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

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

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

 
>>
>> vmemmap_free() {
>> ...
>>     if (altmap) {
>>         alt_start = altmap->base_pfn;
>>         alt_end = altmap->base_pfn + altmap->reserve +
>>               altmap->free + altmap->alloc + altmap->align;
>>     }
>>
>> ...
>>         if (base_pfn >= alt_start && base_pfn < alt_end) {
>>             vmem_altmap_free(altmap, nr_pages);
>>
>> to see whether we did use altmap for the vmemmap allocation.
>>


-aneesh