linux-kernel - Re: [PATCH] arm64: Add memory hotplug support

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Message-ID: <20181211172418.GA29593@edgewater-inn.cambridge.arm.com>
Date:   Tue, 11 Dec 2018 17:24:19 +0000
From:   Will Deacon <will.deacon@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, jonathan.cameron@...wei.com,
        cyrilc@...inx.com, james.morse@....com, anshuman.khandual@....com
Subject: Re: [PATCH] arm64: Add memory hotplug support

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@....com>
> > > ---
> > > 
> > > 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.

Will