Date:   Thu, 18 Mar 2021 12:24:16 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/5] mm,memory_hotplug: Allocate memmap from the added
 memory range

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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