On Thu, 2019-02-07 at 09:48 -0500, Nitesh Narayan Lal wrote: > On 2/4/19 1:15 PM, Alexander Duyck wrote: > > This patch set provides a mechanism by which guests can notify the host of > > pages that are not currently in use. Using this data a KVM host can more > > easily balance memory workloads between guests and improve overall system > > performance by avoiding unnecessary writing of unused pages to swap. > > > > In order to support this I have added a new hypercall to provided unused > > page hints and made use of mechanisms currently used by PowerPC and s390 > > architectures to provide those hints. To reduce the overhead of this call > > I am only using it per huge page instead of of doing a notification per 4K > > page. By doing this we can avoid the expense of fragmenting higher order > > pages, and reduce overall cost for the hypercall as it will only be > > performed once per huge page. > > > > Because we are limiting this to huge pages it was necessary to add a > > secondary location where we make the call as the buddy allocator can merge > > smaller pages into a higher order huge page. > > > > This approach is not usable in all cases. Specifically, when KVM direct > > device assignment is used, the memory for a guest is permanently assigned > > to physical pages in order to support DMA from the assigned device. In > > this case we cannot give the pages back, so the hypercall is disabled by > > the host. > > > > Another situation that can lead to issues is if the page were accessed > > immediately after free. For example, if page poisoning is enabled the > > guest will populate the page *after* freeing it. In this case it does not > > make sense to provide a hint about the page being freed so we do not > > perform the hypercalls from the guest if this functionality is enabled. > > Hi Alexander, > > Did you get a chance to look at my v8 posting of Guest Free Page Hinting > [1]? > Considering both the solutions are trying to solve the same problem. It > will be great if we can collaborate and come up with a unified solution. > > [1] https://lkml.org/lkml/2019/2/4/993 I haven't had a chance to review these yet. I'll try to take a look later today and provide review notes based on what I find. Thanks. - Alex
