Re: [RFC PATCH] Memory hotplug support for arm64 platform

From: Maciej Bielski
Date: Wed Dec 21 2016 - 04:45:10 EST

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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@xxxxxxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Andrea Reale <ar@xxxxxxxxxxxxxxxxxx>
>> ---
>> 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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