[PATCH v2 1/5] mm/hotplug: Embed vmem_altmap details in memory block

David Hildenbrand david at redhat.com
Thu Jul 6 22:59:15 AEST 2023

On 06.07.23 14:32, Aneesh Kumar K V wrote:
> On 7/6/23 4:44 PM, David Hildenbrand wrote:
>> On 06.07.23 11:36, Aneesh Kumar K V wrote:
>>> On 7/6/23 2:48 PM, David Hildenbrand wrote:
>>>> On 06.07.23 10:50, Aneesh Kumar K.V wrote:
>>>>> With memmap on memory, some architecture needs more details w.r.t altmap
>>>>> such as base_pfn, end_pfn, etc to unmap vmemmap memory.
>>>> Can you elaborate why ppc64 needs that and x86-64 + aarch64 don't?
>>>> IOW, why can't ppc64 simply allocate the vmemmap from the start of the memblock (-> base_pfn) and use the stored number of vmemmap pages to calculate the end_pfn?
>>>> To rephrase: if the vmemmap is not at the beginning and doesn't cover full apgeblocks, memory onlining/offlining would be broken.
>>>> [...]
>>> With ppc64 and 64K pagesize and different memory block sizes, we can end up allocating vmemmap backing memory from outside altmap because
>>> a single page vmemmap can cover 1024 pages (64 *1024/sizeof(struct page)). and that can point to pages outside the dev_pagemap range.
>>> So on free we  check
>> So you end up with a mixture of altmap and ordinarily-allocated vmemmap pages? That sound wrong (and is counter-intuitive to the feature in general, where we *don't* want to allocate the vmemmap from outside the altmap).
>> (64 * 1024) / sizeof(struct page) -> 1024 pages
>> 1024 pages * 64k = 64 MiB.
>> What's the memory block size on these systems? If it's >= 64 MiB the vmemmap of a single memory block fits into a single page and we should be fine.
>> Smells like you want to disable the feature on a 64k system.
> But that part of vmemmap_free is common for both dax,dax kmem and the new memmap on memory feature. ie, ppc64 vmemmap_free have checks which require
> a full altmap structure with all the details in. So for memmap on memmory to work on ppc64 we do require similar altmap struct. Hence the idea
> of adding vmemmap_altmap to  struct memory_block

I'd suggest making sure that for the memmap_on_memory case your really 
*always* allocate from the altmap (that's what the feature is about 
after all), and otherwise block the feature (i.e., arch_mhp_supports_... 
should reject it).

Then, you can reconstruct the altmap layout trivially

base_pfn: start of the range to unplug
end_pfn: base_pfn + nr_vmemmap_pages

and pass that to the removal code, which will do the right thing, no?

Sure, remembering the altmap might be a potential cleanup (eventually?), 
but the basic reasoning why this is required as patch #1 IMHO is wrong: 
if you say you support memmap_on_memory for a configuration, then you 
should also properly support it (allocate from the hotplugged memory), 
not silently fall back to something else.


David / dhildenb

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