Re: [PATCH v3 4/5] s390/mm: implement MEM_PREPARE_ONLINE/MEM_FINISH_OFFLINE notifiers

[David Hildenbrand]

On 27.11.23 09:20, Sumanth Korikkar wrote:
> MEM_PREPARE_ONLINE memory notifier makes memory block physical
> accessible via sclp assign command. The notifier ensures self-contained
> memory maps are accessible and hence enabling the "memmap on memory" on
> s390.
>
> MEM_FINISH_OFFLINE memory notifier shifts the memory block to an
> inaccessible state via sclp unassign command.
>
> Implementation considerations:
> * When MHP_MEMMAP_ON_MEMORY is disabled, the system retains the old
>    behavior. This means the memory map is allocated from default memory.
> * If MACHINE_HAS_EDAT1 is unavailable, MHP_MEMMAP_ON_MEMORY is
>    automatically disabled. This ensures that vmemmap pagetables do not
>    consume additional memory from the default memory allocator.
> * The MEM_GOING_ONLINE notifier has been modified to perform no
>    operation, as MEM_PREPARE_ONLINE already executes the sclp assign
>    command.
> * The MEM_CANCEL_ONLINE/MEM_OFFLINE notifier now performs no operation, as
>    MEM_FINISH_OFFLINE already executes the sclp unassign command.
>
> Reviewed-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
> Signed-off-by: Sumanth Korikkar <sumanthk@xxxxxxxxxxxxx>
> ---
>   drivers/s390/char/sclp_cmd.c | 28 ++++++++++++++++++++++------
>   1 file changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/s390/char/sclp_cmd.c b/drivers/s390/char/sclp_cmd.c
> index 355e63e44e95..30b829e4c052 100644
> --- a/drivers/s390/char/sclp_cmd.c
> +++ b/drivers/s390/char/sclp_cmd.c
> @@ -18,6 +18,7 @@
>   #include <linux/mm.h>
>   #include <linux/mmzone.h>
>   #include <linux/memory.h>
> +#include <linux/memory_hotplug.h>
>   #include <linux/module.h>
>   #include <asm/ctlreg.h>
>   #include <asm/chpid.h>
> @@ -26,6 +27,7 @@
>   #include <asm/sclp.h>
>   #include <asm/numa.h>
>   #include <asm/facility.h>
> +#include <asm/page-states.h>
>   
>   #include "sclp.h"
>   
> @@ -319,6 +321,7 @@ static bool contains_standby_increment(unsigned long start, unsigned long end)
>   static int sclp_mem_notifier(struct notifier_block *nb,
>   			     unsigned long action, void *data)
>   {
> +	struct memory_block *memory_block;
>   	unsigned long start, size;
>   	struct memory_notify *arg;
>   	unsigned char id;
> @@ -340,18 +343,29 @@ static int sclp_mem_notifier(struct notifier_block *nb,
>   		if (contains_standby_increment(start, start + size))
>   			rc = -EPERM;
>   		break;
> -	case MEM_GOING_ONLINE:
> +	case MEM_PREPARE_ONLINE:
> +		memory_block = find_memory_block(pfn_to_section_nr(arg->start_pfn));
> +		if (!memory_block) {
> +			rc = -EINVAL;
> +			goto out;
> +		}
>   		rc = sclp_mem_change_state(start, size, 1);
> +		if (rc || !memory_block->altmap)
> +			goto out;
> +		/*
> +		 * Set CMMA state to nodat here, since the struct page memory
> +		 * at the beginning of the memory block will not go through the
> +		 * buddy allocator later.
> +		 */
> +		__arch_set_page_nodat((void *)__va(start), memory_block->altmap->free);

Looking up the memory block and grabbing the altmap from there is a bit 
unfortunate.

Why can't we do that when adding the altmap? Will the hypervisor scream 
at us?

... would we want to communicate any altmap start+size via the memory 
notifier instead?

--
Cheers,

David / dhildenb