[PATCH v3] arm64/mm: add fallback option to allocate virtually contiguous memory

[Anshuman Khandual]

On 10/13/2020 04:35 AM, Sudarshan Rajagopalan wrote:
> When section mappings are enabled, we allocate vmemmap pages from physically
> continuous memory of size PMD_SIZE 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
> memory hot-add procedure, add a fallback option to allocate vmemmap pages from
> discontinuous pages using vmemmap_populate_basepages().

There is a checkpatch warning here, which could be fixed while merging ?

WARNING: Possible unwrapped commit description (prefer a maximum 75 chars per line)
#7: 
When section mappings are enabled, we allocate vmemmap pages from physically

total: 0 errors, 1 warnings, 13 lines checked

> 
> Signed-off-by: Sudarshan Rajagopalan <sudaraja at codeaurora.org>
> Reviewed-by: Gavin Shan <gshan at redhat.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Logan Gunthorpe <logang at deltatee.com>
> Cc: David Hildenbrand <david at redhat.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Steven Price <steven.price at arm.com>

Nonetheless, this looks fine. Did not see any particular problem
while creating an experimental vmemmap with interleaving section
and base page mapping.

Reviewed-by: Anshuman Khandual <anshuman.khandual at arm.com>

> ---
>  arch/arm64/mm/mmu.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 75df62fea1b6..44486fd0e883 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1121,8 +1121,11 @@ 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) {
> +				if (vmemmap_populate_basepages(addr, next, node, altmap))
> +					return -ENOMEM;
> +				continue;
> +			}
>  
>  			pmd_set_huge(pmdp, __pa(p), __pgprot(PROT_SECT_NORMAL));
>  		} else
>