Re: collision between ZONE_MOVABLE and memblock allocations

[Mike Rapoport <rppt@kernel.org>]

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.

> > If
> > we can get it working, this seems like the most correct solution to me, but
> > also the most difficult and risky because it involves significant changes in
> > the code for memory setup at early boot.
> > 
> > Am I missing anything are there other solutions we should consider, or do you
> > have an opinion on which solution we should pursue?
> 
> If this really needs to be addressed than 2) is certainly a more robust
> approach.
> -- 
> Michal Hocko
> SUSE Labs

-- 
Sincerely yours,
Mike.