Re: [PATCH] arm64: Add memory hotplug support

[Robin Murphy]

On 11/12/2018 17:24, Will Deacon wrote:
> On Tue, Dec 11, 2018 at 05:21:24PM +0000, Robin Murphy wrote:
> > On 11/12/2018 16:36, Will Deacon wrote:
> > > On Mon, Dec 10, 2018 at 03:29:01PM +0000, Robin Murphy wrote:
> > > > Wire up the basic support for hot-adding memory. Since memory hotplug
> > > > is fairly tightly coupled to sparsemem, we tweak pfn_valid() to also
> > > > cross-check the presence of a section in the manner of the generic
> > > > implementation, before falling back to memblock to check for no-map
> > > > regions within a present section as before. By having arch_add_memory(()
> > > > create the linear mapping first, this then makes everything work in the
> > > > way that __add_section() expects.
> > > >
> > > > We expect hotplug to be ACPI-driven, so the swapper_pg_dir updates
> > > > should be safe from races by virtue of the global device hotplug lock.
> > > >
> > > > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> > > > ---
> > > >
> > > > Looks like I'm not going to have the whole pte_devmap story figured out
> > > > in time to land any ZONE_DEVICE support this cycle, but since this patch
> > > > also stands alone as a complete feature (and has ended up remarkably
> > > > simple and self-contained), I hope we might consider getting it merged
> > > > on its own merit.
> > > >
> > > > Robin.
> > > >
> > > >    arch/arm64/Kconfig   |  3 +++
> > > >    arch/arm64/mm/init.c |  8 ++++++++
> > > >    arch/arm64/mm/mmu.c  | 12 ++++++++++++
> > > >    arch/arm64/mm/numa.c | 10 ++++++++++
> > > >    4 files changed, 33 insertions(+)
> > > >
> > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > index 6d2b25f51bb3..7b855ae45747 100644
> > > > --- a/arch/arm64/Kconfig
> > > > +++ b/arch/arm64/Kconfig
> > > > @@ -261,6 +261,9 @@ config ZONE_DMA32
> > > >    config HAVE_GENERIC_GUP
> > > >    	def_bool y
> > > > +config ARCH_ENABLE_MEMORY_HOTPLUG
> > > > +	def_bool y
> > > > +
> > > >    config SMP
> > > >    	def_bool y
> > > > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > > > index 2983e0fc1786..82e0b08f2e31 100644
> > > > --- a/arch/arm64/mm/init.c
> > > > +++ b/arch/arm64/mm/init.c
> > > > @@ -291,6 +291,14 @@ int pfn_valid(unsigned long pfn)
> > > >    	if ((addr >> PAGE_SHIFT) != pfn)
> > > >    		return 0;
> > > > +
> > > > +#ifdef CONFIG_SPARSEMEM
> > > > +	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> > > > +		return 0;
> > > > +
> > > > +	if (!valid_section(__nr_to_section(pfn_to_section_nr(pfn))))
> > > > +		return 0;
> > >
> > > I'm a bit nervous about the call to __nr_to_section() here. How do we
> > > ensure that the section number we're passing stays within the bounds of
> > > the mem_section array?
> >
> > The same way every other sparsemem user (apart from arch/arm) does, I guess
> > - this is literally a copy-paste of the generic pfn_valid() implementation
> > :/
>
> I don't trust the generic pfn_valid() at all :)
>
> > Given the implementation of __nr_to_section() respective of how
> > memory_present() and sparse_index_init() set up mem_section in the first
> > place, I can't see how there can be a problem. You did see the bit 4 lines
> > above, right?
>
> D'oh, yes, I read that and then instantly forgot it. Ok, so that should be
> fine.
>
> > > > +#endif
> > > >    	return memblock_is_map_memory(addr);
> > > >    }
> > > >    EXPORT_SYMBOL(pfn_valid);
> > > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > > > index e1b2d58a311a..22379a74d289 100644
> > > > --- a/arch/arm64/mm/mmu.c
> > > > +++ b/arch/arm64/mm/mmu.c
> > > > @@ -1044,3 +1044,15 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
> > > >    	pmd_free(NULL, table);
> > > >    	return 1;
> > > >    }
> > > > +
> > > > +#ifdef CONFIG_MEMORY_HOTPLUG
> > > > +int arch_add_memory(int nid, u64 start, u64 size, struct vmem_altmap *altmap,
> > > > +		    bool want_memblock)
> > > > +{
> > > > +	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
> > > > +			     size, PAGE_KERNEL, pgd_pgtable_alloc, 0);
> > > > +
> > > > +	return __add_pages(nid, start >> PAGE_SHIFT, size >> PAGE_SHIFT,
> > > > +			   altmap, want_memblock);
> > > > +}
> > >
> > > If we're mapping the new memory into the linear map, shouldn't we be
> > > respecting rodata_full and debug page alloc by forcing page granularity
> > > and tweaking the permissions?
> >
> > Bah, James mentioned debug_pagealloc long ago, and I did have a slight
> > nagging feeling that I was still missing something - yes, I need to fix the
> > flags for that case. I'm not sure about rodata_full (do you mean
> > STRICT_KERNEL_RWX?) since a section being added here won't contain kernel
> > text nor data, and I can't seem to find anywhere that rodata options affect
> > the linear mapping of plain free RAM.
>
> Ah, we've got code queued on for-next/core so that changing vmalloc()
> permissions makes the same changes to the linear map.

Gotcha, I see "arm64: mm: apply r/o permissions of VM areas to its 
linear alias as well" now, no wonder I couldn't find anything relevant 
in my rc3-based development branch. I'll rebase and add that case too.

Thanks,
Robin.