* Alexander Duyck (alexander.duyck@xxxxxxxxx) wrote: > On Tue, Sep 10, 2019 at 7:47 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > On Tue 10-09-19 07:42:43, Alexander Duyck wrote: > > > 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. > > > > As I've said I have no idea about virt so this doesn't really tell me > > much. Does that mean that if somebody allocates such a page and tries to > > access it then virt will handle a fault and bring it back? > > Actually I am probably describing too much as the MADV_DONTNEED is the > hypervisor behavior in response to the virtio-balloon notification. A > more thorough explanation of it can be found by just running "man > madvise", probably best just to leave it at that since I am probably > confusing things by describing hypervisor behavior in a kernel patch > set. > > For the most part all the page reporting really does is provide a way > to incrementally identify unused regions of memory in the buddy > allocator. That in turn is used by virtio-balloon in a polling thread > to report to the hypervisor what pages are not in use so that it can > make a decision on what to do with the pages now that it knows they > are unused. > > All this is providing is just a report and it is optional if the > hypervisor will act on it or not. If the hypervisor takes some sort of > action on the page, then the expectation is that the hypervisor will > use some sort of mechanism such as a page fault to discover when the > page is used again. OK, that's interestingly different (but OK) from some other schemes that hav ebeen described which *require* the guest to somehow indicate the page is in use before starting to use the page again. Dave > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@xxxxxxxxxxxxxxxxxxxx > For additional commands, e-mail: virtio-dev-help@xxxxxxxxxxxxxxxxxxxx > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
