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

--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
Linux Foundation Collaborative Project

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