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SubjectRe: [PATCH v4 1/5] mm,memory_hotplug: Allocate memmap from the added memory range
Date
On 18.03.21 11:38, Oscar Salvador wrote:
> On Thu, Mar 18, 2021 at 09:27:48AM +0100, Oscar Salvador wrote:
>>> If we check for
>>>
>>> IS_ALIGNED(nr_vmemmap_pages, PMD_SIZE), please add a proper TODO comment
>>> that this is most probably the wrong place to take care of this.
>>
>> Sure, I will stuff the check in there and place a big TODO comment so we
>> do not forget about addressing this issue the right way.
>
> Ok, I realized something while working on v5.
>
> Here is what I have right now:
>
> bool mhp_supports_memmap_on_memory(unsigned long size)
> {
> /*
> * Note: We calculate for a single memory section. The calculation
> * implicitly covers memory blocks that span multiple sections.
> *
> * Not all archs define SECTION_SIZE, but MIN_MEMORY_BLOCK_SIZE always
> * equals SECTION_SIZE, so use that instead.
> */
> unsigned long nr_vmemmap_pages = MIN_MEMORY_BLOCK_SIZE / PAGE_SIZE;

Even clearer would be just using "size / PAGE_SIZE" here. The you can
even drop the comment.

> unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page);
> unsigned long remaining_size = size - vmemmap_size;
>
> /*
> * Besides having arch support and the feature enabled at runtime, we
> * need a few more assumptions to hold true:
> *
> * a) We span a single memory block: memory onlining/offlinin;g happens
> * in memory block granularity. We don't want the vmemmap of online
> * memory blocks to reside on offline memory blocks. In the future,
> * we might want to support variable-sized memory blocks to make the
> * feature more versatile.
> *
> * b) The vmemmap pages span complete PMDs: We don't want vmemmap code
> * to populate memory from the altmap for unrelated parts (i.e.,
> * other memory blocks)
> *
> * c) The vmemmap pages (and thereby the pages that will be exposed to
> * the buddy) have to cover full pageblocks: memory onlining/offlining
> * code requires applicable ranges to be page-aligned, for example, to
> * set the migratetypes properly.
> *
> * TODO: Although we have a check here to make sure that vmemmap pages
> * fully populate a PMD, it is not the right place to check for
> * this. A much better solution involves improving vmemmap code
> * to fallback to base pages when trying to populate vmemmap using
> * altmap as an alternative source of memory, and we do not exactly
> * populate a single PMD.
> */
> return memmap_on_memory &&
> IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
> size == memory_block_size_bytes() &&
> remaining_size &&
> IS_ALIGNED(remaining_size, pageblock_size) &&
> IS_ALIGNED(vmemmap_size, PMD_SIZE);
> }
>
> Assume we are on x86_64 to simplify the case.
>
> Above, nr_vmemmap_pages would be 32768 and vmemmap_size 2MB (exactly a
> PMD).
>
> Now, although correct, this nr_vmemmap_pages does not match with the
> altmap->alloc.
>
> static void * __meminit altmap_alloc_block_buf(unsigned long size,
> struct altmap)
> {
> ...
> ...
> nr_pfns = size >> PAGE_SHIFT; //size is PMD_SIZE
> altmap->alloc += nr_pfns;
> }
>
> altmap->alloc will be 512, 512 * 4K pages = 2MB.
>
> Of course, the reason they do not match is because in one case, we are
> saying a) how many pfns we need to cover a PMD_SIZE, while in the
> other case we say b) how many pages we need to cover SECTION_SIZE
>
> Then b) multiply for page_size to get the current size of it.

I don't follow. 2MB == 2MB. And if there would be difference then we
would be in the problem I brought up: vmemmap code allocating too much
via the altmap, which can be very bad because might be populating more
vmemmap than we actually need.

>
> So, I have mixed feeling about this.
> Would it be more clear to just do:
>
> bool mhp_supports_memmap_on_memory(unsigned long size)
> {
> /*
> * Note: We calculate for a single memory section. The calculation
> * implicitly covers memory blocks that span multiple sections.
> */

Then this comment is wrong

> unsigned long nr_vmemmap_pages = PMD_SIZE / PAGE_SIZE;

And this stuff just gets confusing.

nr_vmemmap_pages = 2MiB / 4 KiB = 512;

> unsigned long vmemmap_size = nr_vmemmap_pages * PAGE_SIZE;

vmemmap_size = 512 * 4KiB = 2 MiB.

That calculation wasn't very useful (/ PAGE_SIZE * PAGE_SIZE)?

> unsigned long remaining_size = size - vmemmap_size;

And here we could get something like

remaining_size = 2 GiB - 2 MiB

?

Which does not make any sense.

> ...
> ...
>
>


--
Thanks,

David / dhildenb

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