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

["Verma, Vishal L" <vishal.l.verma@intel.com>]

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.

> 
> 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.

> 
> If any adding of memory failed, we remove what we already added. That works, because as
> long as we're holding the relevant locks, memory cannot get onlined in the meantime.
> 
> Then we only have to teach remove_memory() to deal with individual blocks when finding
> blocks that have an altmap.
>