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[RFC PATCH] Memory hotplug support for arm64 platform

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  [RFC PATCH] Memory hotplug support for arm64 platform Maciej Bielski <m.bielski@virtualopensystems.com> - 2016-12-14 13:20 +0100
    Re: [RFC PATCH] Memory hotplug support for arm64 platform Xishi Qiu <qiuxishi@huawei.com> - 2016-12-15 07:30 +0100
      Re: [RFC PATCH] Memory hotplug support for arm64 platform Xishi Qiu <qiuxishi@huawei.com> - 2016-12-15 07:50 +0100
        Re: [RFC PATCH] Memory hotplug support for arm64 platform Andrea Reale <ar@linux.vnet.ibm.com> - 2016-12-15 19:40 +0100
          Re: [RFC PATCH] Memory hotplug support for arm64 platform Xishi Qiu <qiuxishi@huawei.com> - 2016-12-16 02:40 +0100
    Re: [RFC PATCH] Memory hotplug support for arm64 platform Scott Branden <scott.branden@broadcom.com> - 2016-12-20 20:20 +0100
      Re: [RFC PATCH] Memory hotplug support for arm64 platform Maciej Bielski <m.bielski@virtualopensystems.com> - 2016-12-21 10:50 +0100
        Re: [RFC PATCH] Memory hotplug support for arm64 platform Scott Branden <scott.branden@broadcom.com> - 2016-12-22 02:30 +0100

#1541874 — [RFC PATCH] Memory hotplug support for arm64 platform

This patch relates to the work previously announced in [1]. This builds on the
work by Scott Branden [2] and, henceforth, it needs to be applied on top of
Scott's patches [2]. Comments are very welcome.

Changes from the original patchset and known issues: 

- Compared to Scott's original patchset, this work adds the mapping of
  the new hotplugged pages into the kernel page tables. This is done by
  copying the old swapper_pg_dir over a new page, adding the new mappings,
  and then switching to the newly built pg_dir (see `hotplug_paging` in
  arch/arm64/mmu.c). There might be better ways to to this: suggestions
  are more than welcome.

- The stub function for `arch_remove_memory` has been removed for now; we
  are working in parallel on memory hot remove, and we plan to contribute
  it as a separate patch. 
  
- Corresponding Kconfig flags have been added;
  
- Note that this patch does not work when NUMA is enabled; in fact,
  the function `memory_add_physaddr_to_nid` does not have an
  implementation when the NUMA flag is on: this function is supposed to
  return the nid the hotplugged memory should be associated with. However
  it is not really clear to us  yet what the semantics of this function
  in the context of a NUMA system should be. A quick and dirty fix would
  be to always attach to the first available NUMA node.

- In arch/arm64/mm/init.c `arch_add_memory`, we are doing a hack with the
  nomap memory block flags to satisfy preconditions and postconditions of
  `__add_pages` and postconditions of `arch_add_memory`. Compared to
  memory hotplug implementation for other architectures, the "issue"
  seems to be in the implemenation of `pfn_valid`. Suggestions on how
  to cleanly avoid this hack are welcome.

This patchset can be tested by starting the kernel with the `mem=X` flag, where
X is less than the total available physical memory and has to be multiple of
MIN_MEMORY_BLOCK_SIZE. We also tested it on a customised version of QEMU
capable to emulate physical hotplug on arm64 platform.

To enable the feature the CONFIG_MEMORY_HOTPLUG compilation flag
needs to be set to true. Then, after memory is physically hotplugged,
the standard two steps to make it available (as also documented in
Documentation/memory-hotplug.txt) are:

(1) Notify memory hot-add
 	echo '0xYY000000' > /sys/devices/system/memory/probe

where 0xYY000000 is the first physical address of the new memory section.

(2) Online new memory block(s)
    echo online > /sys/devices/system/memory/memoryXXX/state

where XXX corresponds to the ids of newly added blocks.

Onlining can optionally be automatic at hot-add notification by enabling
the global flag:
	echo online > /sys/devices/system/memory/auto_online_blocks
or by setting the corresponding config flag in the kernel build.

Again, any comment is highly appreciated.

[1] https://lkml.org/lkml/2016/11/17/49
[2] https://lkml.org/lkml/2016/12/1/811

Signed-off-by: Maciej Bielski <m.bielski@virtualopensystems.com>
Signed-off-by: Andrea Reale <ar@linux.vnet.ibm.com>
---
 arch/arm64/Kconfig           |  4 +--
 arch/arm64/include/asm/mmu.h |  3 +++
 arch/arm64/mm/init.c         | 58 +++++++++++++++++++++++++++++++-------------
 arch/arm64/mm/mmu.c          | 24 ++++++++++++++++++
 include/linux/memblock.h     |  1 +
 mm/memblock.c                | 10 ++++++++
 6 files changed, 80 insertions(+), 20 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 2482fdd..bd8ddf2 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -577,9 +577,7 @@ config HOTPLUG_CPU
 	  can be controlled through /sys/devices/system/cpu.
 
 config ARCH_ENABLE_MEMORY_HOTPLUG
-	def_bool y
-
-config ARCH_ENABLE_MEMORY_HOTREMOVE
+    depends on !NUMA
 	def_bool y
 
 # Common NUMA Features
diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index 8d9fce0..2499745 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -36,5 +36,8 @@ extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
 			       unsigned long virt, phys_addr_t size,
 			       pgprot_t prot, bool allow_block_mappings);
 extern void *fixmap_remap_fdt(phys_addr_t dt_phys);
+#ifdef CONFIG_MEMORY_HOTPLUG
+extern void hotplug_paging(phys_addr_t start, phys_addr_t size);
+#endif
 
 #endif
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 687d087..a7c740e 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -544,37 +544,61 @@ int arch_add_memory(int nid, u64 start, u64 size, bool for_device)
 	struct zone *zone;
 	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);
+	unsigned long pfn;
 	int ret;
 
+	if (end_pfn > max_sparsemem_pfn) {
+		pr_err("end_pfn too big");
+		return -1;
+	}
+	hotplug_paging(start, size);
+
+	/*
+	 * Mark all the page range as unsuable.
+	 * This is needed because  __add_section (within __add_pages)
+	 * wants pfn_valid to be false, and in arm64 pfn falid is implemented
+	 * by just checking at the nomap flag for existing blocks
+	 */
+	memblock_mark_nomap(start, size);
+
 	pgdat = NODE_DATA(nid);
 
 	zone = pgdat->node_zones +
 		zone_for_memory(nid, start, size, ZONE_NORMAL, for_device);
 	ret = __add_pages(nid, zone, start_pfn, nr_pages);
 
