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

From: Oscar Salvador <osalvador@suse.de>
To: "Björn Töpel" <bjorn@kernel.org>
Cc: "Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alexghiti@rivosinc.com>,
	"Chethan Seshadri" <Chethan.Seshadri@catalinasystems.io>,
	"David Hildenbrand" <david@redhat.com>,
	linux-kernel@vger.kernel.org,
	"Andrew Bresticker" <abrestic@rivosinc.com>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	"Santosh Mamila" <santosh.mamila@catalinasystems.io>,
	linux-mm@kvack.org,
	"Sivakumar Munnangi" <siva.munnangi@catalinasystems.io>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	linux-riscv@lists.infradead.org,
	virtualization@lists.linux-foundation.org,
	"Lorenzo Stoakes" <lstoakes@gmail.com>
Subject: Re: [PATCH v2 4/8] riscv: mm: Add memory hotplugging support
Date: Tue, 14 May 2024 22:49:44 +0200	[thread overview]
Message-ID: <ZkPOaAUZrRWRGke0@localhost.localdomain> (raw)
In-Reply-To: <20240514140446.538622-5-bjorn@kernel.org>

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

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