Re: [PATCH v2] arm64/mm: add fallback option to allocate virtually contiguous memory

[Anshuman Khandual]

On 10/01/2020 04:43 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().
> 
> 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 | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 75df62f..9edbbb8 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1121,8 +1121,18 @@ 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 (altmap)
> +					return -ENOMEM;	/* no fallback */

Why ? If huge pages inside a vmemmap section might have been allocated
from altmap, the base page could also fallback on altmap. If this patch
has just followed the existing x86 semantics, it was written [1] long
back before vmemmap_populate_basepages() supported altmap allocation.
While adding that support [2] recently, it was deliberate not to change 
x86 semantics as it was a platform decision. Nonetheless, it makes sense
to fallback on altmap bases pages if and when required.

[1] 4b94ffdc4163 (x86, mm: introduce vmem_altmap to augment vmemmap_populate())
[2] 1d9cfee7535c (mm/sparsemem: enable vmem_altmap support in vmemmap_populate_basepages())