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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Wednesday 17 May 2017 10:31 AM, Anshuman Khandual wrote:
> 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 ?

That is for pseries.ie, pSeries will get the hugpages reserved by phyp 
and the details of those pages are passed via device tree. Not sure what 
is the conflict here. If we use the above kernel parameter, we will try 
to allocate another 'x' number of hugepages.

> 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 ?
>

There are two ways to specify gpages, one via device tree which is used 
only in case of pseries and other hugepagesz=size hugepags=no-of-hugepages.

-aneesh