Re: [PATCH v2 1/5] mm: memory_hotplug: Memory hotplug (add) support for arm64

[Maciej Bielski]

Hi Robin,

Thank you for your feedback, its highly appreciated. I let myself to add some
comments.

Our primary goal was to have hotplug working even in the basic setup and
publish first working results. Then we want to improve the code building on top
of community comments. This is a general answer for questions about
configuration flags. The working setup is presented, a bit as a hint, and we do
not deem it to be ultimately best at all. The questions about configuration,
IMHO, falls into category of making an agreement on a proper setup (defaults,
dependencies) and, therefore, we strongly rely on the community experience to
advise us how it should be. So, shortly, for some questions "why this is setup
in such a way" the simple anser is that it worked as a first approximation.
Then, I totally agree that for a server-grade system it should be different and
thanks a lot for sharing your opinion on that.

On Mon, Nov 27, 2017 at 03:19:49PM +0000, Robin Murphy wrote:
> Hi Andrea,
> 
> I've also been looking at memory hotplug for arm64, from the perspective of
> enabling ZONE_DEVICE for pmem. May I ask what your use-case for this series
> is? AFAICS the real demand will be coming from server systems, which in
> practice means both ACPI and NUMA, both of which are being resoundingly
> ignored here.
> 

Eventually we aim for aarch64 server system.

> Further review comments inline.
> 
> On 23/11/17 11:13, Maciej Bielski wrote:
> >Introduces memory hotplug functionality (hot-add) for arm64.
> >
> >Changes v1->v2:
> >- swapper pgtable updated in place on hot add, avoiding unnecessary copy:
> >   all changes are additive and non destructive.
> >
> >- stop_machine used to updated swapper on hot add, avoiding races
> >
> >- checking if pagealloc is under debug to stay coherent with mem_map
> >
> >Signed-off-by: Maciej Bielski <m.bielski@xxxxxxxxxxxxxxxxxxxxxx>
> >Signed-off-by: Andrea Reale <ar@xxxxxxxxxxxxxxxxxx>
> >---
> >  arch/arm64/Kconfig           | 12 ++++++
> >  arch/arm64/configs/defconfig |  1 +
> >  arch/arm64/include/asm/mmu.h |  3 ++
> >  arch/arm64/mm/init.c         | 87 ++++++++++++++++++++++++++++++++++++++++++++
> >  arch/arm64/mm/mmu.c          | 39 ++++++++++++++++++++
> >  5 files changed, 142 insertions(+)
> >
> >diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> >index 0df64a6..c736bba 100644
> >--- a/arch/arm64/Kconfig
> >+++ b/arch/arm64/Kconfig
> >@@ -641,6 +641,14 @@ config HOTPLUG_CPU
> >  	  Say Y here to experiment with turning CPUs off and on.  CPUs
> >  	  can be controlled through /sys/devices/system/cpu.
> >+config ARCH_HAS_ADD_PAGES
> >+	def_bool y
> >+	depends on ARCH_ENABLE_MEMORY_HOTPLUG
> >+
> >+config ARCH_ENABLE_MEMORY_HOTPLUG
> >+	def_bool y
> >+    depends on !NUMA
> 
> As above, realistically this seems too limiting to be useful.
> 
> >+
> >  # Common NUMA Features
> >  config NUMA
> >  	bool "Numa Memory Allocation and Scheduler Support"
> >@@ -715,6 +723,10 @@ config ARCH_HAS_CACHE_LINE_SIZE
> >  source "mm/Kconfig"
> >+config ARCH_MEMORY_PROBE
> >+	def_bool y
> >+	depends on MEMORY_HOTPLUG
> 
> I'm particularly dubious about enabling this by default - it's useful for
> development and testing, yes, but I think it's the kind of feature where the
> onus should be on interested developers to turn it on, rather than
> production configs to have to turn it off.
> 
> >+
> >  config SECCOMP
> >  	bool "Enable seccomp to safely compute untrusted bytecode"
> >  	---help---
> >diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
> >index 34480e9..5fc5656 100644
> >--- a/arch/arm64/configs/defconfig
> >+++ b/arch/arm64/configs/defconfig
> >@@ -80,6 +80,7 @@ CONFIG_ARM64_VA_BITS_48=y
> >  CONFIG_SCHED_MC=y
> >  CONFIG_NUMA=y
> >  CONFIG_PREEMPT=y
> >+CONFIG_MEMORY_HOTPLUG=y
> 
> Note that this is effectively pointless, given two lines above...
> 
> >  CONFIG_KSM=y
> >  CONFIG_TRANSPARENT_HUGEPAGE=y
> >  CONFIG_CMA=y
> >diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> >index 0d34bf0..2b3fa4d 100644
> >--- a/arch/arm64/include/asm/mmu.h
> >+++ b/arch/arm64/include/asm/mmu.h
> >@@ -40,5 +40,8 @@ extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
> >  			       pgprot_t prot, bool page_mappings_only);
> >  extern void *fixmap_remap_fdt(phys_addr_t dt_phys);
> >  extern void mark_linear_text_alias_ro(void);
> >+#ifdef CONFIG_MEMORY_HOTPLUG
> >+extern void hotplug_paging(phys_addr_t start, phys_addr_t size);
> 
> Is there any reason for not just implementing all the hotplug code
> self-contained in mmu.c?
> 

