Re: [RFC PATCH v3 1/4] mm,memory_hotplug: Add mhp_supports_memmap_on_memory

[Oscar Salvador]

On Wed, Dec 02, 2020 at 10:36:54AM +0100, David Hildenbrand wrote:
> Instead of adding these arch callbacks, what about a config option
> 
> ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
> 
> that gets selected by the archs with CONFIG_SPARSEMEM_VMEMMAP ?
> 
> The mhp_supports_memmap_on_memory() becomes even more trivial.

I think that would not be enough.
E.g: s390x supports CONFIG_SPARSEMEM_VMEMMAP but it does not support
altmap (and maybe other arches I did not check too).
That is why I was careful in choosing the ones that a) supports
CONFIG_SPARSEMEM_VMEMMAP and b) support altmap

> > Note that mhp_memmap_on_memory kernel boot option will be added in
> > a coming patch.
> 
> I think it makes sense to
> 
> a) separate off the arch changes into separate patches, clarifying why
> it can be used. Move this patches to the end of the series.
> 
> b) Squashing the remainings into patch #2

Ok, I can do that.

> > +/*
> > + * We want memmap (struct page array) to be self contained.
> > + * To do so, we will use the beginning of the hot-added range to build
> > + * the page tables for the memmap array that describes the entire range.
> > + * Only selected architectures support it with SPARSE_VMEMMAP.
> 
> You might want to add how the caller can calculate the necessary size
> and that that this calculated piece of memory to be added will be
> accessed before onlining these pages. This is e.g., relevant if
> virtio-mem, the hyper-v balloon, or xen balloon would want to use this
> mechanism. Also, it's somewhat incompatible with standby memory where
> memory cannot be accessed prior to onlining. So pointing that access out
> might be valuable.

Sure, I will be more verbose.

> You can simplify to
> 
> return arch_support_memmap_on_memory() &&
>        IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
>        size == memory_block_size_bytes();

Yeah, thanks ;-)

-- 
Oscar Salvador
SUSE L3