On Tue, Sep 10, 2019 at 5:42 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > I wanted to review "mm: Introduce Reported pages" just realize that I > have no clue on what is going on so returned to the cover and it didn't > really help much. I am completely unfamiliar with virtio so please bear > with me. > > On Sat 07-09-19 10:25:03, Alexander Duyck wrote: > [...] > > This series provides an asynchronous means of reporting to a hypervisor > > that a guest page is no longer in use and can have the data associated > > with it dropped. To do this I have implemented functionality that allows > > for what I am referring to as unused page reporting > > > > The functionality for this is fairly simple. When enabled it will allocate > > statistics to track the number of reported pages in a given free area. > > When the number of free pages exceeds this value plus a high water value, > > currently 32, it will begin performing page reporting which consists of > > pulling pages off of free list and placing them into a scatter list. The > > scatterlist is then given to the page reporting device and it will perform > > the required action to make the pages "reported", in the case of > > virtio-balloon this results in the pages being madvised as MADV_DONTNEED > > and as such they are forced out of the guest. After this they are placed > > back on the free list, > > And here I am reallly lost because "forced out of the guest" makes me > feel that those pages are no longer usable by the guest. So how come you > can add them back to the free list. I suspect understanding this part > will allow me to understand why we have to mark those pages and prevent > merging. Basically as the paragraph above mentions "forced out of the guest" really is just the hypervisor calling MADV_DONTNEED on the page in question. So the behavior is the same as any userspace application that calls MADV_DONTNEED where the contents are no longer accessible from userspace and attempting to access them will result in a fault and the page being populated with a zero fill on-demand page, or a copy of the file contents if the memory is file backed.
