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

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Message-ID: <87r0e4tkrd.fsf@all.your.base.are.belong.to.us>
Date: Tue, 14 May 2024 18:34:46 +0200
From: Björn Töpel <bjorn@...nel.org>
To: David Hildenbrand <david@...hat.com>, 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

David Hildenbrand <david@...hat.com> writes:

> 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.

I'd say if it can happen on x86-64, it probably can on RISC-V. I'll look
into this for the next spin!

Thanks for spending time on the series!


Cheers,
Björn