Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory

[Anshuman Khandual]

Hello Sudarshan,

On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
> When section mappings are enabled, we allocate vmemmap pages from physically
> continuous memory of size PMD_SZIE using vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB pressure. But when system is highly fragmented
> and memory blocks are being hot-added at runtime, its possible that such
> physically continuous memory allocations can fail. Rather than failing the

Did you really see this happen on a system ?

> memory hot-add procedure, add a fallback option to allocate vmemmap pages from
> discontinuous pages using vmemmap_populate_basepages().

Which could lead to a mixed page size mapping in the VMEMMAP area.
Allocation failure in vmemmap_populate() should just cleanly fail
the memory hot add operation, which can then be retried. Why the
retry has to be offloaded to kernel ?

> 
> Signed-off-by: Sudarshan Rajagopalan <sudaraja@xxxxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Steven Price <steven.price@xxxxxxx>
> ---
>  arch/arm64/mm/mmu.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 75df62f..a46c7d4 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1100,6 +1100,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  	p4d_t *p4dp;
>  	pud_t *pudp;
>  	pmd_t *pmdp;
> +	int ret = 0;
>  
>  	do {
>  		next = pmd_addr_end(addr, end);
> @@ -1121,15 +1122,23 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  			void *p = NULL;
>  
>  			p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
> -			if (!p)
> -				return -ENOMEM;
> +			if (!p) {
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +				vmemmap_free(start, end, altmap);
> +#endif

The mapping was never created in the first place, as the allocation
failed. vmemmap_free() here will free an unmapped area !

> +				ret = -ENOMEM;
> +				break;
> +			}
>  
>  			pmd_set_huge(pmdp, __pa(p), __pgprot(PROT_SECT_NORMAL));
>  		} else
>  			vmemmap_verify((pte_t *)pmdp, node, addr, next);
>  	} while (addr = next, addr != end);
>  
> -	return 0;
> +	if (ret)
> +		return vmemmap_populate_basepages(start, end, node, altmap);
> +	else
> +		return ret;
>  }
>  #endif	/* !ARM64_SWAPPER_USES_SECTION_MAPS */
>  void vmemmap_free(unsigned long start, unsigned long end,
>