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. -- Michal Hocko SUSE Labs
