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    • Oliver O'Halloran's avatar
      powerpc/mm: Fallback to RAM if the altmap is unusable · 9ef34630
      Oliver O'Halloran authored
      The "altmap" is used to provide a pool of memory that is reserved for
      the vmemmap backing of hot-plugged memory. This is useful when adding
      large amount of ZONE_DEVICE memory to a system with a limited amount of
      normal memory.
      
      On ppc64 we use huge pages to map the vmemmap which requires the backing
      storage to be contigious and aligned to the hugepage size. The altmap
      implementation allows for the altmap provider to reserve a few PFNs at
      the start of the range for it's own uses and when this occurs the
      first chunk of the altmap is not usable for hugepage mappings. On hash
      there is no sane way to fall back to a normal sized page mapping so we
      fail the allocation. This results in memory hotplug failing with
      ENOMEM when the new range doesn't fall into an existing vmemmap block.
      
      This patch handles this case by falling back to using system memory
      rather than failing if we cannot allocate from the altmap. This
      fallback should only ever be used for the first vmemmap block so it
      should not cause excess memory consumption.
      
      Fixes: 7b73d978 ("mm: pass the vmem_altmap to vmemmap_populate")
      Signed-off-by: default avatarOliver O'Halloran <oohall@gmail.com>
      Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
      9ef34630