linux-kernel - Re: [PATCH v2 4/8] riscv: mm: Add memory hotplugging support

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Date: Tue, 14 May 2024 22:49:44 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Björn Töpel <bjorn@...nel.org>
Cc: Alexandre Ghiti <alexghiti@...osinc.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	David Hildenbrand <david@...hat.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	linux-riscv@...ts.infradead.org,
	Björn Töpel <bjorn@...osinc.com>,
	Andrew Bresticker <abrestic@...osinc.com>,
	Chethan Seshadri <Chethan.Seshadri@...alinasystems.io>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Santosh Mamila <santosh.mamila@...alinasystems.io>,
	Sivakumar Munnangi <siva.munnangi@...alinasystems.io>,
	Sunil V L <sunilvl@...tanamicro.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 4/8] riscv: mm: Add memory hotplugging support

On Tue, May 14, 2024 at 04:04:42PM +0200, Björn Töpel wrote:
> +static void __meminit free_vmemmap_storage(struct page *page, size_t size,
> +					   struct vmem_altmap *altmap)
> +{
> +	if (altmap)
> +		vmem_altmap_free(altmap, size >> PAGE_SHIFT);
> +	else
> +		free_pages((unsigned long)page_address(page), get_order(size));

David already pointed this out, but can check
arch/x86/mm/init_64.c:free_pagetable().

You will see that we have to do some magic for bootmem memory (DIMMs
which were not hotplugged but already present)

> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> +void __ref vmemmap_free(unsigned long start, unsigned long end, struct vmem_altmap *altmap)
> +{
> +	remove_pgd_mapping(start, end, true, altmap);
> +}
> +#endif /* CONFIG_SPARSEMEM_VMEMMAP */
> +#endif /* CONFIG_MEMORY_HOTPLUG */

I will comment on the patch where you add support for hotplug and the
dependency, but on a track in LSFMM today, we decided that most likely
we will drop memory-hotplug support for !CONFIG_SPARSEMEM_VMEMMAP
environments.
So, since you are adding this plain fresh, please consider to tight the
hotplug dependency to CONFIG_SPARSEMEM_VMEMMAP.
As a bonus, you will only have to maintain one flavour of functions.


-- 
Oscar Salvador
SUSE Labs