-	if (ret)
-		pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
-			__func__, ret);
+	/*
+	 * Make the pages usable after they have been added.
+	 * This will make pfn_valid return true
+	 */
+	memblock_clear_nomap(start, size);
 
-	return ret;
-}
+	/*
+	 * 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.
+	 */
+	for (pfn = start_pfn; pfn < start_pfn + nr_pages; pfn++) {
+		if (!pfn_valid(pfn))
+			continue;
 
-#ifdef CONFIG_MEMORY_HOTREMOVE
-int arch_remove_memory(u64 start, u64 size)
-{
-	unsigned long start_pfn = start >> PAGE_SHIFT;
-	unsigned long nr_pages = size >> PAGE_SHIFT;
-	struct zone *zone;
-	int ret;
+		SetPageReserved(pfn_to_page(pfn));
+	}
 
-	zone = page_zone(pfn_to_page(start_pfn));
-	ret = __remove_pages(zone, start_pfn, nr_pages);
 	if (ret)
-		pr_warn("%s: Problem encountered in __remove_pages() ret=%d\n",
+		pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
 			__func__, ret);
 
 	return ret;
 }
 #endif
-#endif
 
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 05615a3..9efa7d1 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -493,6 +493,30 @@ 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.
+ */
+void hotplug_paging(phys_addr_t start, phys_addr_t size)
+{
+	phys_addr_t pgd_phys = pgd_pgtable_alloc();
+	pgd_t *pgd = pgd_set_fixmap(pgd_phys);
+
+	memcpy(pgd, swapper_pg_dir, PAGE_SIZE);
+
+	__create_pgd_mapping(pgd, start, __phys_to_virt(start), size,
+			PAGE_KERNEL, pgd_pgtable_alloc, false);
+
+	cpu_replace_ttbr1(__va(pgd_phys));
+	memcpy(swapper_pg_dir, pgd, PAGE_SIZE);
+	cpu_replace_ttbr1(swapper_pg_dir);
+
+	pgd_clear_fixmap();
+	memblock_free(pgd_phys, PAGE_SIZE);
+}
+#endif
+
 /*
  * Check whether a kernel address is valid (derived from arch/x86/).
  */
diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index 5b759c9..5f78257 100644
--- a/include/linux/memblock.h
+++ b/include/linux/memblock.h
@@ -92,6 +92,7 @@ int memblock_mark_hotplug(phys_addr_t base, phys_addr_t size);
 int memblock_clear_hotplug(phys_addr_t base, phys_addr_t size);
 int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
 int memblock_mark_nomap(phys_addr_t base, phys_addr_t size);
+int memblock_clear_nomap(phys_addr_t base, phys_addr_t size);
 ulong choose_memblock_flags(void);
 
 /* Low level functions */
diff --git a/mm/memblock.c b/mm/memblock.c
index 7608bc3..05e7676 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -814,6 +814,16 @@ int __init_memblock memblock_mark_nomap(phys_addr_t base, phys_addr_t size)
 }
 
 /**
+ * memblock_clear_nomap - Clear a flag of MEMBLOCK_NOMAP memory region
+ * @base: the base phys addr of the region
+ * @size: the size of the region
+ */
+int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
+{
+	return memblock_setclr_flag(base, size, 0, MEMBLOCK_NOMAP);
+}
+
+/**
  * __next_reserved_mem_region - next function for for_each_reserved_region()
  * @idx: pointer to u64 loop variable
  * @out_start: ptr to phys_addr_t for start address of the region, can be %NULL
-- 
2.7.4

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#1542513

On 2016/12/14 20:16, Maciej Bielski wrote:

> 
>  
> -#ifdef CONFIG_MEMORY_HOTREMOVE
> -int arch_remove_memory(u64 start, u64 size)
> -{
> -	unsigned long start_pfn = start >> PAGE_SHIFT;
> -	unsigned long nr_pages = size >> PAGE_SHIFT;
> -	struct zone *zone;
> -	int ret;
> +		SetPageReserved(pfn_to_page(pfn));
> +	}

Hi Maciej,

Why we need to set reserved here?
I think the new pages are already reserved in __add_zone() -> memmap_init_zone(), right?

Thanks,
Xishi Qiu

>  
> -	zone = page_zone(pfn_to_page(start_pfn));
> -	ret = __remove_pages(zone, start_pfn, nr_pages);
>  	if (ret)
> -		pr_warn("%s: Problem encountered in __remove_pages() ret=%d\n",
> +		pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
>  			__func__, ret);
>  
>  	return ret;
>  }
>  #endif
> -#endif
>  
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 05615a3..9efa7d1 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -493,6 +493,30 @@ 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.
> + */
> +void hotplug_paging(phys_addr_t start, phys_addr_t size)
> +{
> +	phys_addr_t pgd_phys = pgd_pgtable_alloc();
> +	pgd_t *pgd = pgd_set_fixmap(pgd_phys);
> +
> +	memcpy(pgd, swapper_pg_dir, PAGE_SIZE);
> +
> +	__create_pgd_mapping(pgd, start, __phys_to_virt(start), size,
> +			PAGE_KERNEL, pgd_pgtable_alloc, false);
> +
> +	cpu_replace_ttbr1(__va(pgd_phys));
> +	memcpy(swapper_pg_dir, pgd, PAGE_SIZE);
> +	cpu_replace_ttbr1(swapper_pg_dir);
> +
> +	pgd_clear_fixmap();
> +	memblock_free(pgd_phys, PAGE_SIZE);
> +}
> +#endif
> +
>  /*
>   * Check whether a kernel address is valid (derived from arch/x86/).
>   */
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index 5b759c9..5f78257 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -92,6 +92,7 @@ int memblock_mark_hotplug(phys_addr_t base, phys_addr_t size);
>  int memblock_clear_hotplug(phys_addr_t base, phys_addr_t size);
>  int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
>  int memblock_mark_nomap(phys_addr_t base, phys_addr_t size);
> +int memblock_clear_nomap(phys_addr_t base, phys_addr_t size);
>  ulong choose_memblock_flags(void);
>  
>  /* Low level functions */
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 7608bc3..05e7676 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -814,6 +814,16 @@ int __init_memblock memblock_mark_nomap(phys_addr_t base, phys_addr_t size)
>  }
>  
>  /**
> + * memblock_clear_nomap - Clear a flag of MEMBLOCK_NOMAP memory region
> + * @base: the base phys addr of the region
> + * @size: the size of the region
> + */
> +int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
> +{
> +	return memblock_setclr_flag(base, size, 0, MEMBLOCK_NOMAP);
> +}
> +
> +/**
>   * __next_reserved_mem_region - next function for for_each_reserved_region()
>   * @idx: pointer to u64 loop variable
>   * @out_start: ptr to phys_addr_t for start address of the region, can be %NULL

