoming: >> >> No, I think proper free page hinting would be the optimum solution, if >> done right. This would avoid the batch interface and even turn >> virtio-balloon in some sense useless. >> I said "some sense" for a reason. Mainly because other techniques are being worked on that are to fill the holes. > Two reasons why I disagree: > - virtio-balloon is often used as memory hotplug. (e.g. libvirts current/max memory > uses virtio ballon) I know, while one can argue if this real unplug as there are basically no guarantees (see virtio-mem RFC) it is used by people because there is simply no alternative. Still, for now some people use it for that. > - free page hinting will not allow to shrink the page cache of guests (like a ballooner does) There are currently some projects ongoing that try to avoid the page cache in the guest completely. -- Thanks, David _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
