Re: [PATCH v4] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[Zhenhua Huang]

On 2025/1/8 18:52, Anshuman Khandual wrote:
> > I found another bug, that even for early section, when vmemmap_populate is called, SECTION_IS_EARLY is not set. Therefore, early_section() always return false.
> Hmm, well that's unexpected.
>
> > Since vmemmap_populate() occurs during section initialization, it may be hard to say it is a bug..
> > However, should we instead using SECTION_MARKED_PRESENT to check? I tested well in my setup.
> >
> > Hot plug flow:
> > 1. section_activate -> vmemmap_populate
> > 2. mark PRESENT
> >
> > In contrast, the early flow:
> > 1. memblocks_present -> mark PRESENT
> > 2. __populate_section_memmap -> vmemmap_populate
> But from a semantics perspective, should SECTION_MARKED_PRESENT be marked on a
> section before SECTION_IS_EARLY ? Is it really the expected behaviour here or
> that needs to be fixed first ?

The tricky part is vmemmap_populate initializes mem_map, that happens 
during mem_section initialization process. PRESENT or EARLY tag is in 
the same process as well. There doesn't appear to be a compelling reason 
to enforce a specific sequence..

> Although SYSTEM_BOOTING state check might help but section flag seems to be the
> right thing to do here.

Good idea, I prefer to vote for this alternative rather than PRESENT 
tag. As I see we already took this stage to determine whether memmap 
pages are boot pages or not in common mm code:
https://elixir.bootlin.com/linux/v6.13-rc3/source/mm/sparse-vmemmap.c#L465

Would like to hear Catalin's perspective ?:)