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#1542520

On 2016/12/15 14:18, Xishi Qiu wrote:

> On 2016/12/14 20:16, Maciej Bielski wrote:
> 
>>
>>  
>> -#ifdef CONFIG_MEMORY_HOTREMOVE
>> -int arch_remove_memory(u64 start, u64 size)
>> -{
>> -	unsigned long start_pfn = start >> PAGE_SHIFT;
>> -	unsigned long nr_pages = size >> PAGE_SHIFT;
>> -	struct zone *zone;
>> -	int ret;
>> +		SetPageReserved(pfn_to_page(pfn));
>> +	}
> 
> Hi Maciej,
> 
> Why we need to set reserved here?
> I think the new pages are already reserved in __add_zone() -> memmap_init_zone(), right?
> 

Hi Maciej,

The reason is as follows, right?

It's because that in memmap_init_zone() -> early_pfn_valid(), the new page is still
invalid, so we need to init it after memblock_clear_nomap()

So why not use __init_single_page() and set_pageblock_migratetype()?

> Thanks,
> Xishi Qiu
> 

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#1542975

Hi Xishi Qiu,

thanks for your comments. 

The short anwser to your question is the following.  As you hinted,
it is related to the way pfn_valid() is implemented in arm64 when
CONFIG_HAVE_ARCH_PFN_VALID is true (default), i.e., just a check for
the NOMAP flags on the corresponding memblocks.

Since arch_add_memory->__add_pages() expects pfn_valid() to return false
when it is first called, we mark corresponding memory blocks with NOMAP;
however, arch_add_memory->__add_pages()->__add_section()->__add_zone()
expects pfn_valid() to return true when, at the end of its body,
it cycles through pages to call SetPageReserved(). Since blocks are
marked with NOMAP, pages will not be reserved there, henceforth we
need to reserve them after we clear the NOMAP flag inside the body of
arch_add_memory. Having pages reserved at the end of arch_add_memory
is a preconditions for the upcoming onlining of memory blocks (see
memory_block_change_state()).

> It's because that in memmap_init_zone() -> early_pfn_valid(), the new page is still
> invalid, so we need to init it after memblock_clear_nomap()
> 
> So why not use __init_single_page() and set_pageblock_migratetype()?

About your comment on memmap_init_zone()->early_pfn_valid(), I think
that that particular check is never executed in the context of memory
hotplug; in fact, just before the call to early_pfn_valid(), the code
will jump to the `not_early` label, because the context == MEMMAP_HOPTLUG.

Now, the question would be: why there's no need for this hack in
implementation for memory hotplug for other architectures (e.g.,
x86)? The answer we have found lies in the following: as said above,
when CONFIG_HAVE_ARCH_PFN_VALID is true (default), the implementation
of pfn_valid for arm64 just checks the NOMAP flag on the memmblocks,
and returns true if corresponding memblock_add_node() has succeeded (it
seems it does not consider the sparse memory model structures created
by arm64_memory_present() and sparse_init()).

On the contrary, the implementation of pfn_valid() for other
architectures is defined in include/linux/mmzone.h. This
implemenation, among other things, checks for the
SECTION_HAS_MEM_MAP flag on section->section_mem_map. Now,
when we go to memory hotplug, this flag is actually set inside
__add_section()->sparse_add_one_section()->sparse_init_one_section(). This
happens before the call to __add_zone(), so that, when it is eventually
invoked, pfn_valid() would return true and pages would be correctly
reserved.  On the contrary, on arm64, it will keep returning false
because of the different implementation of pfn_valid().

Given the above, we believed it was cleaner to go for this little and
well-confined hack rather then possibly changing the logic of pfn_valid.

However, we are very open for discussion and suggestions for improvement.
In particular, any advices on how to effectively modify pfn_valid()
without breaking existing functionalities are very welcome.

Thanks,
Andrea

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#1543164

On 2016/12/16 2:31, Andrea Reale wrote:

> Hi Xishi Qiu,
> 
> thanks for your comments. 
> 
> The short anwser to your question is the following.  As you hinted,
> it is related to the way pfn_valid() is implemented in arm64 when
> CONFIG_HAVE_ARCH_PFN_VALID is true (default), i.e., just a check for
> the NOMAP flags on the corresponding memblocks.
> 
> Since arch_add_memory->__add_pages() expects pfn_valid() to return false
> when it is first called, we mark corresponding memory blocks with NOMAP;
> however, arch_add_memory->__add_pages()->__add_section()->__add_zone()
> expects pfn_valid() to return true when, at the end of its body,
> it cycles through pages to call SetPageReserved(). Since blocks are
> marked with NOMAP, pages will not be reserved there, henceforth we
> need to reserve them after we clear the NOMAP flag inside the body of
> arch_add_memory. Having pages reserved at the end of arch_add_memory
> is a preconditions for the upcoming onlining of memory blocks (see
> memory_block_change_state()).
> 
>> It's because that in memmap_init_zone() -> early_pfn_valid(), the new page is still
>> invalid, so we need to init it after memblock_clear_nomap()
>>
>> So why not use __init_single_page() and set_pageblock_migratetype()?
> 
> About your comment on memmap_init_zone()->early_pfn_valid(), I think
> that that particular check is never executed in the context of memory
> hotplug; in fact, just before the call to early_pfn_valid(), the code
> will jump to the `not_early` label, because the context == MEMMAP_HOPTLUG.
> 

Hi Andrea,

Thanks for your answer, I notice it now. I thought SetPageReserved() was
done in memmap_init_zone(), but the new kernel change this logic now, so
sorry for the noise.

Thanks,
Xishi Qiu

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#1545393

Hi Maciej,

I have applied that patch ontop of the patches I previously sent out
and tested.

It does recognized the memory in /proc/iomem but I get memory corruption 
of the original system RAM soon after.  It appears the page allocation 
gets corrupted.  I will try to dig into it further but if somebody else 
could try it out in their system to see what results they get it would help.

