On Fri, 17 Nov 2023 00:08:31 +0100 David Hildenbrand <david@xxxxxxxxxx> wrote: > On 14.11.23 19:02, Sumanth Korikkar wrote: > > Hi All, > > > > The patch series implements "memmap on memory" feature on s390 and > > provides the necessary fixes for it. > > Thinking about this, one thing that makes s390x different from all the > other architectures in this series is the altmap handling. > > I'm curious, why is that even required? > > A memmep that is not marked as online in the section should not be > touched by anybody (except memory onlining code :) ). And if we do, it's > usually a BUG because that memmap might contain garbage/be poisoned or > completely stale, so we might want to track that down and fix it in any > case. > > So what speaks against just leaving add_memory() populate the memmap > from the altmap? Then, also the page tables for the memmap are already > in place when onlining memory. Good question, I am not 100% sure if we ran into bugs, or simply assumed that it is not OK to call __add_pages() when the memory for the altmap is not accessible. Maybe there is also already a common code bug with that, s390 might be special but that is often also good for finding bugs in common code ... > Then, adding two new notifier calls on start of memory_block_online() > called something like MEM_PREPARE_ONLINE and end the end of > memory_block_offline() called something like MEM_FINISH_OFFLINE is still > suboptimal, but that's where standby memory could be > activated/deactivated, without messing with the altmap. > > That way, the only s390x specific thing is that the memmap that should > not be touched by anybody is actually inaccessible, and you'd > activate/deactivate simply from the new notifier calls just the way we > used to do. > > It's still all worse than just adding/removing memory properly, using a > proper interface -- where you could alloc/free an actual memmap when the > altmap is not desired. But I know that people don't want to spend time > just doing it cleanly from scratch. Yes, sometimes they need to be forced to do that :-) So, we'll look into defining a "proper interface", and treat patches 1-3 separately as bug fixes? Especially patch 3 might be interesting for arm, if they do not have ZONE_DEVICE, but still use the functions, they might end up with the no-op version, not really freeing any memory.
