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

[Zhenhua Huang]

On 2025/1/9 22:32, Catalin Marinas wrote:
> On Thu, Jan 09, 2025 at 03:04:22PM +0800, Zhenhua Huang wrote:
> > 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.
> [...]
> > > > 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..
>
> The order in which a section is marked as present and vmemmap created
> does seem a bit arbitrary. At least the early code seems to rely on the
> for_each_present_section_nr() loop, so we'll always have this first but
> it's not some internal kernel API that guarantees this.
>
> > > 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
>
> The advantage of SYSTEM_BOOTING is that we don't need to rely on the
> section information at all, though we could add a WARN_ON_ONCE if the
> section is not present.

Hi Catalin,

Sorry, but I don't fully understand your comment here, IIUC we shouldn't 
 add WARN_ON_ONCE in vmemmap_populate(). As you mentioned above, early 
code relies on section present. while the hotplug code does not 
guarantee, it will set PRESENT after calling vmemmap_populate().
By the way, seems you're not opposed to using SYSTEM_BOOTING ? If so, 
please take a look at latest post:
https://lore.kernel.org/linux-mm/20250109093824.452925-1-quic_zhenhuah@xxxxxxxxxxx/
Thanks very much!
>