Regards,
  Scott

On 16-12-14 04:16 AM, Maciej Bielski wrote:
> This patch relates to the work previously announced in [1]. This builds on the
> work by Scott Branden [2] and, henceforth, it needs to be applied on top of
> Scott's patches [2]. Comments are very welcome.
>
> Changes from the original patchset and known issues:
>
> - Compared to Scott's original patchset, this work adds the mapping of
>   the new hotplugged pages into the kernel page tables. This is done by
>   copying the old swapper_pg_dir over a new page, adding the new mappings,
>   and then switching to the newly built pg_dir (see `hotplug_paging` in
>   arch/arm64/mmu.c). There might be better ways to to this: suggestions
>   are more than welcome.
>
> - The stub function for `arch_remove_memory` has been removed for now; we
>   are working in parallel on memory hot remove, and we plan to contribute
>   it as a separate patch.
>
> - Corresponding Kconfig flags have been added;
>
> - Note that this patch does not work when NUMA is enabled; in fact,
>   the function `memory_add_physaddr_to_nid` does not have an
>   implementation when the NUMA flag is on: this function is supposed to
>   return the nid the hotplugged memory should be associated with. However
>   it is not really clear to us  yet what the semantics of this function
>   in the context of a NUMA system should be. A quick and dirty fix would
>   be to always attach to the first available NUMA node.
>
> - In arch/arm64/mm/init.c `arch_add_memory`, we are doing a hack with the
>   nomap memory block flags to satisfy preconditions and postconditions of
>   `__add_pages` and postconditions of `arch_add_memory`. Compared to
>   memory hotplug implementation for other architectures, the "issue"
>   seems to be in the implemenation of `pfn_valid`. Suggestions on how
>   to cleanly avoid this hack are welcome.
>
> This patchset can be tested by starting the kernel with the `mem=X` flag, where
> X is less than the total available physical memory and has to be multiple of
> MIN_MEMORY_BLOCK_SIZE. We also tested it on a customised version of QEMU
> capable to emulate physical hotplug on arm64 platform.
>
> To enable the feature the CONFIG_MEMORY_HOTPLUG compilation flag
> needs to be set to true. Then, after memory is physically hotplugged,
> the standard two steps to make it available (as also documented in
> Documentation/memory-hotplug.txt) are:
>
> (1) Notify memory hot-add
>  	echo '0xYY000000' > /sys/devices/system/memory/probe
>
> where 0xYY000000 is the first physical address of the new memory section.
>
> (2) Online new memory block(s)
>     echo online > /sys/devices/system/memory/memoryXXX/state
>
> where XXX corresponds to the ids of newly added blocks.
>
> Onlining can optionally be automatic at hot-add notification by enabling
> the global flag:
> 	echo online > /sys/devices/system/memory/auto_online_blocks
> or by setting the corresponding config flag in the kernel build.
>
> Again, any comment is highly appreciated.
>
> [1] https://lkml.org/lkml/2016/11/17/49
> [2] https://lkml.org/lkml/2016/12/1/811
>
> Signed-off-by: Maciej Bielski <m.bielski@virtualopensystems.com>
> Signed-off-by: Andrea Reale <ar@linux.vnet.ibm.com>
> ---
>  arch/arm64/Kconfig           |  4 +--
>  arch/arm64/include/asm/mmu.h |  3 +++
>  arch/arm64/mm/init.c         | 58 +++++++++++++++++++++++++++++++-------------
>  arch/arm64/mm/mmu.c          | 24 ++++++++++++++++++
>  include/linux/memblock.h     |  1 +
>  mm/memblock.c                | 10 ++++++++
>  6 files changed, 80 insertions(+), 20 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 2482fdd..bd8ddf2 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -577,9 +577,7 @@ config HOTPLUG_CPU
>  	  can be controlled through /sys/devices/system/cpu.
>
>  config ARCH_ENABLE_MEMORY_HOTPLUG
> -	def_bool y
> -
> -config ARCH_ENABLE_MEMORY_HOTREMOVE
> +    depends on !NUMA
>  	def_bool y
>
>  # Common NUMA Features
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index 8d9fce0..2499745 100644
> --- a/arch/arm64/include/asm/mmu.h
> +++ b/arch/arm64/include/asm/mmu.h
> @@ -36,5 +36,8 @@ extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
>  			       unsigned long virt, phys_addr_t size,
>  			       pgprot_t prot, bool allow_block_mappings);
>  extern void *fixmap_remap_fdt(phys_addr_t dt_phys);
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +extern void hotplug_paging(phys_addr_t start, phys_addr_t size);
> +#endif
>
>  #endif
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 687d087..a7c740e 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -544,37 +544,61 @@ int arch_add_memory(int nid, u64 start, u64 size, bool for_device)
>  	struct zone *zone;
>  	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);
> +	unsigned long pfn;
>  	int ret;
>
> +	if (end_pfn > max_sparsemem_pfn) {
> +		pr_err("end_pfn too big");
> +		return -1;
> +	}
> +	hotplug_paging(start, size);
> +
> +	/*
> +	 * Mark all the page range as unsuable.
> +	 * This is needed because  __add_section (within __add_pages)
> +	 * wants pfn_valid to be false, and in arm64 pfn falid is implemented
> +	 * by just checking at the nomap flag for existing blocks
> +	 */
> +	memblock_mark_nomap(start, size);
> +
>  	pgdat = NODE_DATA(nid);
>
>  	zone = pgdat->node_zones +
>  		zone_for_memory(nid, start, size, ZONE_NORMAL, for_device);
>  	ret = __add_pages(nid, zone, start_pfn, nr_pages);
>
> -	if (ret)
> -		pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
> -			__func__, ret);
> +	/*
> +	 * Make the pages usable after they have been added.
> +	 * This will make pfn_valid return true
> +	 */
> +	memblock_clear_nomap(start, size);
>
> -	return ret;
> -}
> +	/*
> +	 * 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.
> +	 */
> +	for (pfn = start_pfn; pfn < start_pfn + nr_pages; pfn++) {
> +		if (!pfn_valid(pfn))
> +			continue;
>
> -#ifdef CONFIG_MEMORY_HOTREMOVE
> -int arch_remove_memory(u64 start, u64 size)
> -{
> -	unsigned long start_pfn = start >> PAGE_SHIFT;
> -	unsigned long nr_pages = size >> PAGE_SHIFT;
> -	struct zone *zone;
> -	int ret;
> +		SetPageReserved(pfn_to_page(pfn));
> +	}
>
> -	zone = page_zone(pfn_to_page(start_pfn));
> -	ret = __remove_pages(zone, start_pfn, nr_pages);
>  	if (ret)
> -		pr_warn("%s: Problem encountered in __remove_pages() ret=%d\n",
> +		pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
>  			__func__, ret);
>
>  	return ret;
>  }
>  #endif
> -#endif
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 05615a3..9efa7d1 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -493,6 +493,30 @@ 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.
> + */
> +void hotplug_paging(phys_addr_t start, phys_addr_t size)
> +{
> +	phys_addr_t pgd_phys = pgd_pgtable_alloc();
> +	pgd_t *pgd = pgd_set_fixmap(pgd_phys);
> +
> +	memcpy(pgd, swapper_pg_dir, PAGE_SIZE);
> +
> +	__create_pgd_mapping(pgd, start, __phys_to_virt(start), size,
> +			PAGE_KERNEL, pgd_pgtable_alloc, false);
> +
> +	cpu_replace_ttbr1(__va(pgd_phys));
> +	memcpy(swapper_pg_dir, pgd, PAGE_SIZE);
> +	cpu_replace_ttbr1(swapper_pg_dir);
> +
> +	pgd_clear_fixmap();
> +	memblock_free(pgd_phys, PAGE_SIZE);
> +}
> +#endif
> +
>  /*
>   * Check whether a kernel address is valid (derived from arch/x86/).
>   */
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index 5b759c9..5f78257 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -92,6 +92,7 @@ int memblock_mark_hotplug(phys_addr_t base, phys_addr_t size);
>  int memblock_clear_hotplug(phys_addr_t base, phys_addr_t size);
>  int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
>  int memblock_mark_nomap(phys_addr_t base, phys_addr_t size);
> +int memblock_clear_nomap(phys_addr_t base, phys_addr_t size);
>  ulong choose_memblock_flags(void);
>
>  /* Low level functions */
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 7608bc3..05e7676 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -814,6 +814,16 @@ int __init_memblock memblock_mark_nomap(phys_addr_t base, phys_addr_t size)
>  }
>
>  /**
> + * memblock_clear_nomap - Clear a flag of MEMBLOCK_NOMAP memory region
> + * @base: the base phys addr of the region
> + * @size: the size of the region
> + */
> +int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
> +{
> +	return memblock_setclr_flag(base, size, 0, MEMBLOCK_NOMAP);
> +}
> +
> +/**
>   * __next_reserved_mem_region - next function for for_each_reserved_region()
>   * @idx: pointer to u64 loop variable
>   * @out_start: ptr to phys_addr_t for start address of the region, can be %NULL
>

