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

[Gerald Schaefer <gerald.schaefer@linux.ibm.com>]

On Thu, 16 Nov 2023 20:16:02 +0100
David Hildenbrand <david@redhat.com> wrote:

> 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.

Just for better understanding, are page tables for identity and also
vmemmap mapping not allocated from system memory on other archs? I.e.
no altmap support for that, only for struct pages (so far)?

> 
> > 
> > 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.

I would say it is special :-). And again, for understanding, all other
things apart from struct pages, still would need to be allocated from
system memory on other archs?

Of course, struct pages would be by far the biggest part, so having
altmap support for that helps a lot. Doing the other allocations also
via altmap would feel natural, but it is not possible yet, or did we
miss something?

> 
> > And with all the disadvantage of pre-allocating struct pages from system
> > memory.  
> 
> Jep. It never should have been done like that.

At that time, it gave the benefit over all others, that we do not need
to allocate struct pages from system memory, at the time of memory online,
when memory pressure might be high and such allocations might fail.

I guess you can say that it should have been done "right" at that time,
e.g. by already adding something like altmap support, instead of our own
hack.

> 
> > 
> > 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.

Ok, sounds reasonable, let's try that. Introducing some new s390-specific
interface also feels a bit hacky, or ugly, but we'll see if we find a
way so that it is only "special" :-)
Reminds me a bit of that "probe" attribute, that also was an arch-specific
hack initially, IIRC, and is now to be deprecated...