Re: [PATCH 1/2] arm64: mm: vmemmap populate to page level if not section aligned

[Zhenhua Huang]

Thanks Catalin for review!

On 2024/12/7 1:13, Catalin Marinas wrote:
> On Thu, Nov 21, 2024 at 03:12:55PM +0800, Zhenhua Huang wrote:
> > Commit 2045a3b8911b ("mm/sparse-vmemmap: generalise vmemmap_populate_hugepages()")
> > optimizes the vmemmap to populate at the PMD section level.
>
> Wasn't the above commit just a non-functional change making the code
> generic? If there was a functional change, it needs to be spelt out. It
> also implies that the code prior to the above commit needs fixing.

Oh... right. I looked up your change from over a decade ago, identified 
by commit c1cc1552616d ("arm64: MMU initialisation").
However, at that time, there was no support for subsection hotplug, 
which was later introduced by commit ba72b4c8cf60 ("mm/sparsemem: 
support sub-section hotplug").

> > However, if start
> > or end is not aligned to a section boundary, such as when a subsection is hot
> > added, populating the entire section is inefficient and wasteful. In such
> > cases, it is more effective to populate at page granularity.
>
> Do you have any numbers to show how inefficient it is? We trade some
> memory for less TLB pressure by using huge pages for vmemmap.

I see.. thanks, yeah. TLB efficiency will benefit.
I want to express even one subsection hot-added, current code logic 
still populate 2M backup metadata, although only 2M/64 = 32K needs.

> > This change also addresses misalignment issues during vmemmap_free(). When
> > pmd_sect() is true, the entire PMD section is cleared, even if only a
> > subsection is mapped. For example, if subsections pagemap1 and pagemap2 are
> > added sequentially and then pagemap1 is removed, vmemmap_free() will clear the
> > entire PMD section, even though pagemap2 is still active.
>
> What do you mean by a PMD section? The whole PAGE_SIZE *
> PAGES_PER_SECTION range or a single pmd entry? I couldn't see how the

I am referring to a single pmd entry, but the buffer it points to manage 
whole PAGE_SIZE * PAGES_PER_SECTION physical memory. for arm64,4K pages:
pmd entry(2M, struct page metadata) -> PAGE_SIZE * 
PAGES_PER_SECTION(128M physical memory)

pagemap1(Where a subsection equals to 2M/64 = 32K) and pagemap2 are part 
of a single PMD entry. When pagemap1 is removed, vmemmap_free() will 
clear the entire PMD section. IOW, total 128M physical memory will 
become unusable.

> former happens in the core code but I only looked briefly. If it's just
> a pmd entry, I think it's fair to require a 2MB alignment of hotplugged
> memory ranges.

Agree that 2MB alignment of hotplugged memory is fair, commit 
ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") supported it. 
The issue here I want to address is for its backup struct page metadata.

>