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

[Steven Price <steven.price@arm.com>]

On 10/09/2020 07:05, 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
> 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@codeaurora.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Steven Price <steven.price@arm.com>
> ---
>   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
> +				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;

Style comment: I find this usage of 'ret' confusing. When we assign 
-ENOMEM above that is never actually the return value of the function 
(in that case vmemmap_populate_basepages() provides the actual return 
value).

Also the "return ret" is misleading since we know by that point that 
ret==0 (and the 'else' is redundant).

Can you not just move the call to vmemmap_populate_basepages() up to 
just after the (possible) vmemmap_free() call and remove the 'ret' variable?

AFAICT the call to vmemmap_free() also doesn't need the #ifdef as the 
function is a no-op if CONFIG_MEMORY_HOTPLUG isn't set. I also feel you 
need at least a comment to explain Anshuman's point that it looks like 
you're freeing an unmapped area. Although if I'm reading the code 
correctly it seems like the unmapped area will just be skipped.

Steve

>   }
>   #endif	/* !ARM64_SWAPPER_USES_SECTION_MAPS */
>   void vmemmap_free(unsigned long start, unsigned long end,
>