Re: collision between ZONE_MOVABLE and memblock allocations

[Ross Zwisler <zwisler@google.com>]

On Wed, Jul 19, 2023 at 10:14:59AM +0200, David Hildenbrand wrote:
> 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

I've previously been reconfiguring devdax memory like this:

  ndctl create-namespace --reconfig=namespace0.0 -m devdax -f
  daxctl reconfigure-device --mode=system-ram dax0.0

Is this how you've been doing it, or is there something else I should
consider?

I just sent mail to Michal outlining my use case, hopefully it makes sense.

I had thought about using 'memmap=' in the first kernel and the worry was that
I'd have to support many different machines with different memory
configurations, and have to hard-code memory offsets and lengths for the
various memmap= kernel command line parameters.  If I can make ZONE_MOVABLE
work that's preferable because the kernel will choose the correct usermem-only
region for me, and then I can just use that region for the crash kernel and
3rd kernel boots.