On 10.09.19 18:18, Dr. David Alan Gilbert wrote: > * 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. > virtio-balloon also has a mode where the guest would not have to indicate to the host before re-using a page. Only VIRTIO_BALLOON_F_MUST_TELL_HOST enforces this. So it's not completely new. > Dave -- Thanks, David / dhildenb
