On 07.07.20 14:13, David Hildenbrand wrote: > On 07.07.20 14:08, Heiko Carstens wrote: >> On Fri, Jul 03, 2020 at 03:39:08PM +0200, David Hildenbrand wrote: >>> This series is based on the latest s390/features branch [1]. It implements >>> vmemmap_free(), consolidating it with vmem_add_range(), and optimizes it by >>> - Freeing empty page tables (now also done for idendity mapping). >>> - Handling cases where the vmemmap of a section does not fill huge pages >>> completely. >>> >>> vmemmap_free() is currently never used, unless adiing standby memory fails >>> (unlikely). This is relevant for virtio-mem, which adds/removes memory >>> in memory block/section granularity (always removes memory in the same >>> granularity it added it). >>> >>> I gave this a proper test with my virtio-mem prototype (which I will share >>> once the basic QEMU implementation is upstream), both with 56 byte memmap >>> per page and 64 byte memmap per page, with and without huge page support. >>> In both cases, removing memory (routed through arch_remove_memory()) will >>> result in >>> - all populated vmemmap pages to get removed/freed >>> - all applicable page tables for the vmemmap getting removed/freed >>> - all applicable page tables for the idendity mapping getting removed/freed >>> Unfortunately, I don't have access to bigger and z/VM (esp. dcss) >>> environments. >>> >>> This is the basis for real memory hotunplug support for s390x and should >>> complete my journey to s390x vmem/vmemmap code for now :) >>> >>> What needs double-checking is tlb flushing. AFAIKS, as there are no valid >>> accesses, doing a single range flush at the end is sufficient, both when >>> removing vmemmap pages and the idendity mapping. >>> >>> Along, some minor cleanups. >> >> Hmm.. I really would like to see if there would be only a single page >> table walker left in vmem.c, which handles both adding and removing >> things. >> Now we end up with two different page table walk implementations >> within the same file. However not sure if it is worth the effort to >> unify them though. > > I tried to unify vmemmap_populate() and vmem_add_range() already and > didn't like the end result ... so, unifying these along with the removal > part won't be any better - most probably. Open for suggestions :) > > (at least arm64 and x86-64 handle it similarly) > I'll play with something like static void modify_pagetable(unsigned long start, unsigned long end, bool direct, bool add) and see how it turns out. -- Thanks, David / dhildenb
