On 23.09.19 13:34, David Hildenbrand wrote: > On 23.09.19 13:15, Michal Hocko wrote: >> On Mon 23-09-19 11:31:30, David Hildenbrand wrote: >>> On 23.09.19 10:58, Michal Hocko wrote: >>>> On Fri 20-09-19 10:17:54, David Hildenbrand wrote: >>>>> On 09.09.19 13:48, David Hildenbrand wrote: >>>>>> Based on linux/next + "[PATCH 0/3] Remove __online_page_set_limits()" >>>>>> >>>>>> Let's replace the __online_page...() functions by generic_online_page(). >>>>>> Hyper-V only wants to delay the actual onlining of un-backed pages, so we >>>>>> can simpy re-use the generic function. >>>>>> >>>>>> Only compile-tested. >>>>>> >>>>>> Cc: Souptick Joarder <jrdr.linux@xxxxxxxxx> >>>>>> >>>>>> David Hildenbrand (3): >>>>>> mm/memory_hotplug: Export generic_online_page() >>>>>> hv_balloon: Use generic_online_page() >>>>>> mm/memory_hotplug: Remove __online_page_free() and >>>>>> __online_page_increment_counters() >>>>>> >>>>>> drivers/hv/hv_balloon.c | 3 +-- >>>>>> include/linux/memory_hotplug.h | 4 +--- >>>>>> mm/memory_hotplug.c | 17 ++--------------- >>>>>> 3 files changed, 4 insertions(+), 20 deletions(-) >>>>>> >>>>> >>>>> Ping, any comments on this one? >>>> >>>> Unification makes a lot of sense to me. You can add >>>> Acked-by: Michal Hocko <mhocko@xxxxxxxx> >>>> >>>> I will most likely won't surprise if I asked for more here though ;) >>> >>> I'm not surprised, but definitely not in a negative sense ;) I was >>> asking myself if we could somehow rework this, too. >>> >>>> I have to confess I really detest the whole concept of a hidden callback >>>> with a very weird API. Is this something we can do about? I do realize >>>> that adding a callback would require either cluttering the existing APIs >>>> but maybe we can come up with something more clever. Or maybe existing >>>> external users of online callback can do that as a separate step after >>>> the online is completed - or is this impossible due to locking >>>> guarantees? >>>> >>> >>> The use case of this (somewhat special) callback really is to avoid >>> selected (unbacked in the hypervisor) pages to get put to the buddy just >>> now, but instead to defer that (sometimes, defer till infinity ;) ). >>> Especially, to hinder these pages from getting touched at all. Pages >>> that won't be put to the buddy will usually get PG_offline set (e.g., >>> Hyper-V and XEN) - the only two users I am aware of. >>> >>> For Hyper-V (and also eventually virtio-mem), it is important to set >>> PG_offline before marking the section to be online (SECTION_IS_ONLINE). >>> Only this way, PG_offline is properly set on all pfn_to_online_page() >>> pages, meaning "don't touch this page" - e.g., used to skip over such >>> pages when suspending or by makedumpfile to skip over such offline pages >>> when creating a memory dump. >> >> Thanks for the clarification. I have never really studied what those >> callbacks are doing really. >> >>> So if we would e.g., try to piggy-back onto the memory_notify() >>> infrastructure, we could >>> 1. Online all pages to the buddy (dropping the callback) >>> 2. E.g., memory_notify(MEM_ONLINE_PAGES, &arg); >>> -> in the notifier, pull pages from the buddy, mark sections online >>> 3. Set all involved sections online (online_mem_sections()) >> >> This doesn't really sound any better. For one pages are immediately >> usable when they hit the buddy allocator so this is racy and thus not >> reliable. >> >>> However, I am not sure what actually happens after 1. - we are only >>> holding the device hotplug lock and the memory hotplug lock, so the >>> pages can just get allocated. Also, it sounds like more work and code >>> for the same end result (okay, if the rework is really necessary, though). >>> >>> So yeah, while the current callback might not be optimal, I don't see an >>> easy and clean way to rework this. With the change in this series we are >>> at least able to simply defer doing what would have been done without >>> the callback - not perfect but better. >>> >>> Do you have anything in mind that could work out and make this nicer? >> >> I am wondering why those pages get onlined when they are, in fact, >> supposed to be offline. >> > > It's the current way of emulating sub-memory-block hotplug on top of the > memory bock device API we have. Hyper-V and XEN have been using that for > a long time. > So one idea would be to let clients set pages to PG_offline during MEM_GOING_ONLINE. We could then skip any PG_offline pages when onlining pages, not onlining them to the buddy. But then, there still has to be a way to online pages when required - e.g., generic_online_page(). At least the callback could go. -- Thanks, David / dhildenb
