On 08.04.19 22:10, Alexander Duyck wrote: > On Mon, Apr 8, 2019 at 11:40 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >>>>> >>>>> In addition we will need some way to identify which pages have been >>>>> hinted on and which have not. The way I believe easiest to do this >>>>> would be to overload the PageType value so that we could essentially >>>>> have two values for "Buddy" pages. We would have our standard "Buddy" >>>>> pages, and "Buddy" pages that also have the "Offline" value set in the >>>>> PageType field. Tracking the Online vs Offline pages this way would >>>>> actually allow us to do this with almost no overhead as the mapcount >>>>> value is already being reset to clear the "Buddy" flag so adding a >>>>> "Offline" flag to this clearing should come at no additional cost. >>>> >>>> Just nothing here that this will require modifications to kdump >>>> (makedumpfile to be precise and the vmcore information exposed from the >>>> kernel), as kdump only checks for the the actual mapcount value to >>>> detect buddy and offline pages (to exclude them from dumps), they are >>>> not treated as flags. >>>> >>>> For now, any mapcount values are really only separate values, meaning >>>> not the separate bits are of interest, like flags would be. Reusing >>>> other flags would make our life a lot easier. E.g. PG_young or so. But >>>> clearing of these is then the problematic part. >>>> >>>> Of course we could use in the kernel two values, Buddy and BuddyOffline. >>>> But then we have to check for two different values whenever we want to >>>> identify a buddy page in the kernel. >>> >>> Actually this may not be working the way you think it is working. >> >> Trust me, I know how it works. That's why I was giving you the notice. >> >> Read the first paragraph again and ignore the others. I am only >> concerned about makedumpfile that has to be changed. >> >> PAGE_OFFLINE_MAPCOUNT_VALUE >> PAGE_BUDDY_MAPCOUNT_VALUE >> >> Once you find out how these values are used, you should understand what >> has to be changed and where. > > Ugh. Is there an official repo I am supposed to refer to for makedumpfile? > > As far as the changes needed I don't think this would necessitate > additional exports. We could probably just get away with having > makedumpfile generate a new value by simply doing an "&" of the two > values to determine what an offline buddy would be. If need be I can > submit a patch for that. I find it kind of annoying that the kernel is > handling identifying these bits one way, and makedumpfile is doing it > another way. It should have been setup to handle this all the same > way. > >> >>>>> >>>>> Lastly we would need to create a specialized function for allocating >>>>> the non-"Offline" pages, and to tweak __free_one_page to tail enqueue >>>>> "Offline" pages. I'm thinking the alloc function it would look >>>>> something like __rmqueue_smallest but without the "expand" and needing >>>>> to modify the !page check to also include a check to verify the page >>>>> is not "Offline". As far as the changes to __free_one_page it would be >>>>> a 2 line change to test for the PageType being offline, and if it is >>>>> to call add_to_free_area_tail instead of add_to_free_area. >>>> >>>> As already mentioned, there might be scenarios where the additional >>>> hinting thread might consume too much CPU cycles, especially if there is >>>> little guest activity any you mostly spend time scanning a handful of >>>> free pages and reporting them. I wonder if we can somehow limit the >>>> amount of wakeups/scans for a given period to mitigate this issue. >>> >>> That is why I was talking about breaking nr_free into nr_freed and >>> nr_bound. By doing that I can record the nr_free value to a >>> virtio-balloon specific location at the start of any walk and should >>> know exactly now many pages were freed between that call and the next >>> one. By ordering things such that we place the "Offline" pages on the >>> tail of the list it should make the search quite fast since we would >>> just be always allocating off of the head of the queue until we have >>> hinted everything int he queue. So when we hit the last call to alloc >>> the non-"Offline" pages and shut down our thread we can use the >>> nr_freed value that we recorded to know exactly how many pages have >>> been added that haven't been hinted. >>> >>>> One main issue I see with your approach is that we need quite a lot of >>>> core memory management changes. This is a problem. I wonder if we can >>>> factor out most parts into callbacks. >>> >>> I think that is something we can't get away from. However if we make >>> this generic enough there would likely be others beyond just the >>> virtualization drivers that could make use of the infrastructure. For >>> example being able to track the rate at which the free areas are >>> cycling in and out pages seems like something that would be useful >>> outside of just the virtualization areas. >> >> Might be, but might be the other extreme, people not wanting such >> special cases in core mm. I assume the latter until I see a very clear >> design where such stuff has been properly factored out. > > The only real pain point I am seeing right now is the assumptions > makedumpfile is currently making about how mapcount is being used to > indicate pagetype. If we patch it to fix it most of the other bits are > minor. I'll be curious how splitting etc. will be handled. Especially if you want to set Offline for all affected sub pages. -- Thanks, David / dhildenb
