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SubjectRe: [PATCH v3 2/4] x86/mm/memory_hotplug: determine block size based on the end of boot memory
On Tue 13-02-18 14:31:57, Pavel Tatashin wrote:
> Memory sections are combined into "memory block" chunks. These chunks are
> the units upon which memory can be added and removed.
>
> On x86, the new memory may be added after the end of the boot memory,
> therefore, if block size does not align with end of boot memory, memory
> hot-plugging/hot-removing can be broken.
>
> Currently, whenever machine is boot with more than 64G the block size
> unconditionally increased to 2G from the base 128M in order to reduce
> number of memory devices in sysfs:
> /sys/devices/system/memory/memoryXXX
>
> But, we must use the largest allowed block size that aligns to the next
> address to be able to hotplug the next block of memory.
>
> So, when memory is larger than 64G, we check the end address and find the
> largest block size that is still power of two but smaller or equal to 2G.
>
> Before, the fix:
> Run qemu with:
> -m 64G,slots=2,maxmem=66G -object memory-backend-ram,id=mem1,size=2G
>
> (qemu) device_add pc-dimm,id=dimm1,memdev=mem1
> Block size [0x80000000] unaligned hotplug range: start 0x1040000000,
> size 0x80000000
> acpi PNP0C80:00: add_memory failed
> acpi PNP0C80:00: acpi_memory_enable_device() error
> acpi PNP0C80:00: Enumeration failure
>
> With the fix memory is added successfully, as the block size is set to 1G,
> and therefore aligns with start address 0x1040000000.

I dunno. If x86 maintainers are OK with this then why not, but I do not
like how this is x86 specific. I would much rather address this by
making the memblock user interface more sane.

> Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
> ---
> arch/x86/mm/init_64.c | 33 +++++++++++++++++++++++++++++----
> 1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 1ab42c852069..f7dc80364397 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1326,14 +1326,39 @@ int kern_addr_valid(unsigned long addr)
> return pfn_valid(pte_pfn(*pte));
> }
>
> +/*
> + * Block size is the minimum quantum of memory which can be hot-plugged or
> + * hot-removed. It must be power of two, and must be equal or larger than
> + * MIN_MEMORY_BLOCK_SIZE.
> + */
> +#define MAX_BLOCK_SIZE (2UL << 30)
> +
> +/* Amount of ram needed to start using large blocks */
> +#define MEM_SIZE_FOR_LARGE_BLOCK (64UL << 30)
> +
> static unsigned long probe_memory_block_size(void)
> {
> - unsigned long bz = MIN_MEMORY_BLOCK_SIZE;
> + unsigned long boot_mem_end = max_pfn << PAGE_SHIFT;
> + unsigned long bz;
>
> - /* if system is UV or has 64GB of RAM or more, use large blocks */
> - if (is_uv_system() || ((max_pfn << PAGE_SHIFT) >= (64UL << 30)))
> - bz = 2UL << 30; /* 2GB */
> + /* If this is UV system, always set 2G block size */
> + if (is_uv_system()) {
> + bz = MAX_BLOCK_SIZE;
> + goto done;
> + }
>
> + /* Use regular block if RAM is smaller than MEM_SIZE_FOR_LARGE_BLOCK */
> + if (boot_mem_end < MEM_SIZE_FOR_LARGE_BLOCK) {
> + bz = MIN_MEMORY_BLOCK_SIZE;
> + goto done;
> + }
> +
> + /* Find the largest allowed block size that aligns to memory end */
> + for (bz = MAX_BLOCK_SIZE; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
> + if (IS_ALIGNED(boot_mem_end, bz))
> + break;
> + }
> +done:
> pr_info("x86/mm: Memory block size: %ldMB\n", bz >> 20);
>
> return bz;
> --
> 2.16.1
>

--
Michal Hocko
SUSE Labs

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