Simply, in the first version we were supposed to built on top of the patch by
Scott Branden, who put a mock implementation of arch_add_memory() in
arch/arm64/mm/init.c, this is why hotplug_paging() needed to be announced
outside. Quickly looking on the code now I agree that it would be more clean to
put everything in arch/arm64/mm/mmu.c. I will test that.

> >+#endif
> >  #endif
> >diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> >index 5960bef..e96e7d3 100644
> >--- a/arch/arm64/mm/init.c
> >+++ b/arch/arm64/mm/init.c
> >@@ -722,3 +722,90 @@ static int __init register_mem_limit_dumper(void)
> >  	return 0;
> >  }
> >  __initcall(register_mem_limit_dumper);
> >+
> >+#ifdef CONFIG_MEMORY_HOTPLUG
> >+int add_pages(int nid, unsigned long start_pfn,
> >+		unsigned long nr_pages, bool want_memblock)
> >+{
> >+	int ret;
> >+	u64 start_addr = start_pfn << PAGE_SHIFT;
> >+	/*
> >+	 * Mark the first page in the range as unusable. This is needed
> >+	 * because __add_section (within __add_pages) wants pfn_valid
> >+	 * of it to be false, and in arm64 pfn falid is implemented by
> >+	 * just checking at the nomap flag for existing blocks.
> >+	 *
> >+	 * A small trick here is that __add_section() requires only
> >+	 * phys_start_pfn (that is the first pfn of a section) to be
> >+	 * invalid. Regardless of whether it was assumed (by the function
> >+	 * author) that all pfns within a section are either all valid
> >+	 * or all invalid, it allows to avoid looping twice (once here,
> >+	 * second when memblock_clear_nomap() is called) through all
> >+	 * pfns of the section and modify only one pfn. Thanks to that,
> >+	 * further, in __add_zone() only this very first pfn is skipped
> >+	 * and corresponding page is not flagged reserved. Therefore it
> >+	 * is enough to correct this setup only for it.
> >+	 *
> >+	 * When arch_add_memory() returns the walk_memory_range() function
> >+	 * is called and passed with online_memory_block() callback,
> >+	 * which execution finally reaches the memory_block_action()
> >+	 * function, where also only the first pfn of a memory block is
> >+	 * checked to be reserved. Above, it was first pfn of a section,
> >+	 * here it is a block but
> >+	 * (drivers/base/memory.c):
> >+	 *     sections_per_block = block_sz / MIN_MEMORY_BLOCK_SIZE;
> >+	 * (include/linux/memory.h):
> >+	 *     #define MIN_MEMORY_BLOCK_SIZE     (1UL << SECTION_SIZE_BITS)
> >+	 * so we can consider block and section equivalently
> >+	 */
> >+	memblock_mark_nomap(start_addr, 1<<PAGE_SHIFT);
> >+	ret = __add_pages(nid, start_pfn, nr_pages, want_memblock);
> >+
> >+	/*
> >+	 * Make the pages usable after they have been added.
> >+	 * This will make pfn_valid return true
> >+	 */
> >+	memblock_clear_nomap(start_addr, 1<<PAGE_SHIFT);
> >+
> >+	/*
> >+	 * This is a hack to avoid having to mix arch specific code
> >+	 * into arch independent code. SetPageReserved is supposed
> >+	 * to be called by __add_zone (within __add_section, within
> >+	 * __add_pages). However, when it is called there, it assumes that
> >+	 * pfn_valid returns true.  For the way pfn_valid is implemented
> >+	 * in arm64 (a check on the nomap flag), the only way to make
> >+	 * this evaluate true inside __add_zone is to clear the nomap
> >+	 * flags of blocks in architecture independent code.
> >+	 *
> >+	 * To avoid this, we set the Reserved flag here after we cleared
> >+	 * the nomap flag in the line above.
> >+	 */
> >+	SetPageReserved(pfn_to_page(start_pfn));
> 
> This whole business is utterly horrible. I really think we need to revisit
> why arm64 isn't using the normal sparsemem pfn_valid() implementation. If
> there are callers misusing pfn_valid() where they really want page_is_ram()
> or similar, or missing further pfn_valid_within() checks, then it's surely
> time to fix those at the source rather than adding to the Jenga pile of
> hacks in this area. I've started digging into it myself, but don't have any
> answers yet.
> 

