Re: [PATCH 3/3] dax/kmem: Always enroll hotplugged memory for memmap_on_memory

[David Hildenbrand]

On 13.07.23 17:15, Verma, Vishal L wrote:
> On Thu, 2023-07-13 at 09:23 +0200, David Hildenbrand wrote:
> > On 13.07.23 08:45, Verma, Vishal L wrote:
> > > I'm taking a shot at implementing the splitting internally in
> > > memory_hotplug.c. The caller (kmem) side does become trivial with this
> > > approach, but there's a slight complication if I don't have the module
> > > param override (patch 1 of this series).
> > >
> > > The kmem diff now looks like:
> > >
> > >      diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
> > >      index 898ca9505754..8be932f63f90 100644
> > >      --- a/drivers/dax/kmem.c
> > >      +++ b/drivers/dax/kmem.c
> > >      @@ -105,6 +105,8 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
> > >              data->mgid = rc;
> > >       
> > >              for (i = 0; i < dev_dax->nr_range; i++) {
> > >      +               mhp_t mhp_flags = MHP_NID_IS_MGID | MHP_MEMMAP_ON_MEMORY |
> > >      +                                 MHP_SPLIT_MEMBLOCKS;
> > >                      struct resource *res;
> > >                      struct range range;
> > >       
> > >      @@ -141,7 +143,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
> > >                       * this as RAM automatically.
> > >                       */
> > >                      rc = add_memory_driver_managed(data->mgid, range.start,
> > >      -                               range_len(&range), kmem_name, MHP_NID_IS_MGID);
> > >      +                               range_len(&range), kmem_name, mhp_flags);
> > >       
> > >                      if (rc) {
> > >                              dev_warn(dev, "mapping%d: %#llx-%#llx memory add failed\n",
> > >      
> >
> > Why do we need the MHP_SPLIT_MEMBLOCKS?
>
> I thought we still wanted either an opt-in or opt-out for the kmem
> driver to be able to do memmap_on_memory, in case there were
> performance implications or the lack of 1GiB PUDs. I haven't
> implemented that yet, but I was thinking along the lines of a sysfs
> knob exposed by kmem, that controls setting of this new
> MHP_SPLIT_MEMBLOCKS flag.

Why is MHP_MEMMAP_ON_MEMORY not sufficient for that?

> > In add_memory_driver_managed(), if memmap_on_memory = 1 AND is effective for a
> > single memory block, you can simply split up internally, no?
> >
> > Essentially in add_memory_resource() something like
> >
> > if (mhp_flags & MHP_MEMMAP_ON_MEMORY &&
> >       mhp_supports_memmap_on_memory(memory_block_size_bytes())) {
> >          for (cur_start = start, cur_start < start + size;
> >               cur_start += memory_block_size_bytes()) {
> >                  mhp_altmap.free = PHYS_PFN(memory_block_size_bytes());
> >                  mhp_altmap.base_pfn = PHYS_PFN(start);
> >                  params.altmap = &mhp_altmap;
> >
> >                  ret = arch_add_memory(nid, start,
> >                                        memory_block_size_bytes(), &params);
> >                  if (ret < 0) ...
> >
> >                  ret = create_memory_block_devices(start, memory_block_size_bytes(),
> >                                                    mhp_altmap.alloc, group);
> >                  if (ret) ...
> >                  
> >          }
> > } else {
> >          /* old boring stuff */
> > }
> >
> > Of course, doing it a bit cleaner, factoring out adding of mem+creating devices into
> > a helper so we can use it on the other path, avoiding repeating memory_block_size_bytes()
> > ...
>
> My current approach was looping a level higher, on the call to
> add_memory_resource, but this looks reasonable too, and I can switch to
> this. In fact it is better as in my case I had to loop twice, once for
> the regular add_memory() path and once for the _driver_managed() path.
> Yours should avoid that.

As we only care about the altmap here, handling it for arch_add_memory() 
+ create_memory_block_devices() should be sufficient.

--
Cheers,

David / dhildenb