Re: [PATCH 0/8] implement "memmap on memory" feature on s390

David Hildenbrand
2023-11-16T23:08:40Z

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.

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.

--
Cheers,

David / dhildenb