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#1545684

Hi Scott,

Thanks for testing it and providing us the feedback. For replicating the problem you have reported,
could you provide more info on your system configuration, please?
Among others, few questions that we have in mind are:
* What is the RAM size at boot? Is it multiple of 1GB (or precisely `1<<MIN_MEMORY_BLOCK_SIZE`) ?
* Do you run the kernel with `mem=YYY` option?
* Do you follow steps (1) and (2) for performing the hotplug?
* Do you have any other configuration flags enabled except for CONFIG_MEMORY_HOTPLUG?
* Any other non-defaults?

Up to now the patch was passing our simple tests but if we miss something we will be very thankful
if you could help us to sort it out. If you have found something after your deeper analysis, please
keep us informed.


On 20/12/2016 20:12, Scott Branden wrote:
> Hi Maciej,
>
> I have applied that patch ontop of the patches I previously sent out
> and tested.
>
> It does recognized the memory in /proc/iomem but I get memory corruption of the original system
> RAM soon after.  It appears the page allocation gets corrupted.  I will try to dig into it further
> but if somebody else could try it out in their system to see what results they get it would help.
>
> Regards,
>  Scott
>
> On 16-12-14 04:16 AM, Maciej Bielski wrote:
>> This patch relates to the work previously announced in [1]. This builds on the
>> work by Scott Branden [2] and, henceforth, it needs to be applied on top of
>> Scott's patches [2]. Comments are very welcome.
>>
>> Changes from the original patchset and known issues:
>>
>> - Compared to Scott's original patchset, this work adds the mapping of
>>   the new hotplugged pages into the kernel page tables. This is done by
>>   copying the old swapper_pg_dir over a new page, adding the new mappings,
>>   and then switching to the newly built pg_dir (see `hotplug_paging` in
>>   arch/arm64/mmu.c). There might be better ways to to this: suggestions
>>   are more than welcome.
>>
>> - The stub function for `arch_remove_memory` has been removed for now; we
>>   are working in parallel on memory hot remove, and we plan to contribute
>>   it as a separate patch.
>>
>> - Corresponding Kconfig flags have been added;
>>
>> - Note that this patch does not work when NUMA is enabled; in fact,
>>   the function `memory_add_physaddr_to_nid` does not have an
>>   implementation when the NUMA flag is on: this function is supposed to
>>   return the nid the hotplugged memory should be associated with. However
>>   it is not really clear to us  yet what the semantics of this function
>>   in the context of a NUMA system should be. A quick and dirty fix would
>>   be to always attach to the first available NUMA node.
>>
>> - In arch/arm64/mm/init.c `arch_add_memory`, we are doing a hack with the
>>   nomap memory block flags to satisfy preconditions and postconditions of
>>   `__add_pages` and postconditions of `arch_add_memory`. Compared to
>>   memory hotplug implementation for other architectures, the "issue"
>>   seems to be in the implemenation of `pfn_valid`. Suggestions on how
>>   to cleanly avoid this hack are welcome.
>>
>> This patchset can be tested by starting the kernel with the `mem=X` flag, where
>> X is less than the total available physical memory and has to be multiple of
>> MIN_MEMORY_BLOCK_SIZE. We also tested it on a customised version of QEMU
>> capable to emulate physical hotplug on arm64 platform.
>>
>> To enable the feature the CONFIG_MEMORY_HOTPLUG compilation flag
>> needs to be set to true. Then, after memory is physically hotplugged,
>> the standard two steps to make it available (as also documented in
>> Documentation/memory-hotplug.txt) are:
>>
>> (1) Notify memory hot-add
>>      echo '0xYY000000' > /sys/devices/system/memory/probe
>>
>> where 0xYY000000 is the first physical address of the new memory section.
>>
>> (2) Online new memory block(s)
>>     echo online > /sys/devices/system/memory/memoryXXX/state
>>
>> where XXX corresponds to the ids of newly added blocks.
>>
>> Onlining can optionally be automatic at hot-add notification by enabling
>> the global flag:
>>     echo online > /sys/devices/system/memory/auto_online_blocks
>> or by setting the corresponding config flag in the kernel build.
>>
>> Again, any comment is highly appreciated.
>>
>> [1] https://lkml.org/lkml/2016/11/17/49
>> [2] https://lkml.org/lkml/2016/12/1/811
>>
>> Signed-off-by: Maciej Bielski <m.bielski@virtualopensystems.com>
>> Signed-off-by: Andrea Reale <ar@linux.vnet.ibm.com>
>> ---
>>  arch/arm64/Kconfig           |  4 +--
>>  arch/arm64/include/asm/mmu.h |  3 +++
>>  arch/arm64/mm/init.c         | 58 +++++++++++++++++++++++++++++++-------------
>>  arch/arm64/mm/mmu.c          | 24 ++++++++++++++++++
>>  include/linux/memblock.h     |  1 +
>>  mm/memblock.c                | 10 ++++++++
>>  6 files changed, 80 insertions(+), 20 deletions(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 2482fdd..bd8ddf2 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -577,9 +577,7 @@ config HOTPLUG_CPU
>>        can be controlled through /sys/devices/system/cpu.
>>
>>  config ARCH_ENABLE_MEMORY_HOTPLUG
>> -    def_bool y
>> -
>> -config ARCH_ENABLE_MEMORY_HOTREMOVE
>> +    depends on !NUMA
>>      def_bool y
>>
>>  # Common NUMA Features
>> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
>> index 8d9fce0..2499745 100644
>> --- a/arch/arm64/include/asm/mmu.h
>> +++ b/arch/arm64/include/asm/mmu.h
>> @@ -36,5 +36,8 @@ extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
