Re: [PATCH v3] arm64/mm: add fallback option to allocate virtually contiguous memory

[Gavin Shan]

On 10/6/20 2:36 PM, Anshuman Khandual wrote:
> On 10/02/2020 01:46 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 | 11 +++++++++--
> >   1 file changed, 9 insertions(+), 2 deletions(-)

It looks good to me with Anshuman's comments fixed:

Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>

> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 75df62f..11f8639 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -1121,8 +1121,15 @@ 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) {
> > +				/*
> > +				 * fallback allocating with virtually
> > +				 * contiguous memory for this section
> > +				 */
>
> Mapping is always virtually contiguous with or without huge pages.
> Please drop this comment here, as it's obvious.
>
> > +				if (vmemmap_populate_basepages(addr, next, node, NULL))
> > +					return -ENOMEM;
>
> Please send in the 'altmap' instead of NULL for allocation from
> device memory if and when requested.
>
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