[PATCH v8 0/5] Use pageblock_order for cma and alloc_contig_range alignment.

[Zi Yan]

From: Zi Yan <ziy@xxxxxxxxxx>

Hi David,

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-2022-03-16-17-42.

Changelog
===
V8
---
1. Cleaned up has_unmovable_pages() to remove page argument.

V7
---
1. Added page validity check in isolate_single_pageblock() to avoid out
   of zone pages.
2. Fixed a bug in split_free_page() to split and free pages in correct
   page order.

V6
---
1. Resolved compilation error/warning reported by kernel test robot.
2. Tried to solve the coding concerns from Christophe Leroy.
3. Shortened lengthy lines (pointed out by Christoph Hellwig).

V5
---
1. Moved isolation address alignment handling in start_isolate_page_range().
2. Rewrote and simplified how alloc_contig_range() works at pageblock
   granularity (Patch 3). Only two pageblock migratetypes need to be saved and
   restored. start_isolate_page_range() might need to migrate pages in this
   version, but it prevents the caller from worrying about
   max(MAX_ORDER_NR_PAEGS, pageblock_nr_pages) alignment after the page range
   is isolated.

V4
---
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. isolates pages at pageblock granularity instead of
   max(MAX_ORDER_NR_PAEGS, pageblock_nr_pages);
2. splits free pages across the specified range or migrates in-use pages
   across the specified range then splits the freed page to avoid free page
   accounting issues (it happens when multiple pageblocks within a single page
   have different migratetypes);
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.
4. returns pages not in the range as it did before.

One optimization might come later:
1. make MIGRATE_ISOLATE a separate bit to be able to restore the original
   migratetypes when isolation fails in the middle of the range.

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 (5):
  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.

 drivers/virtio/virtio_mem.c    |   6 +-
 include/linux/cma.h            |   4 +-
 include/linux/mmzone.h         |   5 +-
 include/linux/page-isolation.h |  14 +-
 mm/internal.h                  |   6 +
 mm/memory_hotplug.c            |   3 +-
 mm/page_alloc.c                | 246 +++++++--------------------
 mm/page_isolation.c            | 296 +++++++++++++++++++++++++++++++--
 8 files changed, 367 insertions(+), 213 deletions(-)

-- 
2.35.1