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

From: Sudarshan Rajagopalan <[email-masked]>
Date: Thu, 01 Oct 2020 12:51:09 -0700

On 2020-09-30 17:30, Anshuman Khandual wrote:
> 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 <[email-masked]>
>> Cc: Catalin Marinas <[email-masked]>
>> Cc: Will Deacon <[email-masked]>
>> Cc: Anshuman Khandual <[email-masked]>
>> Cc: Mark Rutland <[email-masked]>
>> Cc: Logan Gunthorpe <[email-masked]>
>> Cc: David Hildenbrand <[email-masked]>
>> Cc: Andrew Morton <[email-masked]>
>> Cc: Steven Price <[email-masked]>
>> ---
>>  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())

Yes agreed. We can allow fallback on altmap as well. I did indeed follow 
x86 semantics. Will send the updated patch.

Sudarshan

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