Re: [PATCH v5 4/7] mm/hotplug: Support memmap_on_memory when memmap is not aligned to pageblocks

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

On 7/26/23 2:34 PM, David Hildenbrand wrote:
> 
>>>>    /*
>>>> @@ -1310,7 +1400,10 @@ int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
>>>>    {
>>>>        struct mhp_params params = { .pgprot = pgprot_mhp(PAGE_KERNEL) };
>>>>        enum memblock_flags memblock_flags = MEMBLOCK_NONE;
>>>> -    struct vmem_altmap mhp_altmap = {};
>>>> +    struct vmem_altmap mhp_altmap = {
>>>> +        .base_pfn =  PHYS_PFN(res->start),
>>>> +        .end_pfn  =  PHYS_PFN(res->end),
>>>
>>> Is it required to set .end_pfn, and if so, shouldn't we also set it to
>>> base_pfn + memory_block_memmap_on_memory_pages()) ?
>>>
>>
>> We use that in ppc64 for checking altmap boundary condition. As we
>> discussed earlier, ppc64 due to vmemmap mapping size restrictions can't
>> always allocate vmemmap pages from altmap area even if requested. We
>> fallback to regular memory alocation in that case (only used now with
>> pmem). We use altmap.end_pfn for that boundary check. You can refer to
>> altmap_cross_boundary() for more details.
> 
> But even then, setting the end to the end of the resource size is wrong, no? We don't want anybody to allocate beyond base_pfn + memory_block_memmap_on_memory_pages().
> 

altmap.end is the end pfn of the resource

__nvdimm_setup_pfn()
...
        resource_size_t end = nsio->res.end - end_trunc;
	struct vmem_altmap __altmap = {
		.base_pfn = init_altmap_base(base),
		.reserve = init_altmap_reserve(base),
		.end_pfn = PHYS_PFN(end),
	};


And we use it to find that the page_to_pfn mapping we use by allocating a page from altmap doesn't point to a pfn that is outside the resource range.

-aneesh