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Date:   Thu, 16 Nov 2023 20:16:02 +0100
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Subject: Re: [PATCH 6/8] s390/mm: implement MEM_PHYS_ONLINE MEM_PHYS_OFFLINE
 memory notifiers
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To:     Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc:     linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        linux-s390 <linux-s390@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20231114180238.1522782-1-sumanthk@linux.ibm.com>
 <20231114180238.1522782-7-sumanthk@linux.ibm.com>
 <458da84d-3838-4c5d-abda-1aebba676186@redhat.com>
 <ZVTTs2H6DgNGFPkF@li-2b55cdcc-350b-11b2-a85c-a78bff51fc11.ibm.com>
From:   David Hildenbrand <david@redhat.com>
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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