>>                     unsigned long virt, phys_addr_t size,
>>                     pgprot_t prot, bool allow_block_mappings);
>>  extern void *fixmap_remap_fdt(phys_addr_t dt_phys);
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +extern void hotplug_paging(phys_addr_t start, phys_addr_t size);
>> +#endif
>>
>>  #endif
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index 687d087..a7c740e 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -544,37 +544,61 @@ int arch_add_memory(int nid, u64 start, u64 size, bool for_device)
>>      struct zone *zone;
>>      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);
>> +    unsigned long pfn;
>>      int ret;
>>
>> +    if (end_pfn > max_sparsemem_pfn) {
>> +        pr_err("end_pfn too big");
>> +        return -1;
>> +    }
>> +    hotplug_paging(start, size);
>> +
>> +    /*
>> +     * Mark all the page range as unsuable.
>> +     * This is needed because  __add_section (within __add_pages)
>> +     * wants pfn_valid to be false, and in arm64 pfn falid is implemented
>> +     * by just checking at the nomap flag for existing blocks
>> +     */
>> +    memblock_mark_nomap(start, size);
>> +
>>      pgdat = NODE_DATA(nid);
>>
>>      zone = pgdat->node_zones +
>>          zone_for_memory(nid, start, size, ZONE_NORMAL, for_device);
>>      ret = __add_pages(nid, zone, start_pfn, nr_pages);
>>
>> -    if (ret)
>> -        pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
>> -            __func__, ret);
>> +    /*
>> +     * Make the pages usable after they have been added.
>> +     * This will make pfn_valid return true
>> +     */
>> +    memblock_clear_nomap(start, size);
>>
>> -    return ret;
>> -}
>> +    /*
>> +     * 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.
>> +     */
>> +    for (pfn = start_pfn; pfn < start_pfn + nr_pages; pfn++) {
>> +        if (!pfn_valid(pfn))
>> +            continue;
>>
>> -#ifdef CONFIG_MEMORY_HOTREMOVE
>> -int arch_remove_memory(u64 start, u64 size)
>> -{
>> -    unsigned long start_pfn = start >> PAGE_SHIFT;
>> -    unsigned long nr_pages = size >> PAGE_SHIFT;
>> -    struct zone *zone;
>> -    int ret;
>> +        SetPageReserved(pfn_to_page(pfn));
>> +    }
>>
>> -    zone = page_zone(pfn_to_page(start_pfn));
>> -    ret = __remove_pages(zone, start_pfn, nr_pages);
>>      if (ret)
>> -        pr_warn("%s: Problem encountered in __remove_pages() ret=%d\n",
>> +        pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
>>              __func__, ret);
>>
>>      return ret;
>>  }
>>  #endif
>> -#endif
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 05615a3..9efa7d1 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -493,6 +493,30 @@ 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.
>> + */
>> +void hotplug_paging(phys_addr_t start, phys_addr_t size)
>> +{
>> +    phys_addr_t pgd_phys = pgd_pgtable_alloc();
>> +    pgd_t *pgd = pgd_set_fixmap(pgd_phys);
>> +
>> +    memcpy(pgd, swapper_pg_dir, PAGE_SIZE);
>> +
>> +    __create_pgd_mapping(pgd, start, __phys_to_virt(start), size,
>> +            PAGE_KERNEL, pgd_pgtable_alloc, false);
>> +
>> +    cpu_replace_ttbr1(__va(pgd_phys));
>> +    memcpy(swapper_pg_dir, pgd, PAGE_SIZE);
>> +    cpu_replace_ttbr1(swapper_pg_dir);
>> +
>> +    pgd_clear_fixmap();
>> +    memblock_free(pgd_phys, PAGE_SIZE);
>> +}
>> +#endif
>> +
>>  /*
>>   * Check whether a kernel address is valid (derived from arch/x86/).
>>   */
>> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
>> index 5b759c9..5f78257 100644
>> --- a/include/linux/memblock.h
>> +++ b/include/linux/memblock.h
>> @@ -92,6 +92,7 @@ int memblock_mark_hotplug(phys_addr_t base, phys_addr_t size);
>>  int memblock_clear_hotplug(phys_addr_t base, phys_addr_t size);
>>  int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
>>  int memblock_mark_nomap(phys_addr_t base, phys_addr_t size);
>> +int memblock_clear_nomap(phys_addr_t base, phys_addr_t size);
>>  ulong choose_memblock_flags(void);
>>
>>  /* Low level functions */
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index 7608bc3..05e7676 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -814,6 +814,16 @@ int __init_memblock memblock_mark_nomap(phys_addr_t base, phys_addr_t size)
>>  }
>>
>>  /**
>> + * memblock_clear_nomap - Clear a flag of MEMBLOCK_NOMAP memory region
>> + * @base: the base phys addr of the region
>> + * @size: the size of the region
>> + */
>> +int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>> +{
>> +    return memblock_setclr_flag(base, size, 0, MEMBLOCK_NOMAP);
>> +}
>> +
>> +/**
>>   * __next_reserved_mem_region - next function for for_each_reserved_region()
>>   * @idx: pointer to u64 loop variable
>>   * @out_start: ptr to phys_addr_t for start address of the region, can be %NULL
>>

BR,

-- 
Maciej Bielski

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#1546114

Hi Maciej,

On 16-12-21 01:44 AM, Maciej Bielski wrote:
> Hi Scott,
>
> Thanks for testing it and providing us the feedback. For replicating the problem you have reported,
> could you provide more info on your system configuration, please?
> Among others, few questions that we have in mind are:
> * What is the RAM size at boot? Is it multiple of 1GB (or precisely `1<<MIN_MEMORY_BLOCK_SIZE`) ?
RAM size at boot is 64MB, not a multiple of 1GB.  I will try next week 
on a system with multi-GB at boot to see if it makes a difference. 
SECTION_SIZE_BITS in sparsemem.h has been modified to 28 to allow for 
256MB hotplug regions to be added.

