On Mon, 2019-02-04 at 11:40 -0800, Dave Hansen wrote: > > +void __arch_merge_page(struct zone *zone, struct page *page, > > + unsigned int order) > > +{ > > + /* > > + * The merging logic has merged a set of buddies up to the > > + * KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER. Since that is the case, take > > + * advantage of this moment to notify the hypervisor of the free > > + * memory. > > + */ > > + if (order != KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER) > > + return; > > + > > + /* > > + * Drop zone lock while processing the hypercall. This > > + * should be safe as the page has not yet been added > > + * to the buddy list as of yet and all the pages that > > + * were merged have had their buddy/guard flags cleared > > + * and their order reset to 0. > > + */ > > + spin_unlock(&zone->lock); > > + > > + kvm_hypercall2(KVM_HC_UNUSED_PAGE_HINT, page_to_phys(page), > > + PAGE_SIZE << order); > > + > > + /* reacquire lock and resume freeing memory */ > > + spin_lock(&zone->lock); > > +} > > Why do the lock-dropping on merge but not free? What's the difference? The lock has not yet been acquired in the free path. The arch_free_page call is made from free_pages_prepare, whereas the arch_merge_page call is made from within __free_one_page which has the requirement that the zone lock be taken before calling the function. > This makes me really nervous. You at *least* want to document this at > the arch_merge_page() call-site, and perhaps even the __free_one_page() > call-sites because they're near where the zone lock is taken. Okay, that makes sense. I would probably look at adding the documentation to the arch_merge_page call-site. > The place you are calling arch_merge_page() looks OK to me, today. But, > it can't get moved around without careful consideration. That also > needs to be documented to warn off folks who might move code around. Agreed. > The interaction between the free and merge hooks is also really > implementation-specific. If an architecture is getting order-0 > arch_free_page() notifications, it's probably worth at least documenting > that they'll *also* get arch_merge_page() notifications. If an architecture is getting order-0 notifications then the merge notifications would be pointless since all the pages would be already hinted. I can add documentation that explains that in the case where we are only hinting on non-zero order pages then arch_merge_page should provide hints for when a page is merged above that threshold. > The reason x86 doesn't double-hypercall on those is not broached in the > descriptions. That seems to be problematic. I will add more documentation to address that.