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. -- Thanks, David / dhildenb
