Re: [PATCH] powerpc/mm/hugetlb: Add support for reserving gigantic huge pages via kernel command line

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

On 05/16/2017 02:54 PM, Aneesh Kumar K.V wrote:
> +void __init reserve_hugetlb_gpages(void)
> +{
> +	char buf[10];
> +	phys_addr_t base;
> +	unsigned long gpage_size = 1UL << 34;
> +	static __initdata char cmdline[COMMAND_LINE_SIZE];
> +
> +	if (radix_enabled())
> +		gpage_size = 1UL << 30;
> +
> +	strlcpy(cmdline, boot_command_line, COMMAND_LINE_SIZE);
> +	parse_args("hugetlb gpages", cmdline, NULL, 0, 0, 0,
> +		   NULL, &do_gpage_early_setup);
> +
> +	if (!gpage_npages)
> +		return;
> +
> +	string_get_size(gpage_size, 1, STRING_UNITS_2, buf, sizeof(buf));
> +	pr_info("Trying to reserve %ld %s pages\n", gpage_npages, buf);
> +
> +	/* Allocate one page at a time */
> +	while(gpage_npages) {
> +		base = memblock_alloc_base(gpage_size, gpage_size,
> +					   MEMBLOCK_ALLOC_ANYWHERE);
> +		add_gpage(base, gpage_size, 1);

For 16GB pages (1UL << 34) on POWER8, we already do these functions
inside htab_dt_scan_hugepage_blocks(). IIUC this happens just by
scanning DT without even specifying any gpages in kernel command
line.

memblock_reserve()
add_gpage()

Then attempting to allocate from memblock and adding it again into
gigantic pages list wont collide ? More over its trying to allocate
across the RAM not specifically on the gpages mentioned in device
tree by the platform. Are we trying to support 16GB pages just from
any memory without platform notification through DT ?