Re: [PATCH 6/8] s390/mm: implement MEM_PHYS_ONLINE MEM_PHYS_OFFLINE memory notifiers

From: David Hildenbrand
Date: Thu Nov 16 2023 - 14:16:11 EST

On 15.11.23 15:20, Sumanth Korikkar wrote:
On Tue, Nov 14, 2023 at 07:39:40PM +0100, David Hildenbrand wrote:
On 14.11.23 19:02, Sumanth Korikkar wrote:
Implement MEM_PHYS_ONLINE and MEM_PHYS_OFFLINE memory notifiers on s390

...
arch/s390/mm/init.c | 16 +++++++++++++++-
drivers/s390/char/sclp_cmd.c | 33 ++++++++++++++++++++++++++++++---
2 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
index 8d9a60ccb777..db505ed590b2 100644
--- a/arch/s390/mm/init.c
+++ b/arch/s390/mm/init.c
@@ -288,6 +288,12 @@ int arch_add_memory(int nid, u64 start, u64 size,
rc = vmem_add_mapping(start, size);
if (rc)
return rc;
+ /*
+ * If MHP_MEMMAP_ON_MEMORY is enabled, perform __add_pages() during memory
+ * onlining phase
+ */
+ if (params->altmap)
+ return 0;


So we'd have added memory blocks without a memmap? Sorry, but this seems to
further hack into the s390x direction.

This new approach has the advantage that we do not need to allocate any
additional memory during online phase, neither for direct mapping page
tables nor struct pages, so that memory hotplug can never fail.

Right, just like any other architecture that (triggered by whatever mechanism) ends up calling add_memory() and friends.


The old approach (without altmap) is already a hack, because we add
the memmap / struct pages, but for memory that is not really accessible.

Yes, it's disgusting. And you still allocate other things like memory block devices or the identify map.

And with all the disadvantage of pre-allocating struct pages from system
memory.

Jep. It never should have been done like that.


The new approach allows to better integrate s390 to the existing
interface, and also make use of altmap support, which would eliminate
the major disadvantage of the old behaviour. So from s390 perspective,
this new mechanism would be preferred, provided that there is no
functional issue with the "added memory blocks without a memmap"
approach.

It achieves that by s390x specific hacks in common code :) Instead of everybody else that simply uses add_memory() and friends.


Do you see any functional issues, e.g. conflict with common
code?

I don't see functional issues right now, just the way it is done to implement support for a new feature is a hack IMHO. Replacing hack #1 by hack #2 is not really something reasonable. Let's try to remove hacks.


Maybe s390x should just provide a dedicate interface to add these memory
blocks instead of adding them during boot and then relying on the old way of
using online/offline set them online/offline.

Existing behavior:
The current 'lsmem -a' command displays both online and standby memory.

interface changes:
If a new interface is introduced and standby memory is no longer listed,
the following consequences might occur:

1. Running 'lsmem -a' would only show online memory, potentially leading
to user complaints.

That's why the new, clean way of doing it will require a world switch. If the admin wants the benefits of altmap/memmap allocation, it can be enabled.

2. standby memory addition would need:
* echo "standby memory addr" > /sys/devices/system/memory/probe
As far as I understand, this interface is already deprecated.

It should actually be an s390x specific interface where people are able to query the standby ranges, and request to add/remove them. There, s390x can perform checks and setup everything accordingly before calling add_memory() and have the memory onlined.

We do have something comparable with the dax/kmem infrastructure: users configure the available memory to hotplug, and then hotplug it. Tooling onlines that memory automatically.

Ideally they will add ranges, not memory blocks.


3. To remove standby memory, a new interface probe_remove is needed
* echo "standby memory addr" > /sys/devices/system/memory/probe_remove


Similarly, an s390x specific interface that performs checks and properly tears everything s390x-specifc down -- for example, turning system RAM into standby RAM again.


4. Users may express a need to identify standby memory addresses,
resulting in the creation of another interface to list these standby
memory ranges.

Exactly. Memory that is not added to the system that does not consume any resources, but can be added on demand using an interface that is not the second stage (onlining/offlining) of memory hot(un)plug.

--
Cheers,

David / dhildenb