Re: [PATCH v3 1/5] mm/memory_hotplug: introduce MEM_PREPARE_ONLINE/MEM_FINISH_OFFLINE notifiers

[David Hildenbrand]

On 27.11.23 09:20, Sumanth Korikkar wrote:
> Introduce  MEM_PREPARE_ONLINE/MEM_FINISH_OFFLINE memory notifiers to
> prepare the transition of memory to and from a physically accessible
> state. This enhancement is crucial for implementing the "memmap on
> memory" feature for s390 in a subsequent patch.
>
> Platforms such as x86 can support physical memory hotplug via ACPI. When
> there is physical memory hotplug, ACPI event leads to the memory
> addition with the following callchain:
> acpi_memory_device_add()
>    -> acpi_memory_enable_device()
>       -> __add_memory()
>
> After this, the hotplugged memory is physically accessible, and altmap
> support prepared, before the "memmap on memory" initialization in
> memory_block_online() is called.
>
> On s390, memory hotplug works in a different way. The available hotplug
> memory has to be defined upfront in the hypervisor, but it is made
> physically accessible only when the user sets it online via sysfs,
> currently in the MEM_GOING_ONLINE notifier. This is too late and "memmap
> on memory" initialization is performed before calling MEM_GOING_ONLINE
> notifier.
>
> During the memory hotplug addition phase, altmap support is prepared and
> during the memory onlining phase s390 requires memory to be physically
> accessible and then subsequently initiate the "memmap on memory"
> initialization process.
>
> The memory provider will handle new MEM_PREPARE_ONLINE /
> MEM_FINISH_OFFLINE notifications and make the memory accessible.
>
> The mhp_flag MHP_OFFLINE_INACCESSIBLE is introduced and is relevant when
> used along with MHP_MEMMAP_ON_MEMORY, because the altmap cannot be
> written (e.g., poisoned) when adding memory -- before it is set online.
> This allows for adding memory with an altmap that is not currently made
> available by a hypervisor. When onlining that memory, the hypervisor can
> be instructed to make that memory accessible via the new notifiers and
> the onlining phase will not require any memory allocations, which is
> helpful in low-memory situations.
>
> All architectures ignore unknown memory notifiers.  Therefore, the
> introduction of these new notifiers does not result in any functional
> modifications across architectures.
>
> Suggested-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Sumanth Korikkar <sumanthk@xxxxxxxxxxxxx>
> ---
>   drivers/base/memory.c          | 21 +++++++++++++++++++--
>   include/linux/memory.h         |  2 ++
>   include/linux/memory_hotplug.h | 18 +++++++++++++++++-
>   include/linux/memremap.h       |  1 +
>   mm/memory_hotplug.c            | 30 ++++++++++++++++++++++++++++--
>   mm/sparse.c                    |  3 ++-
>   6 files changed, 69 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index 8a13babd826c..5c6b2af75db4 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -188,6 +188,7 @@ static int memory_block_online(struct memory_block *mem)
>   	unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
>   	unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
>   	unsigned long nr_vmemmap_pages = 0;
> +	struct memory_notify arg;
>   	struct zone *zone;
>   	int ret;
>   
> @@ -197,6 +198,14 @@ static int memory_block_online(struct memory_block *mem)
>   	zone = zone_for_pfn_range(mem->online_type, mem->nid, mem->group,
>   				  start_pfn, nr_pages);
>   
> +	arg.start_pfn = start_pfn;
> +	arg.nr_pages = nr_pages;


Thinking about it, it's possibly cleanest to send the altmap range along 
here.

Later memory onlining notifiers will only be notified about the actual 
!altmap part.

arg.altmap_start_pfn = start_pfn;
arg.altmap_nr_pages = nr_vmemmap_pages;
arg.start_pfn = start_pfn + nr_vmemmap_pages;
arg.nr_pages = nr_pages - nr_vmemmap_pages;

Use that for the two new notifiers only and document it. Thoughts?

--
Cheers,

David / dhildenb