[RFC PATCH 0/3] Use pageblock_order for cma and alloc_contig_range alignment.

From: Zi Yan <ziy@nvidia.com>

Hi David,

You suggested to make alloc_contig_range() deal with pageblock_order instead of
MAX_ORDER - 1 and get rid of MAX_ORDER - 1 dependency in virtio_mem[1]. This
patchset is my attempt to achieve that. Please take a look and let me know if
I am doing it correctly or not.

From what my understanding, cma required alignment of
max(MAX_ORDER - 1, pageblock_order), because when MIGRATE_CMA was introduced,
__free_one_page() does not prevent merging two different pageblocks, when
MAX_ORDER - 1 > pageblock_order. But current __free_one_page() implementation
does prevent that. It should be OK to just align cma to pageblock_order.
alloc_contig_range() relies on MIGRATE_CMA to get free pages, so it can use
pageblock_order as alignment too.

In terms of virtio_mem, if I understand correctly, it relies on
alloc_contig_range() to obtain contiguous free pages and offlines them to reduce
guest memory size. As the result of alloc_contig_range() alignment change,
virtio_mem should be able to just align PFNs to pageblock_order.

Thanks.


[1] https://lore.kernel.org/linux-mm/28b57903-fae6-47ac-7e1b-a1dd41421349@redhat.com/ (local)

Zi Yan (3):
  mm: cma: alloc_contig_range: use pageblock_order as the single
    alignment.
  drivers: virtio_mem: use pageblock size as the minimum virtio_mem
    size.
  arch: powerpc: adjust fadump alignment to be pageblock aligned.

 arch/powerpc/include/asm/fadump-internal.h |  4 +---
 drivers/virtio/virtio_mem.c                |  6 ++----
 include/linux/mmzone.h                     |  5 +----
 kernel/dma/contiguous.c                    |  2 +-
 mm/cma.c                                   |  6 ++----
 mm/page_alloc.c                            | 12 +++++-------
 6 files changed, 12 insertions(+), 23 deletions(-)

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2.33.0