> * Do you run the kernel with `mem=YYY` option?
no, memory is defined in dts file with a few reserved regions:
/memreserve/ 0x75000000 0x00800000;
/memreserve/ 0x77f00000 0x00100000;
&memory {
	reg = <0x00000000 0x74000000 0x0 0x04000000>; /* 64MB */
};

> * Do you follow steps (1) and (2) for performing the hotplug?
no, only step 1 is done, auto-online is enabled.  I'll try turning that 
off and see if it makes a difference.

> * Do you have any other configuration flags enabled except for CONFIG_MEMORY_HOTPLUG?
> * Any other non-defaults?
Yes, a lot of config options have been disabled to reduce kernel size. 
The memory auto-online is also enabled.  I'll try with the default 
defconfig to see if it makes a difference.
>
> Up to now the patch was passing our simple tests but if we miss something we will be very thankful
> if you could help us to sort it out. If you have found something after your deeper analysis, please
> keep us informed.
I think the path forward is to get a normal multi-GB system going and 
then reduce the memory on boot and then hotplug it in after to see if 
that works.  Or, if you can reduce your memory down and give it a try 
that might uncover a problem as well.
>
>
> On 20/12/2016 20:12, Scott Branden wrote:
>> Hi Maciej,
>>
>> I have applied that patch ontop of the patches I previously sent out
>> and tested.
>>
>> It does recognized the memory in /proc/iomem but I get memory corruption of the original system
>> RAM soon after.  It appears the page allocation gets corrupted.  I will try to dig into it further
>> but if somebody else could try it out in their system to see what results they get it would help.
>>
>> Regards,
>>  Scott
>>
>> On 16-12-14 04:16 AM, Maciej Bielski wrote:
>>> This patch relates to the work previously announced in [1]. This builds on the
>>> work by Scott Branden [2] and, henceforth, it needs to be applied on top of
>>> Scott's patches [2]. Comments are very welcome.
>>>
>>> Changes from the original patchset and known issues:
>>>
>>> - Compared to Scott's original patchset, this work adds the mapping of
>>>   the new hotplugged pages into the kernel page tables. This is done by
>>>   copying the old swapper_pg_dir over a new page, adding the new mappings,
>>>   and then switching to the newly built pg_dir (see `hotplug_paging` in
>>>   arch/arm64/mmu.c). There might be better ways to to this: suggestions
>>>   are more than welcome.
>>>
>>> - The stub function for `arch_remove_memory` has been removed for now; we
>>>   are working in parallel on memory hot remove, and we plan to contribute
>>>   it as a separate patch.
>>>
>>> - Corresponding Kconfig flags have been added;
>>>
>>> - Note that this patch does not work when NUMA is enabled; in fact,
>>>   the function `memory_add_physaddr_to_nid` does not have an
>>>   implementation when the NUMA flag is on: this function is supposed to
>>>   return the nid the hotplugged memory should be associated with. However
>>>   it is not really clear to us  yet what the semantics of this function
>>>   in the context of a NUMA system should be. A quick and dirty fix would
>>>   be to always attach to the first available NUMA node.
>>>
>>> - In arch/arm64/mm/init.c `arch_add_memory`, we are doing a hack with the
>>>   nomap memory block flags to satisfy preconditions and postconditions of
>>>   `__add_pages` and postconditions of `arch_add_memory`. Compared to
>>>   memory hotplug implementation for other architectures, the "issue"
>>>   seems to be in the implemenation of `pfn_valid`. Suggestions on how
>>>   to cleanly avoid this hack are welcome.
>>>
>>> This patchset can be tested by starting the kernel with the `mem=X` flag, where
>>> X is less than the total available physical memory and has to be multiple of
>>> MIN_MEMORY_BLOCK_SIZE. We also tested it on a customised version of QEMU
>>> capable to emulate physical hotplug on arm64 platform.
>>>
>>> To enable the feature the CONFIG_MEMORY_HOTPLUG compilation flag
>>> needs to be set to true. Then, after memory is physically hotplugged,
>>> the standard two steps to make it available (as also documented in
>>> Documentation/memory-hotplug.txt) are:
>>>
>>> (1) Notify memory hot-add
>>>      echo '0xYY000000' > /sys/devices/system/memory/probe
>>>
>>> where 0xYY000000 is the first physical address of the new memory section.
>>>
>>> (2) Online new memory block(s)
>>>     echo online > /sys/devices/system/memory/memoryXXX/state
>>>
>>> where XXX corresponds to the ids of newly added blocks.
>>>
>>> Onlining can optionally be automatic at hot-add notification by enabling
>>> the global flag:
>>>     echo online > /sys/devices/system/memory/auto_online_blocks
>>> or by setting the corresponding config flag in the kernel build.
>>>
>>> Again, any comment is highly appreciated.
>>>
>>> [1] https://lkml.org/lkml/2016/11/17/49
>>> [2] https://lkml.org/lkml/2016/12/1/811
>>>
>>> Signed-off-by: Maciej Bielski <m.bielski@virtualopensystems.com>
>>> Signed-off-by: Andrea Reale <ar@linux.vnet.ibm.com>
>>> ---
>>>  arch/arm64/Kconfig           |  4 +--
>>>  arch/arm64/include/asm/mmu.h |  3 +++
>>>  arch/arm64/mm/init.c         | 58 +++++++++++++++++++++++++++++++-------------
>>>  arch/arm64/mm/mmu.c          | 24 ++++++++++++++++++
>>>  include/linux/memblock.h     |  1 +
>>>  mm/memblock.c                | 10 ++++++++
>>>  6 files changed, 80 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 2482fdd..bd8ddf2 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -577,9 +577,7 @@ config HOTPLUG_CPU
