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

[Sudarshan Rajagopalan]

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@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())

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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