Re: [PATCH] arm64: Add memory hotplug support

From: Robin Murphy
Date: Tue Dec 11 2018 - 12:37:50 EST

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.


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
def_bool y
+ 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;
+ 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

return memblock_is_map_memory(addr);
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;
+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.