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

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Date: Tue, 14 May 2024 18:04:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Björn Töpel <bjorn@...nel.org>,
 Alexandre Ghiti <alexghiti@...osinc.com>, Albert Ou <aou@...s.berkeley.edu>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, linux-riscv@...ts.infradead.org
Cc: Björn Töpel <bjorn@...osinc.com>,
 Andrew Bresticker <abrestic@...osinc.com>,
 Chethan Seshadri <Chethan.Seshadri@...alinasystems.io>,
 Lorenzo Stoakes <lstoakes@...il.com>, Oscar Salvador <osalvador@...e.de>,
 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 14.05.24 16:04, Björn Töpel wrote:
> From: Björn Töpel <bjorn@...osinc.com>
> 
> For an architecture to support memory hotplugging, a couple of
> callbacks needs to be implemented:
> 
>   arch_add_memory()
>    This callback is responsible for adding the physical memory into the
>    direct map, and call into the memory hotplugging generic code via
>    __add_pages() that adds the corresponding struct page entries, and
>    updates the vmemmap mapping.
> 
>   arch_remove_memory()
>    This is the inverse of the callback above.
> 
>   vmemmap_free()
>    This function tears down the vmemmap mappings (if
>    CONFIG_SPARSEMEM_VMEMMAP is enabled), and also deallocates the
>    backing vmemmap pages. Note that for persistent memory, an
>    alternative allocator for the backing pages can be used; The
>    vmem_altmap. This means that when the backing pages are cleared,
>    extra care is needed so that the correct deallocation method is
>    used.
> 
>   arch_get_mappable_range()
>    This functions returns the PA range that the direct map can map.
>    Used by the MHP internals for sanity checks.
> 
> The page table unmap/teardown functions are heavily based on code from
> the x86 tree. The same remove_pgd_mapping() function is used in both
> vmemmap_free() and arch_remove_memory(), but in the latter function
> the backing pages are not removed.
> 
> Signed-off-by: Björn Töpel <bjorn@...osinc.com>
> ---
>   arch/riscv/mm/init.c | 242 +++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 242 insertions(+)
> 
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 6f72b0b2b854..7f0b921a3d3a 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -1493,3 +1493,245 @@ void __init pgtable_cache_init(void)
>   	}
>   }
>   #endif
> +
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +static void __meminit free_pte_table(pte_t *pte_start, pmd_t *pmd)
> +{
> +	pte_t *pte;
> +	int i;
> +
> +	for (i = 0; i < PTRS_PER_PTE; i++) {
> +		pte = pte_start + i;
> +		if (!pte_none(*pte))
> +			return;
> +	}
> +
> +	free_pages((unsigned long)page_address(pmd_page(*pmd)), 0);
> +	pmd_clear(pmd);
> +}
> +
> +static void __meminit free_pmd_table(pmd_t *pmd_start, pud_t *pud)
> +{
> +	pmd_t *pmd;
> +	int i;
> +
> +	for (i = 0; i < PTRS_PER_PMD; i++) {
> +		pmd = pmd_start + i;
> +		if (!pmd_none(*pmd))
> +			return;
> +	}
> +
> +	free_pages((unsigned long)page_address(pud_page(*pud)), 0);
> +	pud_clear(pud);
> +}
> +
> +static void __meminit free_pud_table(pud_t *pud_start, p4d_t *p4d)
> +{
> +	pud_t *pud;
> +	int i;
> +
> +	for (i = 0; i < PTRS_PER_PUD; i++) {
> +		pud = pud_start + i;
> +		if (!pud_none(*pud))
> +			return;
> +	}
> +
> +	free_pages((unsigned long)page_address(p4d_page(*p4d)), 0);
> +	p4d_clear(p4d);
> +}
> +
> +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));

If you unplug a DIMM that was added during boot (can happen on x86-64, 
can it happen on riscv?), free_pages() would not be sufficient. You'd be 
freeing a PG_reserved page that has to be freed differently.

-- 
Cheers,

David / dhildenb