On 7/7/23 5:47 PM, David Hildenbrand wrote:
> On 06.07.23 18:06, Aneesh Kumar K V wrote:
>> On 7/6/23 6:29 PM, David Hildenbrand wrote:
>>> 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).
>>>
>>
>> Sure. How about?
>>
>> bool mhp_supports_memmap_on_memory(unsigned long size)
>> {
>>
>> unsigned long nr_pages = size >> PAGE_SHIFT;
>> unsigned long vmemmap_size = nr_pages * sizeof(struct page);
>>
>> if (!radix_enabled())
>> return false;
>> /*
>> * memmap on memory only supported with memory block size add/remove
>> */
>> if (size != memory_block_size_bytes())
>> return false;
>> /*
>> * Also make sure the vmemmap allocation is fully contianed
>> * so that we always allocate vmemmap memory from altmap area.
>> */
>> if (!IS_ALIGNED(vmemmap_size, PAGE_SIZE))
>> return false;
>> /*
>> * The pageblock alignment requirement is met by using
>> * reserve blocks in altmap.
>> */
>> return true;
>> }
>
> Better, but the PAGE_SIZE that could be added to common code as well.
>
> ... but, the pageblock check in common code implies a PAGE_SIZE check, so why
> do we need any other check besides the radix_enabled() check for arm64 and
> just keep all the other checks in common code as they are?
>
> If your vmemmap does not cover full pageblocks (which implies full pages),
> the feature cannot be used *unless* we'd waste altmap space in the vmemmap to
> cover one pageblock.
>
> Wasting hotplugged memory certainly sounds wrong?
>
>
> So I appreciate if you could explain why the pageblock check should not be
> had for ppc64?
>
If we want things to be aligned to pageblock (2M) we will have to use 2M
vmemmap space and that implies a memory block of 2G with 64K page size. That
requirements makes the feature not useful at all
on power. The compromise i came to was what i mentioned in the commit message
for enabling the feature on ppc64.
We use altmap.reserve feature to align things correctly at pageblock
granularity. We can end up loosing some pages in memory with this. For ex: with
256MB memory block
size, we require 4 pages to map vmemmap pages, In order to align things
correctly we end up adding a reserve of 28 pages. ie, for every 4096 pages
28 pages get reserved.
-aneesh
