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 10:53:03PM +0200, David Hildenbrand wrote:
> On 03.06.24 12:43, Mike Rapoport wrote:
> > 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
> 
> I'll try once I understood what changed ever since you documented that in
> 2018 -- or if we missed that detail back then already.
 
> > > 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.
> 
> There are more details [1] but I also did not figure out why the
> memblock_free() was really required to resolve that issue.
> 
> [1] https://marc.info/?l=linux-kernel&m=142961156129456&w=2
 
The tinkering with memblock there and in f9126ab9241f seem bogus in the
context of memory hotplug on x86.

I believe that dropping that memblock_phys_free() is right thing to do
regardless of this series. There's no corresponding memblock_alloc() and it
was added as part of a fix for hotunplug on x86 that anyway had memblock
discarded at that point.

> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.

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