>>>        can be controlled through /sys/devices/system/cpu.
>>>
>>>  config ARCH_ENABLE_MEMORY_HOTPLUG
>>> -    def_bool y
>>> -
>>> -config ARCH_ENABLE_MEMORY_HOTREMOVE
>>> +    depends on !NUMA
>>>      def_bool y
>>>
>>>  # Common NUMA Features
>>> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
>>> index 8d9fce0..2499745 100644
>>> --- a/arch/arm64/include/asm/mmu.h
>>> +++ b/arch/arm64/include/asm/mmu.h
>>> @@ -36,5 +36,8 @@ extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
>>>                     unsigned long virt, phys_addr_t size,
>>>                     pgprot_t prot, bool allow_block_mappings);
>>>  extern void *fixmap_remap_fdt(phys_addr_t dt_phys);
>>> +#ifdef CONFIG_MEMORY_HOTPLUG
>>> +extern void hotplug_paging(phys_addr_t start, phys_addr_t size);
>>> +#endif
>>>
>>>  #endif
>>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>>> index 687d087..a7c740e 100644
>>> --- a/arch/arm64/mm/init.c
>>> +++ b/arch/arm64/mm/init.c
>>> @@ -544,37 +544,61 @@ int arch_add_memory(int nid, u64 start, u64 size, bool for_device)
>>>      struct zone *zone;
>>>      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);
>>> +    unsigned long pfn;
>>>      int ret;
>>>
>>> +    if (end_pfn > max_sparsemem_pfn) {
>>> +        pr_err("end_pfn too big");
>>> +        return -1;
>>> +    }
>>> +    hotplug_paging(start, size);
>>> +
>>> +    /*
>>> +     * Mark all the page range as unsuable.
>>> +     * This is needed because  __add_section (within __add_pages)
>>> +     * wants pfn_valid to be false, and in arm64 pfn falid is implemented
>>> +     * by just checking at the nomap flag for existing blocks
>>> +     */
>>> +    memblock_mark_nomap(start, size);
>>> +
>>>      pgdat = NODE_DATA(nid);
>>>
>>>      zone = pgdat->node_zones +
>>>          zone_for_memory(nid, start, size, ZONE_NORMAL, for_device);
>>>      ret = __add_pages(nid, zone, start_pfn, nr_pages);
>>>
>>> -    if (ret)
>>> -        pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
>>> -            __func__, ret);
>>> +    /*
>>> +     * Make the pages usable after they have been added.
>>> +     * This will make pfn_valid return true
>>> +     */
>>> +    memblock_clear_nomap(start, size);
>>>
>>> -    return ret;
>>> -}
>>> +    /*
>>> +     * 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.
>>> +     */
>>> +    for (pfn = start_pfn; pfn < start_pfn + nr_pages; pfn++) {
>>> +        if (!pfn_valid(pfn))
>>> +            continue;
>>>
>>> -#ifdef CONFIG_MEMORY_HOTREMOVE
>>> -int arch_remove_memory(u64 start, u64 size)
>>> -{
>>> -    unsigned long start_pfn = start >> PAGE_SHIFT;
>>> -    unsigned long nr_pages = size >> PAGE_SHIFT;
>>> -    struct zone *zone;
>>> -    int ret;
>>> +        SetPageReserved(pfn_to_page(pfn));
>>> +    }
>>>
>>> -    zone = page_zone(pfn_to_page(start_pfn));
>>> -    ret = __remove_pages(zone, start_pfn, nr_pages);
>>>      if (ret)
>>> -        pr_warn("%s: Problem encountered in __remove_pages() ret=%d\n",
>>> +        pr_warn("%s: Problem encountered in __add_pages() ret=%d\n",
>>>              __func__, ret);
>>>
>>>      return ret;
>>>  }
>>>  #endif
>>> -#endif
>>>
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index 05615a3..9efa7d1 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -493,6 +493,30 @@ 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.
>>> + */
>>> +void hotplug_paging(phys_addr_t start, phys_addr_t size)
>>> +{
>>> +    phys_addr_t pgd_phys = pgd_pgtable_alloc();
>>> +    pgd_t *pgd = pgd_set_fixmap(pgd_phys);
>>> +
>>> +    memcpy(pgd, swapper_pg_dir, PAGE_SIZE);
>>> +
>>> +    __create_pgd_mapping(pgd, start, __phys_to_virt(start), size,
>>> +            PAGE_KERNEL, pgd_pgtable_alloc, false);
>>> +
>>> +    cpu_replace_ttbr1(__va(pgd_phys));
>>> +    memcpy(swapper_pg_dir, pgd, PAGE_SIZE);
>>> +    cpu_replace_ttbr1(swapper_pg_dir);
>>> +
>>> +    pgd_clear_fixmap();
>>> +    memblock_free(pgd_phys, PAGE_SIZE);
>>> +}
>>> +#endif
>>> +
>>>  /*
>>>   * Check whether a kernel address is valid (derived from arch/x86/).
>>>   */
>>> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
>>> index 5b759c9..5f78257 100644
>>> --- a/include/linux/memblock.h
>>> +++ b/include/linux/memblock.h
>>> @@ -92,6 +92,7 @@ int memblock_mark_hotplug(phys_addr_t base, phys_addr_t size);
>>>  int memblock_clear_hotplug(phys_addr_t base, phys_addr_t size);
>>>  int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
>>>  int memblock_mark_nomap(phys_addr_t base, phys_addr_t size);
>>> +int memblock_clear_nomap(phys_addr_t base, phys_addr_t size);
>>>  ulong choose_memblock_flags(void);
>>>
>>>  /* Low level functions */
>>> diff --git a/mm/memblock.c b/mm/memblock.c
>>> index 7608bc3..05e7676 100644
>>> --- a/mm/memblock.c
>>> +++ b/mm/memblock.c
>>> @@ -814,6 +814,16 @@ int __init_memblock memblock_mark_nomap(phys_addr_t base, phys_addr_t size)
>>>  }
>>>
>>>  /**
>>> + * memblock_clear_nomap - Clear a flag of MEMBLOCK_NOMAP memory region
>>> + * @base: the base phys addr of the region
>>> + * @size: the size of the region
>>> + */
>>> +int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>>> +{
>>> +    return memblock_setclr_flag(base, size, 0, MEMBLOCK_NOMAP);
>>> +}
>>> +
>>> +/**
>>>   * __next_reserved_mem_region - next function for for_each_reserved_region()
>>>   * @idx: pointer to u64 loop variable
>>>   * @out_start: ptr to phys_addr_t for start address of the region, can be %NULL
>>>
>
> BR,
>

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