Re: [RFC PATCH 8/8] HACK: mm: memory_hotplug: Drop memblock_phys_free() call in try_remove_memory()

[Mike Rapoport <rppt@kernel.org>]

On Mon, Jun 03, 2024 at 11:14:00AM +0200, David Hildenbrand wrote:
> On 03.06.24 09:57, Mike Rapoport wrote:
> > On Fri, May 31, 2024 at 09:49:32AM +0200, David Hildenbrand wrote:
> > > On 29.05.24 19:12, Jonathan Cameron wrote:
> > > > I'm not sure what this is balancing, but it if is necessary then the reserved
> > > > memblock approach can't be used to stash NUMA node assignments as after the
> > > > first add / remove cycle the entry is dropped so not available if memory is
> > > > re-added at the same HPA.
> > > > 
> > > > This patch is here to hopefully spur comments on what this is there for!
> > > > 
> > > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > > ---
> > > >    mm/memory_hotplug.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > > > index 431b1f6753c0..3d8dd4749dfc 100644
> > > > --- a/mm/memory_hotplug.c
> > > > +++ b/mm/memory_hotplug.c
> > > > @@ -2284,7 +2284,7 @@ static int __ref try_remove_memory(u64 start, u64 size)
> > > >    	}
> > > >    	if (IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK)) {
> > > > -		memblock_phys_free(start, size);
> > > > +		//		memblock_phys_free(start, size);
> > > >    		memblock_remove(start, size);
> > > >    	}
> > > 
> > > memblock_phys_free() works on memblock.reserved, memblock_remove() works  on
> > > memblock.memory.
> > > 
> > > If you take a look at the doc at the top of memblock.c:
> > > 
> > > memblock.memory: physical memory available to the system
> > > memblock.reserved: regions that were allocated [during boot]
> > > 
> > > 
> > > memblock.memory is supposed to be a superset of memblock.reserved. Your
> > 
> > No it's not.
> > memblock.reserved is more of "if there is memory, don't touch it".
> 
> Then we should certainly clarify that in the comments! :P

You are welcome to send a patch :-P
 
> But for the memory hotunplug case, that's most likely why that code was
> added. And it only deals with ordinary system RAM, not weird reservations
> you describe below.

The commit that added memblock_free() at the first place (f9126ab9241f
("memory-hotplug: fix wrong edge when hot add a new node")) does not really
describe why that was required :(

But at a quick glance it looks completely spurious.
 
> > Some regions in memblock.reserved are boot time allocations and they are indeed a
> > subset of memblock.memory, but some are reservations done by firmware (e.g.
> > reserved memory in DT) that just might not have a corresponding regions in
> > memblock.memory. It can happen for example, when the same firmware runs on
> > devices with different memory configuration, but still wants to preserve
> > some physical addresses.
> 
> Could this happen with a good old BIOS as well? Just curious.
 
Yes. E.g. for E820_TYPE_SOFT_RESERVED.
 
> > > "hack" here indicates that you somehow would be relying on the opposite
> > > being true, which indicates that you are doing the wrong thing.
> > I'm not sure about that, I still have to digest the patches :)
> 
> In any case, using "reserved" to store persistent data across plug/unplug
> sounds wrong; but maybe I'm wrong :)
> 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.