[PATCH v4 0/7] Use pageblock_order for cma and alloc_contig_range alignment.

[Zi Yan]

From: Zi Yan <ziy@xxxxxxxxxx>

Hi all,

This patchset tries to remove the MAX_ORDER-1 alignment requirement for CMA
and alloc_contig_range(). It prepares for my upcoming changes to make
MAX_ORDER adjustable at boot time[1]. It is on top of mmotm-2021-12-29-20-07.

Changelog from RFC
===
1. Dropped two irrelevant patches on non-lru compound page handling, as
   it is not supported upstream.
2. Renamed migratetype_has_fallback() to migratetype_is_mergeable().
3. Always check whether two pageblocks can be merged in
   __free_one_page() when order is >= pageblock_order, as the case (not
   mergeable pageblocks are isolated, CMA, and HIGHATOMIC) becomes more common.
3. Moving has_unmovable_pages() is now a separate patch.
4. Removed MAX_ORDER-1 alignment requirement in the comment in virtio_mem code.

Description
===

The MAX_ORDER - 1 alignment requirement comes from that alloc_contig_range()
isolates pageblocks to remove free memory from buddy allocator but isolating
only a subset of pageblocks within a page spanning across multiple pageblocks
causes free page accounting issues. Isolated page might not be put into the
right free list, since the code assumes the migratetype of the first pageblock
as the whole free page migratetype. This is based on the discussion at [2].

To remove the requirement, this patchset:
1. still isolates pageblocks at MAX_ORDER - 1 granularity;
2. but saves the pageblock migratetypes outside the specified range of
   alloc_contig_range() and restores them after all pages within the range
   become free after __alloc_contig_migrate_range();
3. only checks unmovable pages within the range instead of MAX_ORDER - 1 aligned
   range during isolation to avoid alloc_contig_range() failure when pageblocks
   within a MAX_ORDER - 1 aligned range are allocated separately.
3. splits free pages spanning multiple pageblocks at the beginning and the end
   of the range and puts the split pages to the right migratetype free lists
   based on the pageblock migratetypes;
4. returns pages not in the range as it did before.

Isolation needs to be done at MAX_ORDER - 1 granularity, because otherwise
either 1) it is needed to detect to-be-isolated page size (free, PageHuge, THP,
or other PageCompound) to make sure all pageblocks belonging to a single page
are isolated together and later restore pageblock migratetypes outside the
range, or 2) assuming isolation happens at pageblock granularity, a free page
with multi-migratetype pageblocks can seen in free page path and needs
to be split and freed at pageblock granularity.

One optimization might come later:
1. make MIGRATE_ISOLATE a separate bit to avoid saving and restoring existing
   migratetypes before and after isolation respectively.

Feel free to give comments and suggestions. Thanks.

[1] https://lore.kernel.org/linux-mm/20210805190253.2795604-1-zi.yan@xxxxxxxx/
[2] https://lore.kernel.org/linux-mm/d19fb078-cb9b-f60f-e310-fdeea1b947d2@xxxxxxxxxx/


Zi Yan (7):
  mm: page_alloc: avoid merging non-fallbackable pageblocks with others.
  mm: page_isolation: move has_unmovable_pages() to mm/page_isolation.c
  mm: page_isolation: check specified range for unmovable pages
  mm: make alloc_contig_range work at pageblock granularity
  mm: cma: 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                |   7 +-
 include/linux/mmzone.h                     |  16 +-
 include/linux/page-isolation.h             |   3 +-
 kernel/dma/contiguous.c                    |   2 +-
 mm/cma.c                                   |   6 +-
 mm/memory_hotplug.c                        |  12 +-
 mm/page_alloc.c                            | 337 +++++++++++----------
 mm/page_isolation.c                        | 154 +++++++++-
 9 files changed, 352 insertions(+), 189 deletions(-)

-- 
2.34.1