Re: collision between ZONE_MOVABLE and memblock allocations

[David Hildenbrand]

On 19.07.23 10:06, Michal Hocko wrote:
> On Wed 19-07-23 10:59:52, Mike Rapoport wrote:
> > On Wed, Jul 19, 2023 at 08:14:48AM +0200, Michal Hocko wrote:
> > > On Tue 18-07-23 16:01:06, Ross Zwisler wrote:
> > > [...]
> > > > I do think that we need to fix this collision between ZONE_MOVABLE and memmap
> > > > allocations, because this issue essentially makes the movablecore= kernel
> > > > command line parameter useless in many cases, as the ZONE_MOVABLE region it
> > > > creates will often actually be unmovable.
> > >
> > > movablecore is kinda hack and I would be more inclined to get rid of it
> > > rather than build more into it. Could you be more specific about your
> > > use case?
> > >
> > > > Here are the options I currently see for resolution:
> > > >
> > > > 1. Change the way ZONE_MOVABLE memory is allocated so that it is allocated from
> > > > the beginning of the NUMA node instead of the end. This should fix my use case,
> > > > but again is prone to breakage in other configurations (# of NUMA nodes, other
> > > > architectures) where ZONE_MOVABLE and memblock allocations might overlap.  I
> > > > think that this should be relatively straightforward and low risk, though.
> > > >
> > > > 2. Make the code which processes the movablecore= command line option aware of
> > > > the memblock allocations, and have it choose a region for ZONE_MOVABLE which
> > > > does not have these allocations. This might be done by checking for
> > > > PageReserved() as we do with offlining memory, though that will take some boot
> > > > time reordering, or we'll have to figure out the overlap in another way. This
> > > > may also result in us having two ZONE_NORMAL zones for a given NUMA node, with
> > > > a ZONE_MOVABLE section in between them.  I'm not sure if this is allowed?
> > >
> > > Yes, this is no problem. Zones are allowed to be sparse.
> >
> > The current initialization order is roughly
> >
> > * very early initialization with some memblock allocations
> > * determine zone locations and sizes
> > * initialize memory map	
> >    - memblock_alloc(lots of memory)
> > * lots of unrelated initializations that may allocate memory
> > * release free pages from memblock to the buddy allocator
> >
> > With 2) we can make sure the memory map and early allocations won't be in
> > the ZONE_MOVABLE, but we'll still may have reserved pages there.
>
> Yes this will always be fragile. If the spefic placement of the movable
> memory is not important and the only thing that matters is the size and
> numa locality then an easier to maintain solution would be to simply
> offline enough memory blocks very early in the userspace bring up and
> online it back as movable. If offlining fails just try another
> memblock. This doesn't require any kernel code change.

As an alternative, we might use the "memmap=nn[KMG]!ss[KMG]" [1] 
parameter to mark some memory as protected.

That memory can then be configured as devdax device and online to 
ZONE_MOVABLE (dev/dax).

[1] 
https://docs.pmem.io/persistent-memory/getting-started-guide/creating-development-environments/linux-environments/linux-memmap

--
Cheers,

David / dhildenb