I fully agree and this is the exact reaction we hoped for. We just decided to
avoid opening too many fronts at the same time, also that we were not
completely sure what exactly the pfn_valid() is supposed to serve for and what
we can potentially break. We are looking for your findings here.

> >+
> >+	return ret;
> >+}
> >+
> >+int arch_add_memory(int nid, u64 start, u64 size, bool want_memblock)
> >+{
> >+	int ret;
> >+	unsigned long start_pfn = start >> PAGE_SHIFT;
> >+	unsigned long nr_pages = size >> PAGE_SHIFT;
> >+	unsigned long end_pfn = start_pfn + nr_pages;
> >+	unsigned long max_sparsemem_pfn = 1UL << (MAX_PHYSMEM_BITS-PAGE_SHIFT);
> >+
> >+	if (end_pfn > max_sparsemem_pfn) {
> >+		pr_err("end_pfn too big");
> >+		return -1;
> >+	}
> >+	hotplug_paging(start, size);
> >+
> >+	ret = add_pages(nid, start_pfn, nr_pages, want_memblock);
> >+
> >+	if (ret)
> >+		pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
> >+			__func__, ret);
> >+
> >+	return ret;
> >+}
> >+
> >+#endif /* CONFIG_MEMORY_HOTPLUG */
> >diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> >index f1eb15e..d93043d 100644
> >--- a/arch/arm64/mm/mmu.c
> >+++ b/arch/arm64/mm/mmu.c
> >@@ -28,6 +28,7 @@
> >  #include <linux/mman.h>
> >  #include <linux/nodemask.h>
> >  #include <linux/memblock.h>
> >+#include <linux/stop_machine.h>
> >  #include <linux/fs.h>
> >  #include <linux/io.h>
> >  #include <linux/mm.h>
> >@@ -615,6 +616,44 @@ void __init paging_init(void)
> >  		      SWAPPER_DIR_SIZE - PAGE_SIZE);
> >  }
> >+#ifdef CONFIG_MEMORY_HOTPLUG
> >+
> >+/*
> >+ * hotplug_paging() is used by memory hotplug to build new page tables
> >+ * for hot added memory.
> >+ */
> >+
> >+struct mem_range {
> >+	phys_addr_t base;
> >+	phys_addr_t size;
> >+};
> >+
> >+static int __hotplug_paging(void *data)
> >+{
> >+	int flags = 0;
> >+	struct mem_range *section = data;
> >+
> >+	if (debug_pagealloc_enabled())
> >+		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> >+
> >+	__create_pgd_mapping(swapper_pg_dir, section->base,
> >+			__phys_to_virt(section->base), section->size,
> >+			PAGE_KERNEL, pgd_pgtable_alloc, flags);
> >+
> >+	return 0;
> >+}
> >+
> >+inline void hotplug_paging(phys_addr_t start, phys_addr_t size)
> >+{
> >+	struct mem_range section = {
> >+		.base = start,
> >+		.size = size,
> >+	};
> >+
> >+	stop_machine(__hotplug_paging, &section, NULL);
> 
> Why exactly do we need to swing the stop_machine() hammer here? I appreciate
> that separate hotplug events for adjacent sections could potentially affect
> the same top-level entry in swapper_pg_dir, but those should already be
> serialised by the hotplug lock - who else has cause to modify non-leaf
> entries for the linear map at runtime in a manner which might conflict?
> 

The reason for this has been mentioned by Mark Rutland in the previous spin
(https://lkml.org/lkml/2017/4/11/582), please let us know if you have different
point of view.


BR,
Maciej Bielski

> Robin.
> 
> >+}
> >+#endif /* CONFIG_MEMORY_HOTPLUG */
> >+
> >  /*
> >   * Check whether a kernel address is valid (derived from arch/x86/).
> >   */
> >